Wildfire and charcoal enhance nitrification and ammonium-oxidizing bacterial abundance in dry montane forest soils

Effects of the three amendments on NH3 volatilization, N2O emissions, and nitrification at four salinity levels: An indoor experiment

The marked salinity and alkaline pH of coastal saline soil profoundly impact the nitrogen conversion process, leading to a significantly reduced nitrogen utilization efficiency and substantial gaseous nitrogen loss. The application of soil amendments (e.g. biochar, manure, and gypsum) was proved to be effective for the remediation of saline soils. However, the effects of the three amendments on soil nitrogen transformation in soils with various salinity levels, especially on NH3 volatilization and N2O emission, remain elusive. Here, we reported the effects of biochar, manure, and gypsum on NH3 volatilization and N2O emission under four natural salinity gradients in the Yellow River Delta. Also, high-throughput sequencing and qPCR analysis were performed to characterize the response of nitrification (amoA) and denitrification (nirS, nirK, and nosZ) functional genes to the three amendments. The results showed that the three amendments had little effect on NH3 volatilization in low- and moderate-salinity soils, while biochar stimulated NH3 volatilization in high-salinity soils and reduced NH3 volatilization in severe-salinity soils. Spearman correlation analysis demonstrated that AOA was significantly and positively correlated with the NO3--N content (r = 0.137, P < 0.05) and N2O emissions (r = 0.174, P < 0.01), which indicated that AOA dominated N2O emissions from nitrification in saline soils. Structural equation modeling indicated that biochar, manure, and gypsum affected N2O emission by influencing soil pH, conductivity, mineral nitrogen content, and functional genes (AOA-amoA and nosZ). Two-way ANOVA further showed that salinity and amendments (biochar, manure, and gypsum) had significant effects on N2O emissions. In summary, this study provides valuable insights to better understand the effects of gaseous N changes in saline soils, thereby improving the accuracy and validity of future GHG emission predictions and modeling.

Elucidating the impact of biochar with different carbon/nitrogen ratios on soil biochemical properties and rhizosphere bacterial communities of flue-cured tobacco plants

Background and aims

In agriculture, biochar (BC) and nitrogen (N) fertilizers are commonly used for improving soil fertility and crop productivity. However, it remains unclear how different levels of BC and N fertilizer affect soil fertility and crop productivity.

Methods

This study elucidates the impact of different application rates of BC (0, 600, and 1200 kg/ha) and N fertilizer (105 and 126 kg/ha) on biomass accumulation, soil microbial biomass of carbon (SMC) and nitrogen (SMN), and soil biochemical properties, including soil organic carbon (SOC), total nitrogen (TN), soil nitrate nitrogen (NO3--N), ammonium nitrogen (NH4+-N), urease (UE), acid phosphatase (ACP), catalase (CAT), and sucrase (SC) of tobacco plants. In addition, a high throughput amplicon sequencing technique was adopted to investigate the effect of different application rates of BC/N on rhizosphere bacterial communities of tobacco plants.

Results

The results confirm that high dosages of BC and N fertilizer (B1200N126) significantly enhance dry matter accumulation by 31.56% and 23.97% compared with control B0N105 and B0N126 under field conditions and 23.94% and 24.52% under pot experiment, respectively. The soil biochemical properties, SMC, and SMN significantly improved under the high application rate of BC and N fertilizer (B1200N126), while it negatively influenced the soil carbon/nitrogen ratio. Analysis of rhizosphere bacteriome through amplicon sequencing of 16S rRNA revealed that the structure, diversity, and composition of rhizosphere bacterial communities dramatically changed under different BC/N ratios. Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes, and Acidobacteria were highly abundant bacterial phyla in the rhizosphere of tobacco plants under different treatments. Co-occurrence network analysis displayed fewer negative correlations among rhizosphere bacterial communities under high dosages of biochar and nitrogen (B1200N126) than other treatments, which showed less competition for resources among microbes. In addition, a redundancy analysis further proved a significant positive correlation among SMC, SMN, soil biochemical properties, and high dosage of biochar and nitrogen (B1200N126).

Conclusions

Thus, we conclude that a high dosage of BC (1200 kg/ha) under a high application rate of N fertilizer (126 kg/ha) enhances the biomass accumulation of tobacco plants by improving the soil biochemical properties and activities of rhizosphere bacterial communities.

Pathways framework identifies wildfire impacts on agriculture

Wildfires are a growing concern to society and the environment in many parts of the world. Within the United States, the land area burned by wildfires has steadily increased over the past 40 years. Agricultural land management is widely understood as a force that alters fire regimes, but less is known about how wildfires, in turn, impact the agriculture sector. Based on an extensive literature review, we identify three pathways of impact-direct, downwind and downstream-through which wildfires influence agricultural resources (soil, water, air and photosynthetically active radiation), labour (agricultural workers) and products (crops and livestock). Through our pathways framework, we highlight the complexity of wildfire-agriculture interactions and the need for collaborative, systems-oriented research to better quantify the magnitude of wildfire impacts and inform the adaptation of agricultural systems to an increasingly fire-prone future.

Nanobiochar Associated Ammonia Emission Mitigation and Toxicity to Soil Microbial Biomass and Corn Nutrient Uptake from Farmyard Manure

The unique properties of NB, such as its nano-size effect and greater adsorption capacity, have the potential to mitigate ammonia (NH₃) emission, but may also pose threats to soil life and their associated processes, which are not well understood. We studied the influence of different NB concentrations on NH₃ emission, soil microbial biomass, nutrient mineralization, and corn nutrient uptake from farmyard manure (FM). Three different NB concentrations i.e., 12.5 (NB1), 25 (NB2), and 50% (NB3), alone and in a fertilizer mixture with FM, were applied to corn. NB1 alone increased microbial biomass in soil more than control, but other high NB concentrations did not influence these parameters. In fertilizer mixtures, NB2 and NB3 decreased NH₃ emission by 25% and 38%, respectively, compared with FM alone. Additionally, NB3 significantly decreased microbial biomass carbon, N, and soil potassium by 34%, 36%, and 14%, respectively, compared with FM. This toxicity to soil parameters resulted in a 21% decrease in corn K uptake from FM. Hence, a high NB concentration causes toxicity to soil microbes, nutrient mineralization, and crop nutrient uptake from the FM. Therefore, this concentration-dependent toxicity of NB to soil microbes and their associated processes should be considered before endorsing NB use in agroecosystems.

Partial Substitution of Urea with Biochar Induced Improvements in Soil Enzymes Activity, Ammonia-Nitrite Oxidizers, and Nitrogen Uptake in the Double-Cropping Rice System

Biochar is an important soil amendment that can enhance the biological properties of soil, as well as nitrogen (N) uptake and utilization in N-fertilized crops. However, few studies have characterized the effects of urea and biochar application on soil biochemical traits and its effect on paddy rice. Therefore, a field trial was conducted in the early and late seasons of 2020 in a randomized complete block design with two N levels (135 and 180 kg ha^-1) and four levels of biochar (0, 10, 20, and 30 t ha^-1). The treatment combinations were as follows: 135 kg N ha^-1 + 0 t B ha^-1 (T1), 135 kg N ha^-1 + 10 t B ha^-1 (T2), 135 kg N ha^-1 + 20 t B ha^-1 (T3), 135 kg N ha^-1 + 30 t B ha^-1 (T4), 180 kg N ha^-1 + 0 t B ha^-1 (T5), 180 kg N ha^-1 + 10 t B ha^-1 (T6), 180 kg N ha^-1 + 20 t B ha^-1 (T7) and 180 kg N ha^-1 + 30 t B ha^-1 (T8). The results showed that soil amended with biochar had higher soil pH, soil organic carbon content, total nitrogen content, and mineral nitrogen (NH₄⁺-N and NO₃^--N) than soil that had not been amended with biochar. In both seasons, the 20 t ha^-1 and 30 t ha^-1 biochar treatments had the highest an average concentrations of NO₃^--N (10.54 mg kg^-1 and 10.25 mg kg^-1, respectively). In comparison to soil that had not been treated with biochar, the average activity of the enzymes urease, polyphenol oxidase, dehydrogenase, and chitinase was, respectively, 25.28%, 14.13%, 67.76%, and 22.26% greater; however, the activity of the enzyme catalase was 15.06% lower in both seasons. Application of biochar considerably increased the abundance of ammonia-oxidizing bacteria (AOB), which was 48% greater on average in biochar-amended soil than in unamended soil. However, there were no significant variations in the abundances of ammonia-oxidizing archaea (AOA) or nitrite-oxidizing bacteria (NOB) across treatments. In comparison to soil that had not been treated with biochar, the average N content was 24.46%, 20.47%, and 19.08% higher in the stem, leaves, and panicles, respectively. In general, adding biochar at a rate of 20 to 30 t ha^-1 with low-dose urea (135 kg N ha^-1) is a beneficial technique for improving the nutrient balance and biological processes of soil, as well as the N uptake and grain yield of rice plants.

Soil biogeochemistry and microbial community dynamics in Pinus pinaster Ait. forests subjected to increased fire frequency

Fire frequency might increase in many fire-dominated ecosystems of the world due to the combined effects of global warming, land-use change and increased human pressures. Understanding how changes in fire frequency can affect the main soil biogeochemical dynamics, as well as the microbial community, in the long term is utmost important. Here we determined the effect of changes in fire frequency and other fire history characteristics on soil C and N dynamics and the main microbial groups (using soil fatty acid profiles), in Pinus pinaster forests from central Spain. Stands were chosen to differ in the number of fires (1 to 3) occurred between 1976 and 2018, in the time elapsed since the last fire and the interval undergone between the last two consecutive fires. We found that, in general, most of the studied biogeochemical and microbial variables showed clear differences between unburned and burned stands. The time elapsed since the last fire was the most important fire history covariable and governed the main soil nutrient dynamics and microbial groups. Recovery to pre-fire values took 30-40 years. Increased wildfire frequency only modified total C and nitrification rate, but results were not consistent between stands burned twice and thrice. The time interval (years) between the last two fires was not a significant covariable. The fact that some stands burnt up to thrice in a period of 43 years supports the strong capacity of this ecosystem to recover, even under an increased fire frequency.

The effect of biochar on nitrogen availability and bacterial community in farmland

Purpose

Nitrification and denitrification in soil are key components of the global nitrogen cycle. This study was conducted to investigate the effect of biochar application on soil nitrogen and bacterial diversity.

Methods

Pot experiments were conducted to investigate the effects of different biochar-based rates 0% (CK), 0.5% (BC1), 1.0% (BC2), 2.0% (BC3), and 4.0% (BC4) on soil nutrient and bacterial community diversity and composition.

Results

The results indicate that the total nitrogen (TN) and ammonium nitrogen (AN) contents in the soil increased by 4.7–32.3% and 8.3–101.5%, respectively. The microbial biomass nitrogen (MBN) content increased with increased amounts of biochar rate. The application of biochar also significantly changed the soil bacterial community composition. The copy number of 16S marker gene of related enzymes to the nitrification process in BC2 was reduced by 20.1%. However, the gene expressions of nitric oxide reductase and nitrous oxide reductase in BC3 increased by 16.4% and 16.0%, respectively, compared to those in CK. AN, nitrate nitrogen (NN), and NN/TN were the main factors affecting the structure of the soil bacterial community. In addition, the expressions of nitrite reductase, hydroxylamine, and nitric oxide reductase (cytochrome c) were also significantly correlated.

Conclusion

Therefore, the applied biochar improved soil nitrogen availability and which ultimately resulted in an environmental risk decrease by soil nitrogen release inhibition.

Soil nitrogen functional transformation microbial genes response to biochar application in different irrigation paddy field in southern China

Growing evidence points to the controlled irrigation (CI) and biochar application (BA) having agricultural economic value and ecological benefits, but their synergistic effect and microbial mechanism of nitrogen conversion remain unknown in paddy fields. The effects of different BA (0, 20, 40 t/hm²) on the soil nitrogen functional transformation microbial genes (nifH, AOA-amoA, AOB-amoA) in different irrigation (CI, flooding irrigation) were clarified. After one seasonal growth of paddy, the correlation between the abundance of functional genes OUT and soil nitrogen transformation environment factors during the typical growth period was analyzed. High-throughput sequencing results illustrated that the application of CC (40 t/hm² biochar) increased the nifH genes bacterial community abundance; the abundance of dominant microorganism increased by 79.68~86.19%. Because biochar can potentially control the rates of N cycling in soil systems by adsorbing ammonia and increasing NH₄⁺ storage, it increased soil NH₄⁺-N and NO₃^--N content by 60.77% and 26.14%, improving microbial nitrogen fixation. Rare species Nitrosopumilus, Nitrosococcus, and Methylocystis appeared in biochar treatments group, which increased the diversity of microbial in paddy. The combined use of CI and BA affected soil inorganic nitrogen content, temperature (T), pH, Eh, etc., which affected urease, urea hydrolysis, and nitrogen functional transformation microorganism genes. Correlation analysis shows that soil NH₄⁺-N, T, and Eh, respectively, are significant factors for the formation of nifH, AOA-amoA, and AOB-amoA soil bacterial communities, respectively. This study suggests that to maintain the biodiversity of soil and realize the sustainable development of rice cultivation, CI is of great importance in combination with BA.

Effectiveness of Biochar and Zeolite Soil Amendments in Reducing Pollution of Municipal Wastewater from Nitrogen and Coliforms

A greenhouse experiment with soil cores and wastewater application was carried out to investigate the effects of biochar and zeolite on the mobility of nitrogen and coliform bacteria during the leaching of columns repacked by a silty loam soil. Triticum aestivum plants were grown in cores with and without biochar and zeolite irrigated with municipal wastewater for 4 months in the greenhouse. Cores were then flushed with 800 mLof distillate water and, finally, the leachate was collected. Application of biochar or zeolite significantly (p ≤ 0.05) reduced nitrate and ammonium loss in soil after leaching process, compared to their non-treated counterparts. In addition, interactions of biochar and zeolite significantly decreased nitrate and ammonium content in leachate. Biochar had higher removal effects of coliform bacteria in leachate than zeolite. Lower nitrate and ammonium content in leachate was related to the increased retention of soil amendments. Application of 5% w/w of biochar also reduced the volume of leachate by 11% compare to control, but using 5% w/w and 10% w/w of zeolite increased the volume of leachate compared with non-treated columns by 21% and 48%, respectively. Taken together, these data highlight the need to consider the potential benefits of biochar and zeolite as soil amendment to reduce nitrogen mobility and remove coliform bacteria in the leaching process of municipal wastewater in agricultural systems.

Prospects of Biochar for Sustainable Agriculture and Carbon Sequestration: An Overview for Eastern Himalayas

The net arable land area is declining worldwide rapidly due to soil erosion, drought, loss of soil organic carbon, and other forms of degradation. Intense rainfall, cultivation along steep slopes, unscientific land-use changes, shifting cultivation, soil acidity, and nutrient mining in hills and mountains make agriculture unsustainable and less profitable. Hills and mountain ecosystems of the Eastern Himalayan Region (EHR) are further prone to the impact of climate change posing a serious threat to agricultural production and the environment. Increasing soil carbon reserves contributes to multiple ecosystem services, improves soil nutrient and water-holding capacities, and advances climate-resilient agriculture. Thus, carbon sequestration is increasingly becoming an important aspect of farming among researchers in the region. The EHR predominantly practices shifting cultivation that degrades the ecosystem and promotes land degradation and biodiversity loss. Leaching of exchangeable bases is highly favored due to excess rainfall which in turn creates an acidic soil accounting for >84% of the region. Application of lime to raise the soil acidity for the cultivation of crops did not get adequate acceptance among the farming community due to multiple issues such as cost involvement, non-availability in time and place, and transportation issues. The application of biochar as soil amendments is widely known to improve soil’s physical, chemical, and biological properties. Biochar has also emerged as a potential candidate for long-term carbon sequestration due to its inbuilt structure and higher stability. Shift from traditional “slash and burn” culture to “slash and char” might lead to the sequestration of carbon from the atmosphere. Around 0.21 Pg of carbon (12% of the total anthropogenic carbon emissions by land-use change) can be sequestered in the soil if the traditional “slash and burnt” practice is converted to “slash and char”. The objective of this review is to provide detailed information about the role of biochar in altering the soil properties for sustaining agriculture and carbon sequestration, especially for hills and mountain ecosystems.

Microbial Community-Level Physiological Profiles and Genetic Prokaryotic Structure of Burned Soils Under Mediterranean Sclerophyll Forests in Central Chile

Forest fires alter soil microbial communities that are essential to support ecosystem recovery following land burning. These alterations have different responses according to soil abiotic pre- and post-fire conditions and fire severity, among others, and tend to decrease along vegetation recovery over time. Thus, understanding the effects of fires on microbial soil communities is critical to evaluate ecosystem resilience and restoration strategies in fire-prone ecosystems. We studied the state of community-level physiological profiles (CLPPs) and the prokaryotic community structure of rhizosphere and bulk soils from two fire-affected sclerophyll forests (one surveyed 17 months and the other 33 months after fire occurrence) in the Mediterranean climate zone of central Chile. Increases in catabolic activity (by average well color development of CLPPs), especially in the rhizosphere as compared with the bulk soil, were observed in the most recently affected site only. Legacy of land burning was still clearly shaping soil prokaryote community structure, as shown by quantitative PCR (qPCR) and Illumina MiSeq sequencing of the V4 region of the 16S rRNA gene, particularly in the most recent fire-affected site. The qPCR copy numbers and alpha diversity indexes (Shannon and Pielou’s evenness) of sequencing data decreased in burned soils at both locations. Beta diversity analyses showed dissimilarity of prokaryote communities at both study sites according to fire occurrence, and NO₃^– was the common variable explaining community changes for both of them. Acidobacteria and Rokubacteria phyla significantly decreased in burned soils at both locations, while Firmicutes and Actinobacteria increased. These findings provide a better understanding of the resilience of soil prokaryote communities and their physiological conditions in Mediterranean forests of central Chile following different time periods after fire, conditions that likely influence the ecological processes taking place during recovery of fire-affected ecosystems.

Potentially Toxic Substances and Associated Risks in Soils Affected by Wildfires: A Review

The presence of toxic substances is one of the major causes of degradation of soil quality. Wildfires, besides affecting various chemical, physical, and biological soil properties, produce a mixture of potentially toxic substances which can reach the soil and water bodies and cause harm to these media. This review intends to summarise the current knowledge on the generation by wildfires of potentially toxic substances, their effects on soil organisms, and other associated risks, addressing the effects of fire on metal mobilisation, the pyrolytic production of potentially toxic compounds, and the detoxifying effect of charcoal. Numerous studies ascertained inhibitory effects of ash on seed germination and seedling growth as well as its toxicity to soil and aquatic organisms. Abundant publications addressed the mobilisation of heavy metals and trace elements by fire, including analyses of total concentrations, speciation, availability, and risk of exportation to water bodies. Many publications studied the presence of polycyclic aromatic hydrocarbons (PAH) and other organic pollutants in soils after fire, their composition, decline over time, the risk of contamination of surface and ground waters, and their toxicity to plants, soil, and water organisms. Finally, the review addresses the possible detoxifying role of charcoal in soils affected by fire.

Biochar enhances the retention capacity of nitrogen fertilizer and affects the diversity of nitrifying functional microbial communities in karst soil of southwest China

Biochar is usually used as an agricultural soil amendment to improve soil nutrition availability and soil microbial environment. However, the effects of Moutai lees biochar on the migration and retention characteristics of nitrogen fertilizer and the changes of nitrifying microorganisms on yellow soil of southwest China are still not distinct. In this study, the migration distribution characteristics of nitrogen fertilizer, nitrogen retention capacity and microbial community structure were evaluated by a soil column leaching simulated experiment. Five application rates of biochar: 0%(BC₀), 0.5%(BC_0.5), 1.0%(BC_1.0), 2.0%(BC_2.0) and 4.0%(BC_4.0) were respectively tried. The results showed that the application of Moutai lees biochar has significantly increased the total nitrogen (TN) and nitrate (NN) contents in yellow soil, but it has also significantly decreased the microbial biomass nitrogen (MBN) content. When compared with the BC₀ treatment, it was found that the application of biochar increased nitrogen fertilizer retention rate (N_F) to 49.84%-95.23%. Moreover, high biochar application rates (2.0% and 4.0%) were also able to improve the N_F ratio, while low biochar application rates (0.5% and 1.0%) still had the risk of nitrogen leaching losses. Additionally, the application of biochar changed the bacterial community structure and the relative abundance of nitrogen-related microorganisms in yellow soil. Also, it was determined that Nitrite-oxidizing bacteria (NOB) played a major factor in affecting soil nitrogen, instead of ammonia-oxidizing archaea (AOA) and ammonium-oxidizing bacteria (AOB). Overall, research finally concluded that Moutai lees biochar decreased nitrite oxidation effect and changed ammoxidation to affect nitrogen nutrients availability in yellow soil and the biochar application rate of 4% has increased nitrogen fertilizer retention rate and decreased the risk of nitrogen leaching losses in yellow soil.

Start-up of Anammox systems with different biochar amendment: Process characteristics and microbial community

As the 'go-to' process when it comes to biological nitrogen removal from wastewaters in recent years, the Anammox process has undergone lots of investigations in order to optimize its performance. In evaluating the effect of distinct biochar types at different concentrations on the Anammox startup process, as well as analyze their corresponding influence on the microbial community structure, three additives (coconut, peach, and bamboo) at either 5%, 10%, or 15% respectively were amended in various Anammox EGSB setups. (i). The 5% coconut biochar amendment resulted in the fastest startup of 46 days with an average ammonium removal efficiency of 96% whereas the control setup took 69 days. Thus, a more robust and cost effective Anammox process could be realized on an industrial scale. (ii) The Illumina high-throughput sequencing of the collected sludge samples indicated that the amendment with distinct biochar resulted in varied prevailing microbial communities in the respective setups. (iii) Proteobacteria was the dominant microbial community. (iv) However, two Anammox bacteria species, Candidatus Brocadia and Candidatus Jettenia were identified, with relative abundances of 0-4.72% and 0-6.23% respectively. The results from this study illustrate the correlation between Anammox reactor performance (startup and nitrogen removal efficiency), type and concentration of biochar amendment employed, as well as microbial community succession.

The Role of Biochar in Regulating the Carbon, Phosphorus, and Nitrogen Cycles Exemplified by Soil Systems

Biochar is a carbon-rich material prepared from the pyrolysis of biomass under various conditions. Recently, biochar drew great attention due to its promising potential in climate change mitigation, soil amendment, and environmental control. Obviously, biochar can be a beneficial soil amendment in several ways including preventing nutrients loss due to leaching, increasing N and P mineralization, and enabling the microbial mediation of N2O and CO2 emissions. However, there are also conflicting reports on biochar effects, such as water logging and weathering induced change of surface properties that ultimately affects microbial growth and soil fertility. Despite the voluminous reports on soil and biochar properties, few studies have systematically addressed the effects of biochar on the sequestration of carbon, nitrogen, and phosphorus in soils. Information on microbially-mediated transformation of carbon (C), nitrogen (N), and phosphorus (P) species in the soil environment remains relatively uncertain. A systematic documentation of how biochar influences the fate and transport of carbon, phosphorus, and nitrogen in soil is crucial to promoting biochar applications toward environmental sustainability. This report first provides an overview on the adsorption of carbon, phosphorus, and nitrogen species on biochar, particularly in soil systems. Then, the biochar-mediated transformation of organic species, and the transport of carbon, nitrogen, and phosphorus in soil systems are discussed. This review also reports on the weathering process of biochar and implications in the soil environment. Lastly, the current knowledge gaps and priority research directions for the biochar-amended systems in the future are assessed. This review focuses on literatures published in the past decade (2009–2021) on the adsorption, degradation, transport, weathering, and transformation of C, N, and P species in soil systems with respect to biochar applications.

Nitrogen biogeochemistry in a boreal headwater stream network in interior Alaska

High latitude, boreal watersheds are nitrogen (N)-limited ecosystems that export large amounts of organic carbon (C). Key controls on C cycling in these environments are the biogeochemical processes affecting the N cycle. A study was conducted in Nome Creek, an upland tributary of the Yukon River, and two headwater tributaries to Nome Creek, to examine the relation between seasonal and transport-associated changes in C and N pools and N-cycling processes using laboratory bioassays of water and sediment samples and in-stream tracer tests. Dissolved organic nitrogen (DON) exceeded dissolved inorganic nitrogen (DIN) in Nome Creek except late in the summer season, with little variation in organic C:N ratios with time or transport distance. DIN was dominant in the headwater tributaries. Rates of organic N mineralization and denitrification in laboratory incubations were positively correlated with sediment organic C content, while nitrification rates differed greatly between two headwater tributaries with similar drainages. Additions of DIN or urea did not stimulate microbial activity. In-stream tracer tests with nitrate and urea indicated that uptake rates were slow relative to transport rates; simulated rates of uptake in stream storage zones were higher than rates assessed in the laboratory bioassays. In general, N-cycle processes were more active and had a greater overall impact in the headwater tributaries and were minimized in Nome Creek, the larger, higher velocity, transport-dominated stream. Given expectations of permafrost thaw and increased hydrologic cycling that will flush more inorganic N from headwater streams, our results suggest higher N loads from these systems in the future.

Plant material and its biochar differ in their effects on nitrogen mineralization and nitrification in a subtropical forest soil

Natural wildfires have a great effect on soil N transformation in subtropical forest. The pyrogenic organic matter (PyOM) in forest soils is mainly derived from the plant material burnt during forest fires, which affects soil N composition, N mineralization and nitrification. This study examined the effects of typical fresh plant material (leaves and twigs of Castanopsis sclerophylla, representing litter) and its biochar (representing PyOM) on N mineralization and nitrification in a subtropical forest soil. The soils were incubated with the plant material (PM), its biochar (BC) and their combinations for 84 days. Both PM and BC considerably increased soil pH and dissolved organic C, whereas PM decreased NO₃^--N and dissolved organic N. The additions of PM alone, and its combinations with BC resulted in net N immobilization. The rates of net N mineralization rapidly increased in first 14 days and then became stable following the addition of PM to soil. Moreover, the additions of PM and BC increased the abundances of archaeal amoA and bacterial amoA, especially with PM. The abundance of bacterial amoA correlated positively with soil pH and dissolved organic C, while archaeal amoA showed the opposite. Biochar affected soil properties and N transformation more significantly in the presence of PM, highlighting the need for further research on the interactions of plant litter and its biochar.

Successive biochar amendment affected crop yield by regulating soil nitrogen functional microbes in wheat-maize rotation farmland

Biochar has attracted increased attention because of its potential benefits for carbon sequestration, soil fertility, and contaminant immobilization. However, mechanism of long-term successive biochar amendment affected crop yield by regulating soil properties and nitrogen (N) functional microbes is still unclear by now. A field fixed experiment was carried out from 2011 to 2018 that aimed to study the effects of successive biochar on soil properties, soil nitrogen functional microbial genes, and grain yield in wheat and maize rotation farmland in Northern China. Four straw biochar treatments were tested in this study: 0 (BC0, CK), 2.25 (BC2.25), 6.75 (BC6.75), and 11.25 (BC11.25) Mg ha^-1. The results showed that, after seven wheat-maize rotations, the total organic carbon (TOC), total N (TN), NO₃^-, available potassium (AK), and the C/N ratio in 0-20 cm topsoil were increased significantly following biochar application; however, there were no obvious differences in available phosphorus (AP) and NH₄⁺ among biochar treatments. Biochar also resulted in a significant increase in crop yield and NO₃^- accumulation in 0-200 cm soil layer, with the highest yield in BC6.75. Furthermore, a marked increase was found in the amoA gene abundance in topsoil; however, it decreased significantly with excessive biochar application (BC11.25). At wheat maturity, the nirS gene abundance consistently decreased following biochar application, whereas the nosZ gene abundance initially increased and then decreased (peaking in BC6.75); however, no obvious changes in the nirK gene were observed. At maize maturity, biochar significantly increased the nirS and nosZ gene abundance in topsoil, especially in BC6.75. In addition, redundancy analysis indicated that the soil moisture content, AP, AK, TN, TOC, NO₃^-, NH₄⁺, pH, and C/N ratio had markedly effects on the abundance of the amoA, nirK, nirS, and nosZ genes. In general, biochar-induced alterations of soil properties resulted in changes of gene abundance of soil nitrifying and denitrifying bacteria, and eventually affecting crop yields.

The benefits of biochar: Enhanced cadmium remediation, inhibited precursor production of nitrous oxide and a short-term disturbance on rhizosphere microbial community

Biochar has the potential to remediate heavy metals in agricultural soil and mitigate nitrous oxide (N₂O) emissions; however, the effects of biochar on heavy metal remediation, the soil microbial community and N₂O emissions are not completely understood. In this study, we conducted a pot experiment in which Glycine max L. (soybean) was cultivated in two cadmium (Cd)-contaminated soils (low, 3.14 mg kg^-1; high, 10.80 mg kg^-1) to investigate the effects of biochar on the bioremediation of Cd, N₂O emissions and the rhizosphere microbial community structure. The bioaccumulation of Cd in the plant shoots and roots increased with all biochar addition rates (0%, 1%, 5% and 10%); unexpectedly, the translocation capacity of Cd to the edible parts of the plant significantly decreased to 0.58 mg kg^-1, which was close to the edible threshold (0.4 mg kg^-1). The abundance and activities of functional marker genes of microbial nitrification (amoA) and denitrification (nirK, nirS and nosZ) were quantified with quantitative PCR, and we found that biochar addition reduced the precursor production of rhizoshpere N₂O by inhibiting the transcription of the nirK gene. In addition, the nitrogenase activity during anthesis (S) was significantly (P < 0.05) increased with 1% (v/v) biochar addition. Noticeably, biochar addition only changed the microbial community structure in the very first stage before eventually stabilize. This study highlighted that biochar has the potential ability to maintain the quality of agricultural crops, remediate Cd-contaminated soils and may help reduce N₂O emissions without disturbing the microbial community.

Divergent consequences of different biochar amendments on carbon dioxide (CO2) and nitrous oxide (N2O) emissions from the red soil

Climate change due to greenhouse gas (GHG) emissions is one of the global environmental matters of the 21st century. Biochar (BC) amendments have been proposed as a potential solution for improving soil quality and to mitigate GHGs emissions. Therefore, we evaluated the influence of different BCs on soil CO₂ and N₂O emissions in an outdoor pot experiment. The soil was mixed with three different types of BCs; bamboo, hardwood, and rice straw BCs as BB, BH, and BR, respectively, and control as B0 with four levels (0, 5, 20, and 80 g kg^-1 of soil). Gas samples were collected on a bi-monthly basis for six months. A polyvinyl chloride (PVC) static chamber was placed on each replicate to collect the gas samples at 15, 30, 45, and 60 min, respectively. Compared to B0, the lowest cumulative N₂O emissions were observed in BH80 (11%) followed by BH20, BH5, and BR80. However, for cumulative CO₂ emissions, B0 and BC treatments showed no significant differences except for BB80 (>11%) and BB5 (<2%). BC type and level both had a significant (P < 0.001) impact on the cumulative N₂O emissions with a significant interaction (P < 0.001). However, cumulative CO₂ emissions were unaffected by BC type but BC level showed a significant influence on cumulative CO₂ emissions (P < 0.05) and there was a significant (P < 0.001) interaction between the BC type and level on cumulative CO₂ emissions. Overall, higher doses of BR and BB showed a pronounced effect on soil pH over BH. The soil pH and moisture showed a negative correlation with N₂O emissions whereas soil temperature showed a positive correlation with the cumulative fluxes of N₂O. Our results demonstrate that BC incorporation to soil may help to mitigate GHGs emissions but its influence may vary with BC type and level under different conditions and soil type.

Metagenomic analysis reveals the effects of cotton straw-derived biochar on soil nitrogen transformation in drip-irrigated cotton field

Biochar has been widely accepted as a soil amendment to improve nitrogen (N) use efficiency, but the effect of biochar on N transformation metabolic pathways is unclear. A field experiment was conducted to evaluate the effect of biochar on N transformation in drip-irrigated cotton field. Four treatments were set as (1) no N fertilization (CK), (2) N fertilizer application at 300 kg ha^-1 (N300), (3) N fertilizer application plus cotton straw (N300+ST), and (4) N fertilizer application plus cotton straw-derived biochar (N300+BC). Result showed that soil total N in N300+ST and N300+BC was 16.3% and 24.9% higher than that in N300, respectively. Compared with N300+ST, the nitrate N (NO₃^--N) in N300+BC was significantly increased. Acidolyzable N and non-acidolyzable N in N300+ST and N300+BC were higher than those in CK and N300, while N300+BC performed better than N300+ST. Furthermore, the N fertilizer use efficiency of cotton in N300+ST and N300+BC was 15.1% and 23.2% higher than that in N300, respectively. Both N fertilizer incorporations with straw and biochar significantly altered the microbial community structures and N metabolic pathways. Genes related to denitrification and nitrate reduction in N300+ST were higher than those in N300, and N300+BC significantly increased nitrification and glutamate synthesis genes. Therefore, N fertilizer application plus cotton straw-derived biochar changed the microbial community composition, increased nitrification and glutamate synthesis enzyme genes which were beneficial to the accumulation of soil N content, and improved soil N retention capacity thus to increase N fertilizer use efficiency.

Responses of nitrogen transformation and dissolved oxygen in constructed wetland to biochar and earthworm amendment

Many constructed wetland systems are facing the problem of low dissolved oxygen (DO) and reduced nitrogen removal efficiency. In this study, an experimental constructed wetland system is designed and used to investigate the effect of biochar (rice husk biochar (RHB), coconut shell biochar (CSB), and wood biochar (WB) and earthworm on DO concentration, nitrogen transformation, and ammonia nitrogen removal. Specifically, effects of different biochar and earthworm on NH₄⁺-N in wastewater, N content of Phragmites australis, NH₄⁺-N and NO₃^--N content in substrates, microbial nitrification and denitrification potentials, and the DO concentration were investigated. Results show that the addition of biochar and earthworm increased the removal efficiency of NH₄⁺-N from wastewater. The addition of RHB and WB significantly increased the concentration of DO by 21.4% and 25.7% (P < 0.05) respectively in the constructed wetland. The addition of earthworm significantly increased the DO concentration in the constructed wetland system by an average of 30.35% (P < 0.05).The N content of P. australis increased when biochar and earthworm were introduced into the constructed wetland system, with higher relative N content observed in the above-ground biomass. NO₃^--N content increased, but NH₄⁺-N decreased in the substrate. Addition of both biochar and earthworm increased nitrification and denitrification potentials. However, no significant increase in denitrification potential was observed when only biochar was added. The removal efficiency of NH₄⁺-N from wastewater is significantly positively correlated with the DO, nitrification, and denitrification potential, and nitrogen content of above-ground part of P. australis (P < 0.05). Results suggest that the DO concentration in constructed wetland systems could be improved by the addition of biochar and earthworm. These findings imply that both biochar and earthworm could be added into constructed wetlands to solve the low DO concentration and improve the removal efficiency of nitrogen.

Inhibited effect of biochar application on N2O emissions is amount and time-dependent by regulating denitrification in a wheat-maize rotation system in North China

Biochar application is considered an effective method of reducing nitrous oxide (N₂O) emissions in soil. However, the mechanism and temporal effect of different doses of biochar on N₂O emissions is still obscure. Here, we conducted a two-year field experiment to test the effects of different input amounts and frequencies of biochar on soil N₂O emissions in North China. Biochar was applied in six different treatments in a winter wheat and summer maize rotation system: applications of 0 t/ha biochar (C0), 2.25 t/ha biochar (C1), 4.5 t/ha biochar (C2), 9 t/ha biochar (C3), and 13.5 t/ha biochar (C4) each year, and a single application of 13.5 t/ha biochar (CS) in the first year. The results showed that biochar could inhibit N₂O emissions, reaching 20.6% to 60.1% in the wheat season and 18.1% to 39.4% in the maize season. The inhibitory effect of biochar on soil N₂O emissions was dependent on amount and time. C3 had the best results in the wheat season, although its inhibitory effect in the maize season was not as good relative to C4 due to the lower biochar application. In addition, CS significantly reduced (27.7%) the cumulative N₂O emissions in the first year, although the inhibitory effect disappeared in the second year. Biochar increased the nosZ gene copy numbers and promoted a reduction of N₂O in the soil via the denitrification process. In conclusion, the inhibition of N₂O emissions during denitrification is an important factor for reducing soil N₂O emissions by biochar, and the inhibition of biochar is influenced by the input amount and time.

Intra-Annual Variation in Soil C, N and Nutrients Pools after Prescribed Fire in a Mississippi Longleaf Pine (Pinus palustris Mill.) Plantation

Prescribed fire is an essential tool that is widely used for longleaf pine (Pinus palustris Mill.) stand management; periodic burning serves to reduce competition from woody shrubs and fire-intolerant trees and enhance herbaceous diversity. Low-intensity, prescribed burning is thought to have minimal long-term impact on soil chemistry in southern pine forests, although few studies report the intra-annual variation in soil chemistry after burning. We monitored changes in C, N, oxidation resistant C (CR), pH and elemental nutrients in the forest floor and soil (0–5, 5–10 cm depths) before and after burning (1, 3, 6, 12 months) in a mature longleaf pine plantation at the Harrison Experimental Forest, near Saucier, Mississippi. Prescribed fire consumed much of the forest floor (11.3 Mg ha−1; −69%), increased soil pH and caused a pulse of C, N and elemental nutrients to flow to the near surface soils. In the initial one to three months post-burn coinciding with the start of the growing season, retention of nutrients by soil peaked. Most of the N (93%), Ca (88%), K (96%) and Mg (101%), roughly half of the P (48%) and Mn (52%) and 25% of the C lost from the forest floor were detected in the soil and apparently not lost to volatilization. By month 12, soil C and N pools were not different at depths of 0–5 cm but declined significantly below pre-burn levels at depths of 5–10 cm, C −36% (p < 0.0001), N −26% (p = 0.003), contrary to other examples in southern pine ecosystems. In the upper 5 cm of soil, only Cu (−49%) remained significantly lower than pre-burn contents by month 12, at depths of 5–10 cm, Cu (−76%), Fe (−22%), K (−51%), Mg (−57%), Mn (−82%) and P (−52%) remain lower at month 12 than pre-burn contents. Burning did not increase soil CR content, conversely significant declines in CR occurred. It appears that recovery of soil C and N pools post-burn will require more time on this site than other southern pine forests.

Methane and Nitrous Oxide Flux after Biochar Application in Subtropical Acidic Paddy Soils under Tobacco-Rice Rotation

Biochar amendment is a good means of mitigating methane (CH₄) and nitrous oxide (N₂O) emissions. However, the effects of biochar amendment on N₂O and CH₄ reduction in soil under rotation with different soil moisture contents is not well understood. To understand CH₄ and N₂O flux from soil with biochar amendment under water-unsaturated and water-saturated conditions, a field experiment was conducted in a tobacco-rice rotation field in subtropical China to investigate N₂O and CH₄ emissions following soil amendment with tobacco straw biochar at rates of 0, 10, 40 and 80 t·ha⁻¹ (B0, B10, B40 and B80, respectively). N₂O and CH₄ emissions were monitored by a closed-chamber method in the water-unsaturated tobacco (UT) and water-saturated rice (SR) seasons during the 2015 planting season. The soil pH increased from 5.4 in the control to 6.1 in the soil amended with biochar at 80 t·ha⁻¹ in the UT season. During both the UT and SR seasons, with biochar amendment at 40 and 80 t·ha⁻¹, the soil bulk density (BD) decreased, while the soil organic matter (SOM) and available potassium (Av. K) contents increased. N₂O flux was significantly greater in UT than in SR in the controls but decreased with the application of biochar during both the UT and SR seasons. The cumulative CH₄ emission decreased with the rate of biochar application and the methanotroph pmoA gene copy number in soils and increased with the methanogenic archaea 16Sr DNA gene copy number in soils during the rice-cropping season. These results indicated that biochar amendment could decrease methanogenic archaea and increase of methanotroph pmoA gene, which are the mechanistic origin for CH₄ reduction.

Aged biochar stimulated ammonia-oxidizing archaea and bacteria-derived N2O and NO production in an acidic vegetable soil

Both nitrous oxide (N₂O) and nitric oxide (NO) emissions are typically high in greenhouse-based high N input vegetable soils. Biochar amendment has been widely recommended for mitigating soil N₂O emissions in agriculture. However, knowledge of the regulatory mechanisms of fresh and aged biochar for both N₂O and NO production during ammonia oxidation is lacking. Two vegetable soils with different pH values were used in aerobic incubation experiments with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), 1-octyne and acetylene. The relative importance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) to N₂O and NO production was investigated as influenced by fresh and aged biochar amendments. The results showed that AOA dominated N₂O production in acidic soil, while AOB dominated N₂O production in alkaline soil. Aged biochar stimulated both AOA- and AOB-derived N₂O and NO production by 84.8 and 340%, respectively, in acidic soil but only increased AOA-derived N₂O and NO production in alkaline soil. Fresh biochar amendment increased AOA- and AOB-derived NO in acidic soil and AOA-derived NO in alkaline soil but had negligible effects on AOA- and AOB-derived N₂O in both soils. Fresh biochar decreased AOA-amoA but increased AOB-amoA gene abundances in acidic soil, whereas aged biochar increased AOA- and AOB-amoA gene abundances in both soils. These findings improved our understanding of N₂O and NO production mechanisms under different biochar amendments in alkaline and acidic vegetable soils.

Biochar application significantly affects the N pool and microbial community structure in purple and paddy soils

BACKGROUND

The increasing demand for food production has resulted in the use of large quantities of chemical fertilizers. This has created major environmental problems, such as increased ammonia volatilization, N2O emission, and nitrogen (N) leaching from agricultural soil. In particular, the utilization rate of N fertilizer is low in subtropical southern parts of China due to high rainfall. This causes not only large financial losses in agriculture, but also serious environmental pollution.

METHODS

In this study, 16S rDNA-based analysis and static-chamber gas chromatography were used to elucidate the effects of continuous straw biochar application on the N pool and bacteria environment in two typical soil types, purple and paddy soils, in southern China.

RESULTS

Straw biochar application (1) improved the soil N pool in both rhizosphere and non-rhizosphere soils; (2) significantly reduced the emission of N2O, with no difference in emission between 1 and 2 years of application; (3) increased the abundance of N-processing bacteria in the soil and altered the bacterial community structure; and (4) improved the tobacco yield and N use efficiency in paddy soil. These findings suggest that, in southern China, the application of straw biochar can promote N transformation in purple and paddy soils and reduce the emission of the greenhouse gas N2O.

Effects of season and interval of prescribed burns on pyrogenic carbon in ponderosa pine stands in the southern Blue Mountains, Oregon, USA

In ponderosa pine (Pinus ponderosa) forests of the western United States, prescribed burns are used to reduce fuel loads and restore historical fire regimes. The season of and interval between burns can have complex consequences for the ecosystem, including the production of pyrogenic carbon (PyC). PyC plays a crucial role in soil carbon cycling, displaying turnover times that are orders of magnitude longer than unburned organic matter. This work investigated how the season of and interval between prescribed burns affect soil organic matter, including the formation and retention of PyC, in a ponderosa pine forest of eastern Oregon. In 1997 a prescribed burn study was implemented in Malheur National Forest to examine the ecological effects of burning at 5 and 15-year intervals in either the spring or fall. In October 2015, both O-horizon and mineral soil (0-15 cm) samples were collected and analyzed for PyC concentration, content, and structure using the benzene polycarboxylic acid (BPCA) method. O-horizon depth, carbon and nitrogen concentration and content, pH, and bulk density were also measured. Plots burned in the spring and fall had lower C and N stocks in the O-horizon compared to the unburned controls due to a reduction in O-horizon depth; however, we did not observe any differences in O-horizon concentration of C or N. Moreover, the concentration and stock of C and N in the mineral soil of plots burned in the spring or fall was the same as or only very slightly different from the unburned controls, suggesting that the prescribed burns on these sites have not adversely affected SOM quantity. Compared to unburned controls, we estimate that fall burns increased the mean PyC concentration of the mineral soil by 8.42 g BPCA/kg C. We did not detect a difference in mean PyC concentration of the mineral soil between the spring burns and the unburned controls; however, the spring burn plots did contain a number of isolated pockets with very high concentrations of PyC, suggesting a patchier burn pattern for these plots. In general, there was no detectable difference in any of the response variables when comparing the various prescribed burn treatments to one another. The disturbance caused by the reintroduction of fire to this ecosystem may have obscured subtle differences caused by the different seasons and intervals of burn that could become more apparent over time.

Nitrogen turnover and N2O/N2 ratio of three contrasting tropical soils amended with biochar

Biochar has been reported to reduce emission of nitrous oxide (N₂O) from soils, but the mechanisms responsible remain fragmentary. For example, it is unclear how biochar effects on N₂O emissions are mediated through biochar effects on soil gross N turnover rates. Hence, we conducted an incubation study with three contrasting agricultural soils from Kenya (an Acrisol cultivated for 10-years (Acrisol10); an Acrisol cultivated for over 100-years (Acrisol100); a Ferralsol cultivated for over 100 years (Ferralsol)). The soils were amended with biochar at either 2% or 4% w/w. The ¹⁵N pool dilution technique was used to quantify gross N mineralization and nitrification and microbial consumption of extractable N over a 20-day incubation period at 25 °C and 70% water holding capacity of the soil, accompanied by N₂O emissions measurements. Direct measurements of N₂ emissions were conducted using the helium gas flow soil core method. N₂O emissions varied across soils with higher emissions in Acrisols than in Ferralsols. Addition of 2% biochar reduced N₂O emissions in all soils by 53 to 78% with no significant further reduction induced by addition at 4%. Biochar effects on soil nitrate concentrations were highly variable across soils, ranging from a reduction, no effect and an increase. Biochar addition stimulated gross N mineralization in Acrisol-10 and Acrisol-100 soils at both addition rates with no effect observed for the Ferralsol. In contrast, gross nitrification was stimulated in only one soil but only at a 4% application rate. Also, biochar effects on increased NH₄ ⁺ immobilization and NO₃ ^-consumption strongly varied across the three investigated soils. The variable and bidirectional biochar effects on gross N turnover in conjunction with the unambiguous and consistent reduction of N₂O emissions suggested that the inhibiting effect of biochar on soil N₂O emission seemed to be decoupled from gross microbial N turnover processes. With biochar application, N₂ emissions were about an order of magnitude higher for Acrisol-10 soils compared to Acrisol-100 and Ferralsol-100 soils. Our N₂O and N₂ flux data thus support an explanation of direct promotion of gross N₂O reduction by biochar rather than effects on soil extractable N dynamics. Effects of biochar on soil extractable N and gross N turnover, however, might be highly variable across different soils as found here for three typical agricultural soils of Kenya.

Biochar additions alter phosphorus and nitrogen availability in agricultural ecosystems: A meta-analysis

Biochar is a carbon (C) rich product of thermochemical conversion of organic material that is used as a soil amendment due to its resistance to decomposition and its influence on nutrient dynamics; however, individual studies on biochar effects on phosphorus (P) and nitrogen (N) have proven inconsistent. Herein, we performed a meta-analysis of 124 published studies to evaluate the influence of biochar on available P, microbial biomass P (MBP), and inorganic N (NO₃^--N and NH₄⁺-N) in global agricultural ecosystems. Overall, the results showed that biochar applications significantly increased surface soil available P by 45% and MBP by 48% across the full range of biochar characteristics, soil type, or experimental conditions. By contrast, biochar addition to soil reduced NO₃^--N concentrations by 12% and NH₄⁺-N by 11%, but in most cases biochar added in combination with organic fertilizer significantly increased soil NH₄⁺-N compared to controls. Biochar C:N ratio and biochar source (feedstock) strongly influenced soil P availability response to biochar where inorganic N was most influenced by biochar C:N ratio and soil pH. Biochar made from manure or other low C:N ratio materials, generated at low temperatures, or applied at high rates were generally more effective at enhancing soil available P. It is important, however, to note that most negative results were observed in short-term (<6 months) where long-term studies (>12 months) tended to result in neutral to modest positive effects on both P and N. This meta-analysis indicates that biochar generally enhances soil P availability when added to soils alone or in combination with fertilizer. These findings provide a scientific basis for developing more rational strategies toward widespread adoption of biochar as a soil amendment for agricultural P and N management.

Soil carbon in Australian fire-prone forests determined by climate more than fire regimes

Knowledge of global C cycle implications from changes to fire regime and climate are of growing importance. Studies on the role of the fire regime in combination with climate change on soil C pools are lacking. We used Bayesian modelling to estimate the soil % total C (% C_Tot) and % recalcitrant pyrogenic C (% RPC) from field samples collected using a stratified sampling approach. These observations were derived from the following scenarios: 1. Three fire frequencies across three distinctive climate regions in a homogeneous dry sclerophyll forest in south-eastern Australia over four decades. 2. The effects of different fire intensity combinations from successive wildfires. We found climate had a stronger effect than fire frequency on the size of the estimated mineral soil C pool. The largest soil C pool was estimated to occur under a wet and cold (WC) climate, via presumed effects of high precipitation, an adequate growing season temperature (i.e. resulting in relatively high NPP) and winter conditions sufficiently cold to retard seasonal soil respiration rates. The smallest soil C pool was estimated in forests with lower precipitation but warmer mean annual temperature (MAT). The lower precipitation and higher temperature was likely to have retarded NPP and litter decomposition rates but may have had little effect on relative soil respiration. Small effects associated with fire frequency were found, but both their magnitude and direction were climate dependent. There was an increase in soil C associated with a low intensity fire being followed by a high intensity fire. For both fire frequency and intensity the response of % RPC mirrored that of % C_Tot: i.e. it was effectively a constant across all combinations of climate and fire regimes sampled.

Fire intensity drives post-fire temporal pattern of soil carbon accumulation in Australian fire-prone forests

The impact of fire on global C cycles is considerable but complex. Nevertheless, studies on patterns of soil C accumulation following fires of differing intensity over time are lacking. Our study utilised 15 locations last burnt by prescribed fire (inferred low intensity) and 18 locations last burnt by wildfire (inferred high intensity), with time since fire (TSF) up to 43years, in a homogenous forest type in south eastern Australia. Following a stratified approach to mineral soil sampling, the soil % total C (% C_Tot) and % recalcitrant pyrogenic C (% RPC), were estimated. Generalised additive models indicated increases in % C_Tot at TSF >30years in sites last burnt by wildfire. Estimates in sites last subjected to prescribed fire however, remained constant across the TSF chronosequence. There was no significant difference in % C_Tot between the different fire types for the first 20years after fire. In the first 10years after wildfires, % RPC was elevated, declining to a minimum at ca. TSF 25years. After prescribed fires, % RPC was unaffected by TSF. Differences in response of % C_Tot and % RPC to fire type may reflect the strength of stimulation of early successional processes and extent of charring. The divergent response to fire type in % C_Tot was apparent at TSF longer than the landscape average fire return interval (i.e., 15 to 20years). Thus, any attempt to increase C sequestration in soils would require long-term exclusion of fire. Conversely, increased fire frequency is likely to have negligible impact on soil C stocks in these forests. Further investigation of the effects of fire frequency, fire intensity combinations and interaction of fire with other disturbances will enhance prediction of the likely impact of imposed or climatically induced changes to fire regimes on soil C.

Effects of combined application of nitrogen fertilizer and biochar on the nitrification and ammonia oxidizers in an intensive vegetable soil

Soil amended with single biochar or nitrogen (N) fertilizer has frequently been reported to alter soil nitrification process due to its impact on soil properties. However, little is known about the dynamic response of nitrification and ammonia-oxidizers to the combined application of biochar and N fertilizer in intensive vegetable soil. In this study, an incubation experiment was designed to evaluate the effects of biochar and N fertilizer application on soil nitrification, abundance and community shifts of ammonia-oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) in Hangzhou greenhouse vegetable soil. Results showed that single application of biochar had no significant effect on soil net nitrification rates and ammonia-oxidizers. Conversely, the application of only N fertilizer and N fertilizer + biochar significantly increased net nitrification rate and the abundance of AOB rather than AOA, and only AOB abundance was significantly correlated with soil net nitrification rate. Moreover, the combined application of N fertilizer and biochar had greater effect on AOB communities than that of the only N fertilizers, and the relative abundance of 156 bp T-RF (Nitrosospira cluster 3c) decreased but 60 bp T-RF (Nitrosospira cluster 3a and cluster 0) increased to become a single predominant group. Phylogenetic analysis indicated that all the AOB sequences were grouped into Nitrosospira cluster, and most of AOA sequences were clustered within group 1.1b. We concluded that soil nitrification was stimulated by the combined application of N fertilizer and biochar via enhancing the abundance and shifting the community composition of AOB rather than AOA in intensive vegetable soil.

Emissions intensity and carbon stocks of a tropical Ultisol after amendment with Tithonia green manure, urea and biochar

Biochar has been shown to reduce soil emissions of CO₂, CH₄ and N₂O in short-term incubation and greenhouse experiments. Such controlled experiments failed to represent variable field conditions, and rarely included crop growth feedback. The objective of this study was to assess the effect of biochar, in comparison to green manure and mineral nitrogen, on greenhouse gas Emissions Intensity (EI = emissions in CO₂ equivalents per ton of grain yield) in a low-fertility tropical Ultisol. Using a field trial in western Kenya, biochar (0 and 2.5 t ha^-1; made from Eucalyptus wood) was integrated with urea (0 and 120 kg N ha^-1) and green manure (Tithonia diversifolia; 0, 2.5 and 5 t ha^-1) in a factorial design for four consecutive seasons from October 2012 to August 2014. Compared to the control, biochar increased soil CO₂ emissions (9-33%), reduced soil CH₄ uptake (7-59%) and reduced soil N₂O emissions (1-42%) in each season, with no seasonal differences. N₂O emissions increased following amendment with T. diversifolia (6%) and urea (13%) compared to the control. Generally, N₂O emissions decreased where only biochar was applied. The greatest decrease in N₂O (42%) occurred where all three amendments were applied compared to when they were added separately. EI in response to any of the amendments was lower than the control, ranging from 9 to 65% (33.0 ± 3.2 = mean ± SE). The amendments increased SOC stocks by 0.1-1.2 t ha^-1 year^-1 (mean ± SE of 0.8 ± 0.09 t ha^-1 year^-1). The results suggest decreased net EI with biochar in low fertility soils mainly through greater net primary productivity (89% of the decrease).

Plant Community and Nitrogen Deposition as Drivers of Alpha and Beta Diversities of Prokaryotes in Reconstructed Oil Sand Soils and Natural Boreal Forest Soils

The Athabasca oil sand deposit is one of the largest single oil deposits in the world. Following surface mining, companies are required to restore soil-like profiles that can support the previous land capabilities. The objective of this study was to assess whether the soil prokaryotic alpha diversity (α-diversity) and β-diversity in oil sand soils reconstructed 20 to 30 years previously and planted to one of three vegetation types (coniferous or deciduous trees and grassland) were similar to those found in natural boreal forest soils subject to wildfire disturbance. Prokaryotic α-diversity and β-diversity were assessed using massively parallel sequencing of 16S rRNA genes. The β-diversity, but not the α-diversity, differed between reconstructed and natural soils. Bacteria associated with an oligotrophic lifestyle were more abundant in natural forest soils, whereas bacteria associated with a copiotrophic lifestyle were more abundant in reconstructed soils. Ammonia-oxidizing archaea were most abundant in reconstructed soils planted with grasses. Plant species were the main factor influencing α-diversity in natural and in reconstructed soils. Nitrogen deposition, pH, and plant species were the main factors influencing the β-diversity of the prokaryotic communities in natural and reconstructed soils. The results highlight the importance of nitrogen deposition and aboveground-belowground relationships in shaping soil microbial communities in natural and reconstructed soils.IMPORTANCE Covering over 800 km², land disturbed by the exploitation of the oil sands in Canada has to be restored. Here, we take advantage of the proximity between these reconstructed ecosystems and the boreal forest surrounding the oil sand mining area to study soil microbial community structure and processes in both natural and nonnatural environments. By identifying key characteristics shaping the structure of soil microbial communities, this study improved our understanding of how vegetation, soil characteristics and microbial communities interact and drive soil functions.

Potential role of biochars in decreasing soil acidification - A critical review

A large number of soils, worldwide, are acid (normally pH<5.5) and suffering from on-going soil acidification. Acid soils or soils undergoing acidification generally have low fertility and low crop productivity. Biochars have been reported to be of potential value in agriculture for improving soil properties and in reducing the hazards caused by soil acidification and in naturally acidic soils. However, the ameliorant effects of biochars on acid soils and the mechanisms involved have not previously been critically reviewed. Here we summarize the phenomena, and mechanisms involved in the improvement of soil acidity by biochars, the alleviation of aluminum toxicity, the enhancement of nutrient availability, and changes in nitrification by collating data in the literature. In addition, the agronomic effectiveness and environmental concerns in the incorporation of biochar and other soil additives (i.e. lime, industrial by-products, organic wastes and plant residues) to acid soils are systemically compared. We conclude that biochar is a potentially effective amendment to reverse or to prevent acidification in acid soils. Finally, perspectives for further research in terms of soil acidification are presented to address some issues that are still poorly understood and/or highly controversial.

Post-fire soil functionality and microbial community structure in a Mediterranean shrubland subjected to experimental drought

Fire may cause significant alterations in soil properties. Post-fire soil dynamics can vary depending, among other factors, on rainfall patterns. However, little is known regarding variations in response to post-fire drought. This is relevant in arid and semiarid areas with poor soils, like much of the western Mediterranean. Furthermore, climate change projections in such areas anticipate reduced precipitation and longer annual drought periods, together with an increase in fire severity and frequency. This research evaluates the effects of experimental drought after fire on soil dynamics of a Cistus-Erica shrubland (Central Spain). A replicated (n=4) field experiment was conducted in which the total rainfall and its patterns were manipulated by means of a rain-out shelters and irrigation system. The treatments were: environmental control (natural rainfall), historical control (average rainfall, 2months drought), moderate drought (25% reduction of historical control, 5months drought) and severe drought (45% reduction, 7months drought). After one growing season under these rainfall treatments, the plots were burned. One set of unburned plots under natural rainfall served as an additional control. Soils were collected seasonally. Fire increased soil P and N availability. Post-fire drought treatments reduced available soil P but increased N concentration (mainly nitrate). Fire reduced available K irrespective of drought treatments. Fire reduced enzyme activities and carbon mineralization rate, a reduction that was higher in post-fire drought-treated soils. Fire decreased soil microbial biomass and the proportion of fungi, while that of actinomycetes increased. Post-fire drought decreased soil total microbial biomass and fungi, with bacteria becoming more abundant. Our results support that increasing drought after fire could compromise the resilience of Mediterranean ecosystems to fire.

Ash-soil interface: Mineralogical composition and physical structure

Fires exert many changes on the physical, chemical, morphological, mineralogical, and biological properties of soil that, in turn, affect the soil's hydrology and nutrient flux, modifying its ability to support vegetation and resist erosion. The ash produced by forest fires is a complex mixture composed of organic and inorganic particles with varied properties. This research was conducted to study and characterized ash properties produced at different temperatures and with different soil organic matter combinations. The samples, which included two treatments of soils with underlying mixed leaves and branches composed mainly by Pinus halepensis, Pistacia lentiscus, Cistus salviifolius and typical herbaceous vegetation, versus samples of mixed leaves and branches alone. Both were exposed to 400°C and 600°C heat in a muffle furnace for 2h. The residue ash was generally grayish, consisting of mixed-sized particles that preserved almost none of the original characteristics of the fuel, and was deposited in ash layers with diverse physicochemical and textural properties. The results of this study highlight the differences between all examined samples and strongly support the assumption that ash produced from a complex vegetation-soil system is a new substance with unique structural, textural, and mineralogical properties. Moreover, the ash produced at different temperatures appeared in distinct layering patterns.

Forest restoration treatments have subtle long-term effects on soil C and N cycling in mixed conifer forests

Decades of fire suppression following extensive timber harvesting have left much of the forest in the intermountain western United States exceedingly dense, and forest restoration techniques (i.e., thinning and prescribed fire) are increasingly being used in an attempt to mitigate the effects of severe wildfire, to enhance tree growth and regeneration, and to stimulate soil nutrient cycling. While many of the short-term effects of forest restoration have been established, the long-term effects on soil biogeochemical and ecosystem processes are largely unknown. We assessed the effects of commonly used forest restoration treatments (thinning, burning, and thinning + burning) on nutrient cycling and other ecosystem processes 11 yr after restoration treatments were implemented in a ponderosa pine (Pinus ponderosa var. scopulorum)/Douglas fir (Pseudotsuga menziesii var. glauca) forest at the Lubrecht Fire and Fire Surrogates Study (FFS) site in western Montana, USA. Despite short-term (<3 yr) increases in soil inorganic nitrogen (N) pools and N cycling rates following prescribed fire, long-term soil N pools and N mineralization rates showed only subtle differences from untreated control plots. Similarly, despite a persistent positive correlation between fuels consumed in prescribed burns and several metrics of N cycling, variability in inorganic N pools decreased significantly since treatments were implemented, indicating a decline in N spatial heterogeneity through time. However, rates of net nitrification remain significantly higher in a thin + burn treatment relative to other treatments. Short-term declines in forest floor carbon (C) pools have persisted in the thin + burn treatment, but there were no significant long-term differences among treatments in extractable soil phosphorus (P). Finally, despite some short-term differences, long-term foliar nutrient concentrations, litter decomposition rates, and rates of free-living N fixation in the experimental plots were not different from control plots, suggesting nutrient cycles and ecosystem processes in temperate coniferous forests are resilient to disturbance following long periods of fire suppression. Overall, this study provides forest managers and policymakers valuable information showing that the effects of these commonly used restoration prescriptions on soil nutrient cycling are ephemeral and that use of repeated treatments (i.e., frequent fire) will be necessary to ensure continued restoration success.

Bacterial Mobilization of Nutrients From Biochar-Amended Soils

Soil amendments with biochar to improve soil fertility and increase soil carbon stocks have received some high-level attention. Physical and chemical analyses of amended soils and biochars from various feedstocks are reported, alongside some evaluations of plant growth promotion capabilities. Fewer studies investigated the soil microbiota and their potential to increase cycling and mobilization of nutrients in biochar-amended soils. This review is discussing the latest findings in the bacterial contribution to cycling and mobilizing nitrogen, phosphorus, and sulfur in biochar-amended soils and potential contributions to plant growth promotion. Depending on feedstock, pyrolysis, soil type, and plant cover, changes in the bacterial community structure were observed for a majority of the studies using amplicon sequencing or genetic fingerprinting methods. Prokaryotic nitrification largely depends on the availability of ammonium and can vary considerably under soil biochar amendment. However, denitrification to di-nitrogen and in particular, nitrous oxide reductase activity is commonly enhanced, resulting in reduced nitrous oxide emissions. Likewise, bacterial fixation of di-nitrogen appears to be regularly enhanced. A paucity of studies suggests that bacterial mobilization of phosphorus and sulfur is enhanced as well. However, most studies only tested for extracellular sulfatase and phosphatase activity. Further research is needed to reveal details of the bacterial nutrient mobilizing capabilities and this is in particular the case for the mobilization of phosphorus and sulfur.

Ameliorating Effects of Biochar Derived from Poultry Manure and White Clover Residues on Soil Nutrient Status and Plant growth Promotion--Greenhouse Experiments

Biochar application to agricultural soils is rapidly emerging as a new management strategy for its potential role in carbon sequestration, soil quality improvements, and plant growth promotion. The aim of our study was to investigate the effects of biochars derived from white clover residues and poultry manure on soil quality characteristics, growth and N accumulation in maize (Zea mays L.) and wheat (Triticum aestivum L.) grown in a loam soil under greenhouse conditions. Treatments comprised of: untreated control; mineral N fertilizer (urea N, UN) at the rate of 200, and 100 mg N kg^-1, white clover residues biochar (WCRB), poultry manure biochar (PMB) at 30 Mg ha^–1, and the possible combinations of WCRB+PMB (50:50), UN+WCRB (50:50), UN+PMB (50:50), and UN+WCRB+PMB (50:25:25). The treatments were arranged in a completely randomized design with three replications. Results indicated a significant increase in the growth and biomass production of maize and wheat supplemented with biochars alone or mixed with N fertilizer. Biochars treatments showed varying impact on plant growth depended upon the type of the biochar, and in general plant growth under PMB was significantly higher than that recorded under WCRB. The growth characteristics in the combined treatments (half biochar+half N) were either higher or equivalent to that recorded under full fertilizer N treatment (N₂₀₀). The biochar treatments WCRB, PMB, and WCRB+PMB (50:50) increased maize shoot N by 18, 26 and 21%, respectively compared to the control while wheat shoot N did not show positive response. The N-uptake by maize treated with WCRB, PMB, and WCRB+PMB (50:50) was 54, 116, and 90 mg g^-1 compared to the 33 mg g^-1 in the control while the N-uptake by wheat was 41, 60, and 53 mg g^-1 compared to 24 mg g^-1 in the control. The mixed treatments (half biochar+half N) increased N-uptake by 2.3folds in maize and 1.7 to 2.5folds in wheat compared to the N₁₀₀ showing increasing effect of biochar on N use efficiency of applied N. Post-harvest soil analysis indicated a significant increase in pH, organic matter, organic C, total N, C:N, and porosity (% pore space) by the added biochars while bulk density (BD) was significantly decreased. The organic matter content in the soil amended with biochars ranged between 19.5 and 23.2 g kg^-1 compared to 11.7 and 10.2 g kg^-1 in the control and N fertilizer treatments while the BD of biochars amended soils (WCRB, PMB, and WCRB+PMB) was 1.07, 1.17, and 1.11 g cm^-3 compared to 1.28 g cm^-1 in the control. In summary, the results of present study highlight the agronomic benefits of biochars in improving the quality of the soil, and promoting growth, yield and N accumulation of both maize and wheat with a consequent benefit to agriculture.

The impact of biochars on sorption and biodegradation of polycyclic aromatic hydrocarbons in soils--a review

Amending polycyclic aromatic hydrocarbon (PAH)-contaminated soils with biochar may be cheaper and environmentally friendly than other forms of organic materials. This has led to numerous studies on the use of biochar to either bind or stimulate the microbial degradation of organic compounds in soils. However, very little or no attention have been paid to the fact that biochars can give simultaneous impact on PAH fate processes, such as volatilization, sorption and biodegradation. In this review, we raised and considered the following questions: How does biochar affect microbes and microbial activities in the soil? What are the effects of adding biochar on sorption of PAHs? What are the effects of adding biochar on degradation of PAHs? What are the factors that we can manipulate in the laboratory to enhance the capability of biochars to degrade PAHs? A triphasic concept of how biochar can give simultaneous impact on PAH fate processes in soils was proposed, which involves rapid PAH sorption into biochar, subsequent desorption and modification of soil physicochemical properties by biochar, which in turn stimulates microbial degradation of the desorbed PAHs. It is anticipated that biochar can give simultaneous impact on PAH fate processes in soils.

Shifts in the abundance and community structure of soil ammonia oxidizers in a wet sclerophyll forest under long-term prescribed burning

Fire shapes global biome distribution and promotes the terrestrial biogeochemical cycles. Ammonia-oxidizing bacteria (AOB) and archaea (AOA) play a vital role in the biogeochemical cycling of nitrogen (N). However, behaviors of AOB and AOA under long-term prescribed burning remain unclear. This study was to examine how fire affected the abundances and communities of soil AOB and AOA. A long-term repeated forest fire experiment with three burning treatments (never burnt, B0; biennially burnt, B2; and quadrennially burnt, B4) was used in this study. The abundances and community structure of soil AOB and AOA were determined using quantitative PCR, restriction fragment length polymorphism and clone library. More frequent fires (B2) increased the abundance of bacterium amoA gene, but tended to decrease archaeal amoA genes. Fire also modified the composition of AOA and AOB communities. Canonical correspondence analysis showed soil pH and dissolved organic C (DOC) strongly affected AOB genotypes, while nitrate-N and DOC shaped the AOA distribution. The increased abundance of bacterium amoA gene by fires may imply an important role of AOB in nitrification in fire-affected soils. The fire-induced shift in the community composition of AOB and AOA demonstrates that fire can disturb nutrient cycles.

Biochar decelerates soil organic nitrogen cycling but stimulates soil nitrification in a temperate arable field trial

Biochar production and subsequent soil incorporation could provide carbon farming solutions to global climate change and escalating food demand. There is evidence that biochar amendment causes fundamental changes in soil nutrient cycles, often resulting in marked increases in crop production, particularly in acidic and in infertile soils with low soil organic matter contents, although comparable outcomes in temperate soils are variable. We offer insight into the mechanisms underlying these findings by focusing attention on the soil nitrogen (N) cycle, specifically on hitherto unmeasured processes of organic N cycling in arable soils. We here investigated the impacts of biochar addition on soil organic and inorganic N pools and on gross transformation rates of both pools in a biochar field trial on arable land (Chernozem) in Traismauer, Lower Austria. We found that biochar increased total soil organic carbon but decreased the extractable organic C pool and soil nitrate. While gross rates of organic N transformation processes were reduced by 50–80%, gross N mineralization of organic N was not affected. In contrast, biochar promoted soil ammonia-oxidizer populations (bacterial and archaeal nitrifiers) and accelerated gross nitrification rates more than two-fold. Our findings indicate a de-coupling of the soil organic and inorganic N cycles, with a build-up of organic N, and deceleration of inorganic N release from this pool. The results therefore suggest that addition of inorganic fertilizer-N in combination with biochar could compensate for the reduction in organic N mineralization, with plants and microbes drawing on fertilizer-N for growth, in turn fuelling the belowground build-up of organic N. We conclude that combined addition of biochar with fertilizer-N may increase soil organic N in turn enhancing soil carbon sequestration and thereby could play a fundamental role in future soil management strategies.