An estimation of annual nitrous oxide emissions and soil quality following the amendment of high temperature walnut shell biochar and compost to a small scale vegetable crop rotation

Soil N2 O emissions from specialty crop systems: A global estimation and meta-analysis

Nitrous oxide (N2 O) exacerbates the greenhouse effect and thus global warming. Agricultural management practices, especially the use of nitrogen (N) fertilizers and irrigation, increase soil N2 O emissions. As a vital sector of global agriculture, specialty crop systems usually require intensive input and management. However, soil N2 O emissions from global specialty crop systems have not been comprehensively evaluated. Here, we synthesized 1137 observations from 114 published studies, conducted a meta-analysis to evaluate the effects of agricultural management and environmental factors on soil N2 O emissions, and estimated global soil N2 O emissions from specialty crop systems. The estimated global N2 O emission from specialty crop soils was 1.5 Tg N2 O-N year-1 , ranging from 0.5 to 4.5 Tg N2 O-N year-1 . Globally, soil N2 O emissions exponentially increased with N fertilizer rates. The effect size of N fertilizer on soil N2 O emissions generally increased with mean annual temperature, mean annual precipitation, and soil organic carbon concentration but decreased with soil pH. Global climate change will further intensify the effect of N fertilizer on soil N2 O emissions. Drip irrigation, fertigation, and reduced tillage can be used as essential strategies to reduce soil N2 O emissions and increase crop yields. Deficit irrigation and non-legume cover crop can reduce soil N2 O emissions but may also lower crop yields. Biochar may have a relatively limited effect on reducing soil N2 O emissions but be effective in increasing crop yields. Our study points toward effective management strategies that have substantial potential for reducing N2 O emissions from global agricultural soils.

Biochar-compost as a new option for soil improvement: Application in various problem soils

Due to the synergistic effects of biochar and compost/composting, the combined application of biochar and compost (biochar-compost) has been recognized as a highly promising and efficient method of soil improvement. However, the willingness to apply biochar-compost for soil improvement is still low compared to the use of biochar or compost alone. This paper collects data on the application of biochar-compost in several problem soils that are well-known and extensively investigated by agronomists and scientists, and summarizes the effects of biochar-compost application in common problem soils. These typical problem soils are classified based on three different characteristics: climatic zones, abiotic stresses, and contaminants. The improvement effect of biochar-compost in different soils is assessed and directions for further research and suggestions for application are made. Generally, biochar-compost mitigates the high mineralization rate of soil organic matter, phosphorus deficiency and aluminum toxicity, and significantly improves crop yields in most tropical soils. Biochar-compost can help to achieve long-term sustainable management of temperate agricultural soils by sequestering carbon and improving soil physicochemical properties. Biochar-compost has shown positive performance in the remediation of both dry and saline soils by reducing the threat of soil water scarcity or high salinity and improving the consequent deterioration of soil conditions. By combining different mechanisms of biochar and compost to immobilize or remove contaminants, biochar-compost tends to perform better than biochar or compost alone in soils contaminated with heavy metals (HMs) or organic pollutants (OPs). This review aims to improve the practicality and acceptability of biochar-compost and to promote its application in soil. Additionally, the prospects, challenges and future directions for the application of biochar-compost in problem soil improvement were foreseen.

Biochar mitigated more N-related global warming potential in rice season than that in wheat season: An investigation from ten-year biochar-amended rice-wheat cropping system of China

Rice-wheat cropping system (RWCS), the major rice-based cropping system, constitutes a significant source of N-related greenhouse gas (GHG) emission due to the unique wet-dry alternation process. Biochar is often highlighted as a potential solution for reducing fertilizer N losses, hence, understanding its effects on Ngr emissions (mainly NH₃ and N₂O) under wet-dry conditions is critical to inform strategies for GHG mitigation. This study investigated the responses of NH₃ and N₂O emissions to biochar amendments during rice and wheat seasons based on in situ measurements under ten-year successive straw biochar application in RWCS. Our results indicated that 43.7% and 89.9% of N₂O and NH₃ emissions were emitted during rice season and 56.3% and 10.1% during wheat season, respectively. Long-term biochar amendment was found to play significant role in mitigating NH₃ emissions (38.6-43.9%), which could be attributed to the disappearance of liming effect of aged-biochar on flooding water and decreased NH₄⁺ concentrations in the soil. However, considerable variation of N₂O emissions were observed in RWCS. Biochar showed a significant decreasing effect on the net global warming potential related to N₂O and NH₃ emissions (GWP_N) in rice season (16.1-89.6%), and slight increased tendency in wheat season (1.43-13.1%) primarily due to its positive effects on N₂O emission. Biochar amendment mainly BC22.5, significantly increased above-ground yields by 9.22% in rice season. Thus, it is a low carbon-producing and sustainable crop management method that can support crop production, C sequestration, and GHG mitigation in rice season under RWCS from the viewpoint of the Ngr mitigation. Our results suggest that emission patterns of N₂O and NH₃ varied with wet-dry alternation under the disturbance of long-term biochar amendment in RWCS; moreover, long-term biochar application exhibited significant potential for mitigating soil Ngr losses in rice season for RWCS.

Addition of walnut shells biochar to alkaline arable soil caused contradictory effects on CO2 and N2O emissions, nutrients availability, and enzymes activity

Mitigation of greenhouse gas (GHGs) emissions and improving soil health using biochar (BC) shall help achieving the UN-Sustainable Development Goals. The impacts of walnut shells biochar (WSB) pyrolyzed at different temperatures on CO₂ and N₂O emission and soil health have not been yet sufficiently explored. We investigated the effects of addition of WSB pyrolyzed at either 300 °C (WSB-300), 450 °C (WSB-450), or at 600 °C (WSB-600) to alkaline soil on CO₂ and N₂O emissions, nutrients availability, and soil enzymes activities in a 120-day incubation experiment. Cumulative N₂O emissions were reduced significantly as compared to the control, by 64.9%, 50.6%, and 36.4% after WSB-600, WSB-450 and WSB-300, respectively. However, the cumulative CO₂ emissions increased, over the control, as follows: WSB-600 (50.7%), WSB-450 (68.6%), and WSB-300 (73.4%). Biochar addition, particularly WSB-600 significantly increased soil pH (from 8.1 to 8.34), soil organic C (SOC; from 8.6 to 22.3 g kg^-1), available P (from 21.0 to 60.5 mg kg^-1), and K (181.0-480.5 mg kg^-1), and activities of urease, alkaline phosphatase, and invertase. However, an opposite pattern was observed with NH₄⁺, NO₃^-, total N and β-glucosidase activity after WSB application. The WBS produced from high temperature pyrolysis can be used for N₂O emissions mitigation and improvement of soil pH, SOC, available P and K, and activities of urease, alkaline, phosphatase. However, WBS produced from low temperature pyrolysis can be used to promote N availability and β-glucosidase; however, these findings should be verified under different field and climatic conditions.

Effects of different feedstocks-based biochar on soil remediation: A review

As a promising amendment, biochar has excellent characteristics and can be used as a remediation agent for diverse types of soil pollution. Biochar is mostly made from agricultural wastes, forestry wastes, and biosolids (eg, sewage sludge), but not all the biochar has the same performance in the improvement of soil quality. There is a lack of guidelines devoted to the selection of biochar to be used for different types of soil pollution, and this can undermine the remediation efficiency. To shed light on this sensitive issue, this review focus on the following aspects, (i) how feedstocks affect biochar properties, (ii) the effects of biochar on heavy metals and organic pollutants in soil, and (iii) the impact on greenhouse gas emissions from soil. Generally, the biochars produced from crop residue and woody biomass which are composed of lignin, cellulose, and hemicellulose are more suitable for organic pollution remediation and greenhouse gas emission reduction, while biochar with high ash content are more suitable for cationic organic pollutant and heavy metal pollution (manure and sludge, etc.). Additionally, the effect of biochar on soil microorganisms shows that gram-negative bacteria in soil tend to use WB biochar with high lignin content, while biochar from OW (rich in P, K, Mg, and other nutrients) is more able to promote enzyme activity. Finally, our recommendations on feedstocks selection are presented in the form of a flow diagram, which is precisely intended to be used as a support for decisions on the crucial proportioning conditions to be selected for the preparation of biochar having specific properties and to maximize its efficiency in pollution control.

Growth Response of Ginger (Zingiber officinale), Its Physiological Properties and Soil Enzyme Activities after Biochar Application under Greenhouse Conditions

This study aimed to investigate the effects of biochar (1%, 2%, and 3%) on seed germination, plant growth, root morphological characteristics, and physiological properties of ginger (Zingiber officinale) and soil enzymatic activities. Pot experiments under greenhouse conditions at 24 °C (day) and 16 °C (night) showed after six weeks that biochar additions of 2% and 3% significantly increased seed germination, plant height, leaf length, leaf number, as well as shoot and root dry weights compared to the control. Total root length significantly increased by 30%, 47%, and 74%, with increasing biochar contents (1%, 2%, and 3%) compared to the control. Root surface area, projected area, root diameter, and root volume reached a maximum at the 3% biochar treatment. The treatment with 2% biochar significantly increased fluorescein diacetate hydrolase and phenoloxidase activities by 33% and 59% compared to the control; so did the addition of 3% biochar, which significantly increased fluorescein diacetate hydrolases, phenoloxidase, and acid and alkaline phosphomonoesterase activity in soil compared to the control. Treatment with 3% biochar increased relative water content by 8%, chlorophyll content by 35%, and carotenoid content by 43% compared to the control. These results suggest that biochar can improve the performance of the rhizome of ginger and increase the activity of soil enzymes, thereby improving soil nutrient supply.

Corn Cob-Derived Biochar Improves the Growth of Saline-Irrigated Quinoa in Different Orders of Egyptian Soils

Biochar is one of the important recycling methods in sustainable development, as it ensures the transformation of agricultural wastes into fertilizers and conditioners that improve soil properties and fertility. In the current study, corn cob-derived biochar (CB) was used to reduce the negative effects of saline water on quinoa (Chenopodium quinoa cv. Utosaya Q37) grown on Aridisols and Entisols, which are the major soil groups of Egyptian soils. Quinoa plants were cultivated in pot experiment and were irrigated with saline water (EC = 10 dS m−1). The experiment contained three treatments, including control without any treatment, biochar at a rate of 1% (w/w) (BC1), and biochar at a rate of 3% (w/w) (BC3). The findings of the current study showed that BC treatments realized significant effects on soil salinity, pH, soil organic matter (SOM), and plant availability and nutrients’ uptake in the two soils types. BC3 increased the SOM in Entisols and Aridisols by 23 and 44%; moreover, the dry biomass of quinoa plants was ameliorated by 81 and 41%, respectively, compared with the control. Addition of biochar to soil increased the nutrients’ use efficiencies by quinoa plants for the two studied Egyptian soils. Biochar addition caused significant increases in the use efficiency of nitrogen (NUF), phosphorus (PUE), and potassium (KUE) by quinoa plants. BC3 increased NUE, PUE, and KUS by 81, 81, and 80% for Entisols, while these increases were 40, 41, and 42% in the case of Aridisols. Based on the obtained results, the application of corn cob biochar improves the soil quality and alleviates the negative effects of saline irrigation on quinoa plants grown on Aridisols and Entisols Egyptian soils. Biochar can be used as a soil amendment in arid and semi-arid regions to reduce the salinity hazards.

A Sustainable Approach for Improving Soil Properties and Reducing N2O Emissions Is Possible through Initial and Repeated Biochar Application

Recent findings of changing climate, water scarcity, soil degradation, and greenhouse gas emissions have brought major challenges to sustainable agriculture worldwide. Biochar application to soil proves to be a suitable solution to these problems. Although the literature presents the pros and cons of biochar application, very little information is available on the impact of repeated application. In this study, we evaluate and discuss the effects of initial and reapplied biochar (both in rates of 0, 10, and 20 t ha−1) combined with N fertilization (at doses of 0, 40, and 80 kg ha−1) on soil properties and N2O emission from Haplic Luvisol in the temperate climate zone (Slovakia). Results showed that biochar generally improved the soil properties such as soil pH(KCl) (p ≤ 0.05; from acidic towards moderately acidic), soil organic carbon (p ≤ 0.05; an increase from 4% to over 100%), soil water availability (an increase from 1% to 15%), saturated hydraulic conductivity (an increase from 5% to 95%). The effects were more significant in the following cases: repeated rather than single biochar application, higher rather than lower biochar application rates, and higher rather than lower N fertilization levels. Initial and repeated biochar applications, leading to N2O emissions reduction, can be related to increased soil pH(KCl).

Effects of biochar on nitrification and denitrification-mediated N2O emissions and the associated microbial community in an agricultural soil

Nitrous oxide (N₂O) is a strong greenhouse gas, and it is of great significance for N₂O reduction to study the effects of biochar on its production pathway. In this research, the contributions and mechanisms of biochar on autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and denitrification (DF) to N₂O emissions were studied by using ¹⁵N stable isotopes and high-throughput sequencing after laboratory incubation. The results showed that biochar addition at 2% (B2) significantly reduced the N₂O emissions from the ANF by an average of 20.6%, while adding 5% biochar (B5) had no significant effect on the ANF. Both B2 and B5 significantly reduced the N₂O emissions from the HNF by 15.7% and 13.2%, respectively, and reduced the N₂O emissions from the DF by 40.9% and 11.7%, respectively. B2 enhanced the relative contribution rate of the ANF to N₂O emissions by 6.3%, while B5 had little effect on it. Biochar addition significantly changed the copy numbers of the AOA and AOB, as well as the nirK, nirS, and nosZ genes, but it had no significant effect on the community composition of the AOA and had minimal effect on the AOB community. B2 significantly increased the abundance of the genus Rhodococcus of nirK type denitrifiers and had a significant effect on the relative abundance of Cupriavidus and Pseudomonas of the nosZ type denitrifiers. These results revealed that the inhibitory effects of biochar on N₂O emissions from nitrification might be attributed to the direct immobilization and adsorption of inorganic N by biochar and to its promotion of the genus Rhodococcus of nirK-type denitrifiers and the genera Cupriavidus and Pseudomonas of the nosZ-type denitrifiers. The soil exchangeable NH₄⁺-N and NO₃^--N concentrations were the primary factors affecting the N₂O emission rates. These results help to elucidate the effects and mechanisms of biochar on N₂O production pathways in agricultural soil.

Biochar stimulates NH4+ turnover while decreasing NO3- production and N2O emissions in soils under long-term vegetable cultivation

A ¹⁵N-tracer incubation experiment was conducted to investigate the short-term effects of biochar on gross N transformation rates and nitrous oxide (N₂O) emissions in soils under 1-year and 10-year vegetable cultivations. Biochar was applied at three rates: 0 (control), 10, and 30 t ha^-1. Gross N transformation rates in the two vegetable soils varied in response to biochar application. Specifically, organic N oxidation into NO₃^- (O_Norg) was almost negligible in the biochar-amended soils, and biochar application at 10 t ha^-1 did not change either the rate of mineralization of organic N into NH₄⁺ (M_Norg) nor the inorganic N supply capacity (INS, O_Norg + M_Norg) in both soils, when compared to the control. However, 30 t ha^-1 biochar decreased INS significantly, by inhibiting M_Norg, in the 1-year vegetable soil but increased INS, by stimulating M_Norg, in the 10-year vegetable soil. The rates of NH₄⁺ oxidation into NO₃^- (O_NH4), NO₃^- immobilization into organic N, and dissimilatory NO₃^- reduction into NH₄⁺ were not influenced significantly by biochar application in the 1-year vegetable soil, resulting in no significant differences in NO₃^- production potential. Conversely, biochar decreased NO₃^- production potential significantly in the 10-year vegetable soil, by inhibiting O_NH4 and increasing NH₄⁺ immobilization into organic N (I_NH4), with more obvious effects under higher biochar application rates. Overall, the results demonstrate the capacity of biochar to stimulate NH₄⁺ turnover and to decrease NO₃^- production potential in soil under long-term vegetable cultivation; however, the effect is limited under short-term vegetable cultivation. In addition, N₂O emissions decreased significantly in biochar-amended vegetable soils.

Biochar production and applications in agro and forestry systems: A review

Biochar is a product of biomass thermochemical conversion. Its yield and quality vary significantly with the production technology and process parameters, which also affect its performance in agro and forestry systems. In this review, biochar production technologies including slow pyrolysis, fast pyrolysis, gasification, and torrefaction were compared. The yield of biochar was found to decrease with faster heating rate or more oxygen available. The benefits of biochar application to agro and forestry systems were discussed. Improvements in soil health, plant growth, carbon sequestration, and greenhouse gas mitigation are apparent in many cases, but opposite results do exist, indicating that the beneficial aspect of biochar are limited to particular conditions such as the type of biochar used, the rate of application, soil type, climate, and crop species. Limitations of current studies and future research needed on biochar are also discussed. Specifically, the relationships among biochar production technologies, biochar properties, and biochar performance in agro and forestry systems must be better understood.

Nitrogen dynamics affected by biochar and irrigation level in an onion field

Soil amended with biochar has many potential environmental benefits, but its influence on the fate of nitrogen (N) under irrigated conditions is unclear. The objective of this research was to determine the effects of biochar and interactions with irrigation on N movement in soil, gas emissions, and leaching. A three-year study was conducted in an onion field with three main irrigation treatments (50, 75, and 100% of a reference that provided sufficient water for plant growth) and three biochar amendment rates (0 or control, low char - applied first year at 29 Mg ha^-1, and high char - added both first and second year for a total 58 Mg ha^-1) as sub-treatments in a split-plot design. Nitrogen fertilizer was applied three times during first year growing season, but weekly the second year. Ammonia (NH₃) volatilization, nitrous oxide (N₂O) emission, and nitrate (NO₃^-) in soil pore water were monitored during growing season, and annual N (total and NO₃^-) changes in soil profile were determined for first two years. Nitrate leaching was measured in the third year. Ammonia volatilization was affected by fertilization frequency with higher loss (5-8% of total applied) when fertilizer was applied in large doses during the first year compared to the second year (4-5%). Nitrous oxide emissions were ≤0.1% of applied N for both years and not affected by any treatments or fertilization frequency. Nitrate concentration in soil profile increased significantly as irrigation level dropped, but most of the NO₃^- was leached by winter rain. There was no significant biochar effect on total N gas emissions or soil NO₃^- accumulation, but significant irrigation effect and interaction with biochar were determined on soil NO₃^- accumulation. High leaching was associated with biochar amendment and higher irrigation level. Irrigation strategies are the key to improving N management and developing the best practices associated with biochar.

High-Temperature Hay Biochar Application into Soil Increases N2O Fluxes

Biochar has been proposed as an amendment that can improve soil conditions, increase harvest yield, and reduce N losses through NO3− leaching and N2O emissions. We conducted an experiment to test the hay biochar mitigation effect on N2O emissions depending on its production temperature. The pot experiment consisted of the soil amendment with three different production temperature biochars (300 °C, 550 °C, 850 °C) alone and in combination with three different organic fertilizers (cattle slurry, slurry digestate, vinasse), in growth chamber conditions. The effects of biochar and fertilizer were both significant, but the interaction biochar:fertilizer was not. The amendment with the three fertilizer types and with the highest production temperature biochar resulted in significantly higher cumulative N2O fluxes. Biochar did not show a mitigation effect on N2O emissions when applied with organic fertilizer. Cumulative emissions were higher with biochar addition, with increasing emissions for increasing biochar production temperature. Our results support the idea that biochar cannot be considered as a universal tool for the reduction of N2O emissions.

Soil Management Practices to Mitigate Nitrous Oxide Emissions and Inform Emission Factors in Arid Irrigated Specialty Crop Systems

Greenhouse gas (GHG) emissions from arid irrigated agricultural soil in California have been predicted to represent 8% of the state’s total GHG emissions. Although specialty crops compose the majority of the state’s crops in both economic value and land area, the portion of GHG emissions contributed by them is still highly uncertain. Current and emerging soil management practices affect the mitigation of those emissions. Herein, we review the scientific literature on the impact of soil management practices in California specialty crop systems on GHG nitrous oxide emissions. As such studies from most major specialty crop systems in California are limited, we focus on two annual and two perennial crops with the most data from the state: tomato, lettuce, wine grapes and almond. Nitrous oxide emission factors were developed and compared to Intergovernmental Panel on Climate Change (IPCC) emission factors, and state-wide emissions for these four crops were calculated for specific soil management practices. Dependent on crop systems and specific management practices, the emission factors developed in this study were either higher, lower or comparable to IPCC emission factors. Uncertainties caused by low gas sampling frequency in these studies were identified and discussed. These uncertainties can be remediated by robust and standardized estimates of nitrous oxide emissions from changes in soil management practices in California specialty crop systems. Promising practices to reduce nitrous oxide emissions and meet crop production goals, pertinent gaps in knowledge on this topic and limitations of this approach are discussed.

Soil Carbon and Nitrogen Dynamics in Two Agricultural Soils Amended with Manure-Derived Biochar

Biochar has been promoted as a means to sequester C and improve soil quality. Biochar produced from agricultural waste streams and recycled as a soil amendment also provides a strategy for improved nutrient management in agricultural systems. The effects of biochar amendment on soil C and N cycling remain poorly constrained. This study aimed to examine the effects of biochar on soil C and N storage, N mineralization, and soil physiochemical properties. Soils were collected from a field experiment in which biochar derived from poultry manure was applied for 2 yr in two croplands differing in soil texture (sandy and silt-loam). Samples from biochar-amended and control soils were physically fractionated to separate water-stable soil aggregates and analyzed for C and N. Biochar amendments increased total soil C by 16 (sandy soil) and 30% (silt-loam soil). These increases were observed in aggregate size classes associated with short-term C and N storage in silt-loam soils and intermediate-term C and N storage in sandy soils. Net N mineralization rates observed in a short-term incubation were small or negative (1.79 and -24.7 μg N g soil for sandy and silt-loam soils, respectively), indicating little or no new N mineralization from biochar over short timescales. Biochar amendment had a positive impact on cation exchange capacity at both sites, increasing it by 7 and 11% in the silt-loam soil and sandy soil, respectively. These results suggest that biochar amendments to cropping systems can improve the capacity of soil to retain nutrients and store C and N.

Impacts of biochar application on upland agriculture: A review

Soil degradation has become an emerging global problem limiting sustainable upland crop production. Soil erosion, soil acidity, low fertility, inorganic/organic contamination, and salinization challenge food security and lead to severe economic constraints. Therefore, a new research agenda to develop cost-beneficial amendments for improving upland soil quality and productivity is urgently required. Biochar has been used in recent years to mitigate the problems mentioned above. Application of biochar improves the upland soil quality through significant changes in soil physicochemical and biological properties, thereby substantially increasing crop yield. This review article aims to discuss the effects of biochar on upland soil quality and productivity based on biochar-soil interactions. The yield of various upland crops can be enhanced by biochar-induced increases of nutrient availability and topsoil retention/recovery. Furthermore, biochar can assist in controlling unsuitable soil acidity/alkalinity/salinity and remediating a contaminated soil while increasing the retention of soil organic carbon, water content, and thereby high crop yield. Biochar is strongly recommended as one of the best management practices to meet the challenges of upland agriculture. However, the properties of biochar and soil type should be considered carefully prior to application.

Variable effects of biochar application to soils on nitrification-mediated N2O emissions

Although a meta-analysis on biochar's effects on N₂O emission reported an overall reduction in N₂O emission by adding biochar to the soils, there are still variations in the changes in N₂O emission, especially from field results. The objectives of this study are 1) to compare the effects of biochar addition on N₂O emission between three agricultural upland field experiments, where soil water status was dry favoring nitrification and 2) to identify main factors explaining biochar's variable effects on N₂O emission. Three field experiments were conducted: Exp A in the cultivated grassland treated with rice husk biochar at 2 ton ha^-1 + urea (CHAR) and with urea only (CON); Exp B in the cabbage field with CHAR and CON treatments; and Exp C in the pepper field with CHAR, CON, and CHAR + DCD (dicyandiamide, nitrification inhibitor) treatments. In Exp A and C, cumulative N₂O emissions significantly increased by 82.5% and 55.8% in the CHAR than CON treatments, respectively, while in Exp B, there was no difference in cumulative N₂O emission between the CHAR and CON. Based on results from using nitrification inhibitor and soil % water filled pore space (WFPS), we assumed that the main N₂O production mechanism was nitrification. Our results suggest that soil water status right after urea application is the primary determinant of different effects of biochar on N₂O emission in addition to soil C status and biochar's adsorption. Principal component analysis using the 25 compiled data also supported our results. This study identified the specific field conditions under which biochar could have stimulating effects on N₂O emission. Mitigation potential of biochar application should be reconsidered if biochar and urea were amended to dry soils with low C contents.

Offsetting global warming-induced elevated greenhouse gas emissions from an arable soil by biochar application

Global warming will likely enhance greenhouse gas (GHG) emissions from soils. Due to its slow decomposability, biochar is widely recognized as effective in long-term soil carbon (C) sequestration and in mitigation of soil GHG emissions. In a long-term soil warming experiment (+2.5 °C, since July 2008) we studied the effect of applying high-temperature Miscanthus biochar (0, 30 t/ha, since August 2013) on GHG emissions and their global warming potential (GWP) during 2 years in a temperate agroecosystem. Crop growth, physical and chemical soil properties, temperature sensitivity of soil respiration (R_s ), and metabolic quotient (qCO₂ ) were investigated to yield further information about single effects of soil warming and biochar as well as on their interactions. Soil warming increased total CO₂ emissions by 28% over 2 years. The effect of warming on soil respiration did not level off as has often been observed in less intensively managed ecosystems. However, the temperature sensitivity of soil respiration was not affected by warming. Overall, biochar had no effect on most of the measured parameters, suggesting its high degradation stability and its low influence on microbial C cycling even under elevated soil temperatures. In contrast, biochar × warming interactions led to higher total N₂ O emissions, possibly due to accelerated N-cycling at elevated soil temperature and to biochar-induced changes in soil properties and environmental conditions. Methane uptake was not affected by soil warming or biochar. The incorporation of biochar-C into soil was estimated to offset warming-induced elevated GHG emissions for 25 years. Our results highlight the suitability of biochar for C sequestration in cultivated temperate agricultural soil under a future elevated temperature. However, the increased N₂ O emissions under warming limit the GHG mitigation potential of biochar.

Influence of surface chemistry of carbon materials on their interactions with inorganic nitrogen contaminants in soil and water

Inorganic nitrogen contaminants (INC) (NH₄⁺, NO₃^-, NO₂^-, NH₃, NO, NO₂, and N₂O) pose a growing risk to the environment, and their remediation methods are highly sought after. Application of carbon materials (CM), such as biochar and activated carbon, to remediate INC from agricultural fields and wastewater treatment plants has gained a significant interest since past few years. Understanding the role of surface chemistry of CM in adsorption of various INC is highly critical to increase adsorption efficiency as well as to assess the long term impact of using these highly recalcitrant CM for remediation of INC. Critical reviews of adsorption studies related to INC have revealed that carbon surface chemistry (surface functional groups, pH, Eh, elemental composition, and mineral content) has significant influence on adsorption of INC. Compared to basic functional groups, oxygen containing surface functional groups have been found to be more influential for adsorption of INC. However, basic sites on carbon materials still play an important role in chemisorption of anionic INC. Apart from surface functional groups, pH, Eh and pH_zpc of CM and elemental and mineral composition of its surface are important properties capable of altering INC interactions with CM. This review summarizes our current understanding of INC interactions with CM's surface through the known chemisorption mechanisms: electrostatic interaction, hydrogen bonding, electron donor-acceptor mechanism, hydrophobic and hydrophilic interaction, chemisorption aided by minerals, and interactions influenced by pH and elemental composition. Change in surface chemistry of CM in soil during aging is also discussed.

Nitrogen nutrition in cotton and control strategies for greenhouse gas emissions: a review

Cotton (Gossypium hirustum L.) is grown globally as a major source of natural fiber. Nitrogen (N) management is cumbersome in cotton production systems; it has more impacts on yield, maturity, and lint quality of a cotton crop than other primary plant nutrient. Application and production of N fertilizers consume large amounts of energy, and excess application can cause environmental concerns, i.e., nitrate in ground water, and the production of nitrous oxide a highly potent greenhouse gas (GHG) to the atmosphere, which is a global concern. Therefore, improving nitrogen use efficiency (NUE) of cotton plant is critical in this context. Slow-release fertilizers (e.g., polymer-coated urea) have the potential to increase cotton yield and reduce environmental pollution due to more efficient use of nutrients. Limited literature is available on the mitigation of GHG emissions for cotton production. Therefore, this review focuses on the role of N fertilization, in cotton growth and GHG emission management strategies, and will assess, justify, and organize the researchable priorities. Nitrate and ammonium nitrogen are essential nutrients for successful crop production. Ammonia (NH₃) is a central intermediate in plant N metabolism. NH₃ is assimilated in cotton by the mediation of glutamine synthetase, glutamine (z-) oxoglutarate amino-transferase enzyme systems in two steps: the first step requires adenosine triphosphate (ATP) to add NH₃ to glutamate to form glutamine (Gln), and the second step transfers the NH₃ from glutamine (Gln) to α-ketoglutarate to form two glutamates. Once NH₃ has been incorporated into glutamate, it can be transferred to other carbon skeletons by various transaminases to form additional amino acids. The glutamate and glutamine formed can rapidly be used for the synthesis of low-molecular-weight organic N compounds (LMWONCs) such as amides, amino acids, ureides, amines, and peptides that are further synthesized into high-molecular-weight organic N compounds (HMWONCs) such as proteins and nucleic acids.

In-situ biogas upgrading during anaerobic digestion of food waste amended with walnut shell biochar at bench scale

A modified version of an in-situ CO₂ removal process was applied during anaerobic digestion of food waste with two types of walnut shell biochar at bench scale under batch operating mode. Compared with the coarse walnut shell biochar, the fine walnut shell biochar has a higher ash content (43 vs. 36 wt%) and higher concentrations of calcium (31 vs. 19 wt% of ash), magnesium (8.4 vs. 5.6 wt% of ash) and sodium (23.4 vs. 0.3 wt% of ash), but a lower potassium concentration (0.2 vs. 40% wt% of ash). The 0.96-3.83 g biochar (g VS_added)^-1 fine walnut shell biochar amended digesters produced biogas with 77.5%-98.1% CH₄ content by removing 40%-96% of the CO₂ compared with the control digesters at mesophilic and thermophilic temperature conditions. In a direct comparison at 1.83 g biochar (g VS_added)^-1, the fine walnut shell biochar amended digesters (85.7% CH₄ content and 61% CO₂ removal) outperformed the coarse walnut shell biochar amended digesters (78.9% CH₄ content and 51% CO₂ removal). Biochar addition also increased alkalinity as CaCO₃ from 2800 mg L^-1 in the control digesters to 4800-6800 mg L^-1, providing process stability for food waste anaerobic digestion.

Toward a Better Assessment of Biochar-Nitrous Oxide Mitigation Potential at the Field Scale

Through meta-analysis, we synthesize results from field studies on the effect of biochar application on NO emissions and crop yield. We aimed to better constrain the effect of biochar on NO emissions under field conditions, identify significant predictor variables, assess potential synergies and tradeoffs between NO mitigation and yield, and discuss knowledge gaps. The response ratios for yield and NO emissions were weighted by one of two functions: (i) the inverse of the pooled variance or (ii) the inverse of number of observations per field site. Significant emission reductions were observed when weighting by the inverse of the pooled variance (-18.1 to -7.1%) but not when weighting by the number of observations per site (-17.1 to +0.8%), thus revealing a bias in the existing data by sites with more observations. Mean yield increased by 1.7 to 13.8%. Our study shows yield benefits but no robust evidence for NO emission reductions by biochar under field conditions. When weighted by the inverse of the number of observations per site, NO emission reductions were not significantly affected by cropping system, biochar properties of feedstock, pyrolysis temperature, surface area, pH, ash content, application rate, or site characteristics of N rate, N form, or soil pH. Uneven coverage in the range of these predictor variables likely underlies the failure to detect effects. We discuss the need for future biochar field studies to investigate effects of fertilizer N form, sustained and biologically relevant changes in soil moisture, multiple biochars per site, and time since biochar application.

Contrasting effects of biochar on N2O emission and N uptake at different N fertilizer levels on a temperate sandy loam

Biochar has been frequently suggested as an amendment to improve soil quality and mitigate climate change. To investigate the optimal management of nitrogen (N) fertilization, we examined the combined effect of biochar and N fertilizer on plant N uptake and N₂O emissions in a cereal rotation system in a randomized two-factorial field experiment on a sandy loam soil in Brandenburg, Germany. The biochar treatment received 10Mgha^-1 wood-derived biochar in September 2012. Four levels of N fertilizer, corresponding to 0, 50%, 100%, 130% of the recommended fertilizer level, were applied in winter wheat (Triticum aestivum L.)) and winter rye (Secale cereal L.) in 2013 and 2014 followed by the catch crop oil radish (Raphanus sativus L. var. oleiformis). Biomass and N uptake of winter wheat and winter rye were significantly affected by the level of N fertilizer but not by biochar. For N uptake of oil radish an interaction effect was observed for biochar and N fertilizer. Without applied fertilizer, 39% higher N uptake was found in the presence of biochar, accompanied by higher soil NH₄⁺ content and elevated cumulative CO₂ emissions. At 130% of the recommended fertilizer level, 16% lower N uptake and lower cumulative N₂O emissions were found in the biochar-mediated treatment. No significant change in abundance of microbial groups and nosZ gene were observed. Our results highlight that biochar can have a greenhouse gas mitigation effect at high levels of N supply and may stimulate nutrient uptake when no N is supplied.

Stoichiometric ratio of dissolved organic carbon to nitrate regulates nitrous oxide emission from the biochar-amended soils

Biochar has the potential to mitigate nitrous oxide (N₂O) emissions from soils. However, the mechanisms responsible for N₂O emission in biochar-amended soils are yet to be elucidated. In this study, an incubation experiment was carried out to investigate the effects of seven biochars (eucalyptus, softwood mixture, mallee, jarrah, peanut shell, green waste and radiata pine) on the stoichiometric shifts of dissolved organic carbon (DOC), nitrate (NO₃^--N) and N₂O emission in two contrasting soils (Ferrosol with 5.3% total C, 0.46% total N; Tenosol with 0.4% total C, 0.01% total N). All biochar treatments were found to significantly reduce N₂O emission in Tenosol by 61-72%. However, in Ferrosol, biochars' impacts on N₂O emission were variable, with only peanut shell, green waste and radiata pine bicohars significantly reducing N₂O emission by 17-23%. A decrease in NO₃^- availability in most biochar-amended treatments also was observed in both soils compared with the control. The N₂O fluxes in Ferrosol were mainly regulated by the shifts in the availability and stoichiometry of DOC and NO₃^- induced by the biochar amendments. The DOC derived from biochars increased DOC:NO₃^- ratio in Ferrosol at the beginning of the experiment, but these effects disappeared 7days after incubation. Overall, the N₂O fluxes were C-limited due to the presence of high concentrations of NO₃^- in Ferrosol. However, in Tenosol, the relationship between stoichiometry of DOC:NO₃^- and N₂O fluxes was much weaker than Ferrosol and N₂O fluxes mainly limited by the concentration of NO₃^-. This study demonstrated that the mechanisms responsible for biochar effects on soil N₂O fluxes are considered to be soil and biochar specific.

Soil biochar amendment as a climate change mitigation tool: Key parameters and mechanisms involved

Biochar, a solid porous material obtained from the carbonization of biomass under low or no oxygen conditions, has been proposed as a climate change mitigation tool because it is expected to sequester carbon (C) for centuries and to reduce greenhouse gas (GHG) emissions from soils. This review aimed to identify key biochar properties and production parameters that have an effect on these specific applications of the biochar. Moreover, mechanisms involved in interactions between biochar and soils were highlighted. Following a compilation and comparison of the characteristics of 76 biochars from 40 research studies, biochars with a lower N content, and consequently a higher C/N ratio (>30), were found to be more suitable for mitigation of N2O emissions from soils. Moreover, biochars produced at a higher pyrolysis temperature, and with O/C ratio <0.2, H/Corg ratio <0.4 and volatile matter below 80% may have high C sequestration potential. Based on these observations, biochar production and application to the field can be used as a tool to mitigate climate change. However, it is important to determine the pyrolysis conditions and feedstock needed to produce a biochar with the desired properties for a specific application. More research studies are needed to identify the exact mechanisms involved following biochar amendment to soil.

Responses of bacterial community and functional marker genes of nitrogen cycling to biochar, compost and combined amendments in soil

Biochar and compost are seen as two attractive waste management options and are used for soil amendment and pollution remediation. The interaction between biochar and composting may improve the potential benefits of biochar and compost. We investigated soil physicochemical properties, bacterial community, bacterial 16S rRNA, and functional marker genes of nitrogen cycling of the soil remedied with nothing (S), compost (SC), biochar (SB), a mixture of compost and biochar (SBC), composted biochar (SBced), and a composted mixture of biochar and biomass (SBCing). The results were that all amendments (1) increased the bacterial community richness (except SB) and SBCing showed the greatest efficiency; (2) increased the bacterial community diversity (SBCing > SBC > SC > SBced > SB > S); and (3) changed the gene copy numbers of 16S rRNA, nirK, nirS, and nosZ genes of bacteria, ammonia-oxidizing archaea (AOA), and ammonia-oxidizing bacteria (AOB). All amendments (except SB) could increase the gene copy number of 16S rRNA, and SBCing had the greatest efficiency. The changes of soil bacterial community richness and diversity and the gene copy numbers of 16S rRNA, nirK, nirS, nosZ, AOA, and AOB would affect carbon and nitrogen cycling of the ecosystem and also implied that BCing had the greatest efficiency on soil amendment.

Soil properties, greenhouse gas emissions and crop yield under compost, biochar and co-composted biochar in two tropical agronomic systems

The addition of organic amendments to agricultural soils has the potential to increase crop yields, reduce dependence on inorganic fertilizers and improve soil condition and resilience. We evaluated the effect of biochar (B), compost (C) and co-composted biochar (COMBI) on the soil properties, crop yield and greenhouse gas emissions from a banana and a papaya plantation in tropical Australia in the first harvest cycle. Biochar, compost and COMBI organic amendments improved soil properties, including significant increases in soil water content, CEC, K, Ca, NO3, NH4 and soil carbon content. However, increases in soil nutrient content and improvements in physical properties did not translate to improved fruit yield. Counter to our expectations, banana crop yield (weight per bunch) was reduced by 18%, 12% and 24% by B, C and COMBI additions respectively, and no significant effect was observed on the papaya crop yield. Soil efflux of CO2 was elevated by addition of C and COMBI amendments, likely due to an increase in labile carbon for microbial processing. Our data indicate a reduction in N2O flux in treatments containing biochar. The application of B, C and COMBI amendments had a generally positive effect on soil properties, but this did not translate into a crop productivity increase in this study. The benefits to soil nutrient content, soil carbon storage and N2O emission reduction need to be carefully weighed against potentially deleterious effects on crop yield, at least in the short-term.

Carbon Abatement and Emissions Associated with the Gasification of Walnut Shells for Bioenergy and Biochar Production

By converting biomass residue to biochar, we could generate power cleanly and sequester carbon resulting in overall greenhouse gas emissions (GHG) savings when compared to typical fossil fuel usage and waste disposal. We estimated the carbon dioxide (CO₂) abatements and emissions associated to the concurrent production of bioenergy and biochar through biomass gasification in an organic walnut farm and processing facility in California, USA. We accounted for (i) avoided-CO₂ emissions from displaced grid electricity by bioenergy; (ii) CO₂ emissions from farm machinery used for soil amendment of biochar; (iii) CO₂ sequestered in the soil through stable biochar-C; and (iv) direct CO₂ and nitrous oxide (N₂O) emissions from soil. The objective of these assessments was to pinpoint where the largest C offsets can be expected in the bioenergy-biochar chain. We found that energy production from gasification resulted in 91.8% of total C offsets, followed by stable biochar-C (8.2% of total C sinks), offsetting a total of 107.7 kg CO₂-C eq Mg^-1 feedstock. At the field scale, we monitored gas fluxes from soils for 29 months (180 individual observations) following field management and precipitation events in addition to weekly measurements within three growing seasons and two tree dormancy periods. We compared four treatments: control, biochar, compost, and biochar combined with compost. Biochar alone or in combination with compost did not alter total N₂O and CO₂ emissions from soils, indicating that under the conditions of this study, biochar-prompted C offsets may not be expected from the mitigation of direct soil GHG emissions. However, this study revealed a case where a large environmental benefit was given by the waste-to-bioenergy treatment, addressing farm level challenges such as waste management, renewable energy generation, and C sequestration.

Soil respiration characteristics in different land uses and response of soil organic carbon to biochar addition in high-latitude agricultural area

The farmland tillage practices changed the soil chemical properties, which also impacted the soil respiration (R s ) process and the soil carbon conservation. Originally, the farmland in northeast China had high soil carbon content, which was decreased in the recent decades due to the tillage practices. To better understand the R s dynamics in different land use types and its relationship with soil carbon loss, soil samples at two layers (0-15 and 15-30 cm) were analyzed for organic carbon (OC), total nitrogen (TN), total phosphorus (TP), total carbon (TC), available nitrogen (AN), available phosphorus (AP), soil particle size distribution, as well as the R s rate. The R s rate of the paddy land was 0.22 (at 0-15 cm) and 3.01 (at 15-30 cm) times of the upland. The average concentrations of OC and clay content in cultivated areas were much lower than in non-cultivated areas. The partial least squares analysis suggested that the TC and TN were significantly related to the R s process in cultivated soils. The upland soil was further used to test soil CO2 emission response at different biochar addition levels during 70-days incubation. The measurement in the limited incubation period demonstrated that the addition of biochar improved the soil C content because it had high concentration of pyrogenic C, which was resistant to mineralization. The analysis showed that biochar addition can promote soil OC by mitigating carbon dioxide (CO2) emission. The biochar addition achieved the best performance for the soil carbon conservation in high-latitude agricultural area due to the originally high carbon content.

Benefits of biochar, compost and biochar-compost for soil quality, maize yield and greenhouse gas emissions in a tropical agricultural soil

Soil quality decline represents a significant constraint on the productivity and sustainability of agriculture in the tropics. In this study, the influence of biochar, compost and mixtures of the two on soil fertility, maize yield and greenhouse gas (GHG) emissions was investigated in a tropical Ferralsol. The treatments were: 1) control with business as usual fertilizer (F); 2) 10 t ha(-1) biochar (B)+F; 3) 25 t ha(-1) compost (Com)+F; 4) 2.5 t ha(-1) B+25 t ha(-1) Com mixed on site+F; and 5) 25 t ha(-1) co-composted biochar-compost (COMBI)+F. Total aboveground biomass and maize yield were significantly improved relative to the control for all organic amendments, with increases in grain yield between 10 and 29%. Some plant parameters such as leaf chlorophyll were significantly increased by the organic treatments. Significant differences were observed among treatments for the δ(15)N and δ(13)C contents of kernels. Soil physicochemical properties including soil water content (SWC), total soil organic carbon (SOC), total nitrogen (N), available phosphorus (P), nitrate-nitrogen (NO3(-)N), ammonium-nitrogen (NH4(+)-N), exchangeable cations and cation exchange capacity (CEC) were significantly increased by the organic amendments. Maize grain yield was correlated positively with total biomass, leaf chlorophyll, foliar N and P content, SOC and SWC. Emissions of CO2 and N2O were higher from the organic-amended soils than from the fertilizer-only control. However, N2O emissions generally decreased over time for all treatments and emission from the biochar was lower compared to other treatments. Our study concludes that the biochar and biochar-compost-based soil management approaches can improve SOC, soil nutrient status and SWC, and maize yield and may help mitigate greenhouse gas emissions in certain systems.

Effects of different biochars and digestate on N2O fluxes under field conditions

Field studies that have investigated the effects of char materials on the emission of nitrous oxide (N2O) are still scarce. Therefore, we conducted a field trial with bio- and hydrochars and measured N2O emissions for one whole year. It was hypothesised that the incorporation of chars reduces the emissions of N2O. Chars were produced by pyrolysis and hydrothermal carbonisation (HTC) using either maize silage or wood residues as feedstock. In addition, after production chars were post-treated with digestate in order to accelerate the ageing process of the chars. Chars and digestate were applied to the soil to raise the C content. Emissions of N2O were measured weekly and soil samples for inorganic nitrogen (N) and soil water-content were taken once a month. Additionally, the abundance of functional marker genes from denitrification (nosZ) was determined in October 2012 and in June 2013. The treatment with pure digestate emitted the most N2O compared to the control and char treatments. However, this was significant only in one case. There were no great differences between the char treatments due to high spatial variability and gene abundance of nosZ did not differ between treatments. Overall, emissions of N2O were relatively low. This was attributed to the heterogeneous distribution of the chars and the sandy soils that did not favour the production of N2O. To conclude, the emissions of N2O were mainly influenced by temperature and precipitation and to a minor extent by the type of char and post-treatment.