Biochar incorporation increased nitrogen and carbon retention in a waste-derived soil

Investigation of the effects of biochar amendment on soil under freeze‒thaw cycles and the underlying mechanism

Biochar (BC) is widely utilized as a soil amendment; however, for widely distributed seasonally frozen soils, the effect of BC on soil and the optimal utilization of BC during the freeze‒thaw process are still unclear. In this study, the effects of freeze‒thaw aged biochar (FT-BC) and BC on soil properties and wheat cultivation were systematically investigated, and the underlying interaction mechanism between BC and soil was explored. The results show that FT-BC dramatically reduces the adverse effects of freeze‒thaw cycles on soil, enhances wheat growth, and increases dry matter yield by 17.5 %, which is mainly attributed to the ability of FT-BC to maintain soil structure, reduce water loss rates to below 0.20 g/h, and decrease nitrogen leaching by more than 20 % during freeze‒thaw cycles. Additionally, fresh BC had a greater effect on the fixation of cadmium than FT-BC in the soil, reducing its accumulation in wheat by 22.5 %. Multiple characterizations revealed that the freeze‒thaw process increased the porosity and specific surface area of FT-BC, providing more sites for water and nitrogen adsorption, whereas the dissolved organic matter released from fresh BC had a better ability to trap cadmium. These findings provide insights into the interactions between BC and soil components during the freeze‒thaw process and suggest the optimized utilization of fresh BC and FT-BC for different soil repair purposes.

Biochar improves fertility in waste derived manufactured soils, but not resilience to climate change

We present a soil manufactured from waste materials, which could replace the use of peat and topsoil in plant production and reduce the pressure on natural soil resources. We tested the effect of the manufactured soil on ecosystem functions and microbial communities with and without plants present, and with and without biochar addition (Experiment 1). The resilience of the soil in response to drought and flooding, and also the effect of biochar was also tested (Experiment 2). Biochar increased soil C and N regardless of plant presence and negated the effect of the plant on soil peroxidase enzyme activity. The manufactured soil was largely resilient to drought, but not flooding, with negative impacts on microbial communities. Results indicate that biochar could improve soil properties, but not resilience to climatic perturbations. Results suggest that manufactured soils amended with biochar could offer a useful alternative to natural soil in many contexts.

Biochar improves the nutrient cycle in sandy-textured soils and increases crop yield: a systematic review

BACKGROUND

Biochar is a relatively new development in sustainable agricultural management that can be applied to ameliorate degraded and less fertile soils, especially sandy-textured ones, to improve their productivity with respect to crop production through improved nutrient availability. However, as the literature has shown, the response of sandy-textured soils to biochar varies in terms of effect size and direction. Therefore, the present study systematically reviewed the available evidence to synthesize the impact of biochar amendments on aspects of the nutrient cycle of sandy-textured soils.

METHODS

Both peer-reviewed and gray literature were searched in English in bibliographic databases, organizational web pages, and Internet search engines. Articles underwent a two-stage screening (title and abstract, and full-text) based on predefined criteria, with consistency checks. Validity assessments were conducted, utilizing specifically designed tools for study validity. Data extraction involved categorizing the various properties of the nutrient cycle into nine main Soil and Plant Properties (SPPs), each of which was studied independently. Nine meta-analyses were performed using a total of 1609 observations derived from 92 articles. Comparing meta-averages with and without correction for publication bias suggests that publication bias plays a minor role in the literature, while some indication for publication bias is found when accounting for heterogeneity by means of meta-regressions.

REVIEW FINDINGS

According to the results, soil total and available nitrogen [N], phosphorous [P] and potassium [K], plant nutrient level, and potential cation exchange capacity (CEC) increased by 36% (CI [23%, 50%]), 34% (CI [15%, 57%]), 15% (CI [1%, 31%]), and 18% (CI [3%, 36%), respectively, and N2O emission and mineral nutrient leaching decreased by 29% (CI [- 48%, - 3%]) and 38% (CI [- 56%, - 13%). On average, however, biochar had no effect on soil mineral nitrogen and nutrient use efficiency. Publication bias was identified in the response of effective CEC. After corrections for publication bias, the response shifted from 36% to a negative value of - 34% (CI [- 50%, - 14%]). Meta-regression found that the effect modifiers experimental continent, biochar application rate, and soil pH, explain result heterogeneity. Stronger responses came from the continent of South America, higher application rates, and higher pH soils. Overall, biochar is found useful for many SPPs of nutrient cycling of sandy-textured soils, thereby contributing to increased crop yields in such soils.

Nutrient runoff loss from saline-alkali paddy fields in Songnen Plain of Northeast China via different runoff pathways: effects of nitrogen fertilizer types

The application of nitrogen (N) fertilizer aggravates the nutrient runoff loss from paddy, causing serious agricultural non-point source pollution, and leading to a serious decline in water quality. The global area of saline-alkali paddy has expanded, but the response of nutrient loss via runoff to N-fertilizer applications in saline-alkali paddy is still unclear. This study conducted a 147-day field experiment to evaluate the nutrient runoff loss from saline-alkali paddy with different N-fertilizer application strategies in Songnen Plain of Northeast China. Regardless of N-fertilizer types, the nutrient loss via rainfall runoff in the entire rice-growing season was significantly (p < 0.05) higher than that via artificial drainage. The N and phosphorus (P) concentrations in runoff water were correlated with salinity and alkalinity. Especially, pH had a significant positive correlation with total-P (TP) (r = 0.658, p < 0.01). In the entire rice-growing season, the TN runoff losses in urea (U), microbial fertilizer (MF), and inorganic compound fertilizer (ICF) treatments were significantly (p < 0.05) lower compared with carbon-based slow-release fertilizer (CSF) and organic-inorganic compound fertilizer (OCF), respectively. Meanwhile, the TP runoff losses in OCF and ICF treatments were significantly (p < 0.05) lower than U and MF, respectively. Overall, the application of ICF is a better choice to avoid N and P losses via runoff from saline-alkali paddy fields.

Enhancing cation and anion exchange capacity of rice straw biochar by chemical modification for increased plant nutrient retention

Biochar, a potential alternative of infield crop residue burning, can prevent nutrient leaching from soil and augment soil fertility. However, pristine biochar contains low cation (CEC) and anion (AEC) exchange capacity. This study developed fourteen engineered biochar by treating a rice straw biochar (RBC-W) first separately with different CEC and AEC enhancing chemicals, and then with their combined treatments to increase CEC and AEC in the novel biochar composites. Following a screening experiment, promising engineered biochar, namely RBC-W treated with O₃-HCl-FeCl₃ (RBC-O-Cl), H₂SO₄-HNO₃-HCl-FeCl₃ (RBC-A-Cl), and NaOH-Fe(NO₃)₃(RBC-OH-Fe), underwent physicochemical characterization and soil leaching-cum nutrient retention studies. RBC-O-Cl, RBC-A-Cl, and RBC-OH-Fe recorded a spectacular rise in CEC and AEC over RBC-W. All the engineered biochar remarkably reduced the leaching of NH₄⁺-N, NO₃^- -N, PO₄^3--P and K⁺ from a sandy loam soil and increased retention of these nutrients. RBC-O-Cl at 4.46 g kg^-1 dosage emerged as the most effective soil amendment increasing the retention of above ions by 33.7, 27.8, 15.0, and 5.74 % over a comparable dose of RBC-W. The engineered biochar could thus enhance plants' nutrient use efficiency and reduce the use of costly chemical fertilizers that are harmful to environmental quality.

Successive addition of rice straw biochar enhances carbon accumulation in soil irrigated with saline or non-saline water

Biochar has been evaluated globally to improve soil fertility and mitigate climate change. However, the long-term effects of successive biochar application on carbon (C) accumulation in soil irrigated with saline versus non-saline water (canal water) has not been investigated. A field experiment was conducted to study the effects of rice straw biochar addition rates (0, 2, 4, 8 Mg ha^-1) on C storage in soil irrigated with water of different electrical conductivity [EC, dS m^-1]; 0.3 (non-saline canal irrigation water; CIW), 5 (saline irrigation water; SIW₅), 10 (SIW₁₀), and 15 (SIW₁₅) in a cotton-wheat system. Long-term irrigation with saline water of variable EC levels adversely affected soil functions, reducing above-ground biomass in cotton (12-48%) and wheat (5-27%). In contrast, plots irrigated with saline water but amended with rice straw biochar showed significant improvement in aboveground biomass (both in cotton and wheat), possibly due to its beneficial effects on soil properties such as soil EC, organic carbon, microbial population, water and nutrient availability, bulk density, soil aggregation, and proliferation of roots. Interestingly, the change in total organic C (TOC) stocks (8.5, 17, and 27.5 Mg C ha^-1) after 5 years were found to be almost double the amount of biochar C added (4.3, 8.6, 17.2 Mg ha^-1), indicating stabilization of belowground C inputs from the root biomass by the applied biochar and also possibly through its physical interaction with aggregates and minerals. Even though biochar application to saline water irrigated plots increased the contribution of plant-derived C to overall soil TOC stocks, it was still lower compared with canal water irrigated plots. The study conclusively established that the long-term stabilization of biogenic C through biochar has essential implications for mitigating climate change by accumulating additional C beyond the recalcitrant C contained in biochar.

An assessment of biochar as a potential amendment to enhance plant nutrient uptake

In a desperate attempt to find organic alternatives to synthetic fertilizers, agricultural scientists are increasingly using biochar as a soil amendment. Using chemical fertilizers results in enormous financial burdens and chronic health problems for plants and soils. Global concerns have also increased over the prolonged consumption of foods grown with artificial fertilizers and growth promotors. This adversely affects the environment and the welfare of humans, animals, and other living organisms. This way, organic biofertilizers have established a sustainable farming system. In such a context, biochar is gaining much attention among scientists as it may improve the overall performance of plants; in particular, crops have been optimistically cultivated with the addition of various sources. Field experiments have been conducted with multiple plant-based biochars and animal manure-based biochar. Plants receive different essential nutrients from biochar due to their physicochemical properties. Despite extensive research on biochar's effects on plant growth, yield, and development, it is still unknown how biochar promotes such benefits. Plant performance is affected by many factors in response to biochar amendment, but biochar's effect on nutrient uptake is not widely investigated. We attempted this review by examining how biochar affects nutrient uptake in various crop plants based on its amendment, nutrient composition, and physicochemical and biological properties. A greater understanding and optimization of biochar-plant nutrient interactions will be possible due to this study.

Enhancing N uptake and reducing N pollution via green, sustainable N fixation-release model

The overuse of N fertilizers has caused serious environmental problems (e.g., soil acidification, excessive N₂O in the air, and groundwater contamination) and poses a serious threat to human health. Improving N fertilizer utilization efficiency and plant uptake is an alternative for N fertilizers overuses. Enterobacter cloacae is an opportunistic pathogen, also used as plant growth-promoting rhizobacteria (PGPR), has been widely presented in the fields of bioremediation and bioprotection. Here we developed a new N fixation-release model by combining biochar with E. cloacae. The efficiency of the model was evaluated using a greenhouse pot experiment with maize (Zea mays L.) as the test crop. The results showed that biochar combined with E. cloacae significantly increased the N content. The application of biochar combined with E. cloacae increased total N in soil by 33% compared with that of N fertilizers application. The N-uptake and utilization efficiency (NUE) in plant was increased 17.03% and 14.18%, respectively. The activities of urease, dehydrogenase and fluorescein diacetate hydrolase (FDA) was improved, the catalase (CAT) activity decreased. Analysis of the microbial community diversity revealed the abundance of Proteobacteria, Actinobacteria, Firmicutes, and Gemmatimonadetes were significantly improved. The mechanism under the model is that E. cloacae acted as N-fixation by capturing N₂ from air. Biochar served as carrier, supporting better living environment for E. cloacae, also as adsorbent adsorbing N from fertilizer and from fixed N by E. cloacae, the adsorption in turn slower the N release. Altogether, the model promotes N utilization by plants, improves the soil environment, and reduces N pollution.

Effects of biochar on water quality and rice productivity under straw returning condition in a rice-wheat rotation region

Straw returning is helpful to improve soil properties and realize the reutilization of agricultural waste. However, wheat straw returning may result in paddy water quality deterioration in rice-wheat rotation regions. This study conducted pot experiments of rice planting with different biochar application rates (0, 5, 20, and 40 t/hm²) under wheat straw returning conditions. The purposes are to investigate the applicability of biochar mixed with wheat straw returning to paddy fields and explore the effects of biochar on water quality, leaching losses of nitrogen (N) and phosphorus (P), and rice yield components. Results indicated that total straw returning reduced the water quality in paddy surface water and aggravated the leaching losses of N and P. Fortunately, the biochar application improved the negative effects caused by straw returning. 40 t/hm² biochar mixed with straw returning significantly reduced the concentrations of COD and N in paddy surface water and N leaching loss than straw returning treatment (ST), decreased by 48.33%, 41.01%, and 45.73%, respectively. Meanwhile, applying biochar at a rate of 20 t/hm² with straw returning is suitable to control the diffusion of P. In addition, the ST treatment had no significant effect on rice yield, while the proper application rate of biochar under straw returning condition can improve rice yield and promote N utilization. 20 t/hm² biochar treatment is more effective to improving rice yield (16.89%) and N use efficiency (NUE) (10.14%). These findings can provide a new method to solve the negative effects of total straw returning on the water environment and rice growth and guide the utilization of straw resources in the rice-wheat rotation regions.

Combining Rice Straw Biochar With Leguminous Cover Crop as Green Manure and Mineral Fertilizer Enhances Soil Microbial Biomass and Rice Yield in South China

Whether combining rice-straw biochar (RSB) with leguminous cover crop (LCC) has synergistic effects in the rice production system or not, is still unknown. Two pot experiments were conducted to systematically explore the impacts of RSB on mass decomposition and nitrogen (N) release from LCC residues after incorporation into acidic paddy soil. Similarly, the effect of combining these two factors on soil nutrient status and microbial biomasses in the rice production system was also examined. Five treatments, namely, no N fertilizer (CK), 100% N fertilizer (150 kg N ha^-1 as N₁₀₀), 80% N fertilizer plus RSB (N₈₀B), LCC (N₈₀M), and a combination of RSB with LCC (N₈₀BM), were included. The results indicated that biomass decomposition and N release pattern followed a double exponential decay model such that the addition of RSB slightly stimulated the rates of both mass decomposition and N release during the initial rapid phase of decomposition. Thereafter, it notably slowed down the rates of both these parameters during the relatively slower stage of incorporating LCC residues to paddy soil during early rice season. Compared to 100% N, applying 80% N in conjunction with RSB and/or LCC residue increased grain yield and its components (i.e., effective panicles, 1,000-grain weight, and fully filled grains) that subsequently increased N accumulation and its physiological use efficiency (PUE _N ) of rice shoot. Moreover, under 20% N, applying RSB and/or LCC residue remarkably increased the soil organic matter and total N, and soil microbial populations and biomasses, while the contents of NH₄ ⁺ and NO₃ ^- were decreased in RSB-amended paddy soil (N₈₀B and N₈₀BM), in comparison with N₁₀₀. Thus, combining RSB with LCC residue is a novel and promising management intervention for reducing mineral fertilizer use, improving soil fertility and rice production, and consequently minimizing the overall production cost in south China.

Benefits and Limitations of Using Hydrochars from Organic Residues as Replacement for Peat on Growing Media

New technologies for the production of peat-substitutes are required to meet the rising demand for growing media in horticulture and the need to preserve natural peatlands. Hydrothermal conversion of organic residues into char materials, hydrochars, with peat-like properties may produce such substitutes, reducing environmental impacts and CO2 emissions from improper management. To assess their potential as a component in growing media, cress seed germination tests are used to assess hydrochars from digestate (D), spent coffee grounds (SCG), and grape marc (GM). Pre- and post-treatments (extraction, washing, and drying) are applied to remove phytotoxic compounds associated with process waters retained on the hydrochars, and a nitrification bioassay with process water is used to predict their toxicity. All hydrochars achieve similar or better germination results compared to their feedstock, showing a potential to replace at least 5% of peat in growing media. SCG and GM hydrochars show inhibition above 5%, while all post-treated D-hydrochar mixtures produce >3 times longer roots than the control. The nitrification test shows a high sensitivity and good agreement with the high inhibition trends found in the germination tests with process water. Such tests can be a good way to optimize process combinations for the hydrothermal production of peat replacements.

Effects of soil amendments on fractions and stability of soil organic matter in saline-alkaline paddy

Soil amelioration is an effective practice to alleviate the adverse effects of soil salinization. However, increasing the fertility of salt-affected soils has been challenging, particularly in coastal saline-alkaline paddy soils. Here, we carried out a 45-day incubation experiment to evaluate the impacts of soil amendments on fractions and stability of soil organic matter (SOM) in a saline-alkaline paddy. The experiment simulates the flooding-draining practice and consists of CaCO₃, gypsum and biochar amendments using different fertility soils. We measured dissolved organic carbon (DOC) and nitrogen (DON) in supernatant liquids, water-soluble cations, water extractable organic carbon (WEOC) and nitrogen (WEON), and microbial biomass carbon (MBC) and nitrogen (MBN) in soils after the incubation. Results showed that water soluble sodium (Na⁺) was significantly decreased under all amendments (by 17%-32%), except in high fertility soil. We found a significant decrease in DOC (by 36%-47%) under gypsum treatment, but in DON (by 18%-59%) under biochar treatment. However, there was no significant effect on DOC or DON under CaCO₃ treatment. Gypsum treatment led to decreased WEOC content (by 0.067%-5.4%), but increased MBC (by 0.16%-44%) and MBN (by 8.3%-37%) in all soils. Biochar treatment caused a decrease in the ratios of WEOC to soil organic carbon (SOC) and WEON to total nitrogen (TN), and an increase in MBC:SOC and MBN:TN ratios. These results suggest that gypsum and biochar amendments can enhance SOM stability in the saline-alkaline paddy. However, SOM stability was not enhanced under CaCO₃ treatment, probably due to the presence of a large amount of Na⁺ in these soils. Our study highlights that soil amelioration has different effects on soil carbon and nitrogen cycles in the saline-alkaline paddy soils, which is associated with water-logged condition.

Chemical and Biological Enhancement Effects of Biochar on Wheat Growth and Yield under Arid Field Conditions

Nitrogen (N) losses are prevalent under South East Asia’s due to high N fertilizer inputs, but low N fertilizer use efficiency. This leaves a large quantity of reactive N at risk of loss to the environment. Biochar has been found to reduce N losses across a variety of soil types, however, there is limited data available for semi-arid climates, particularly at a field-scale. Herein we present an exploration of the biological and chemical enhancement effects observed of a cotton stalk-based biochar on wheat growth and yield under arid field conditions. The biochar was treated with urea-N and biofertilizer (bio-power) in different treatment setups. The six experimental treatments included; (i) a full N dose “recommended for wheat crops in the region” (104 kg N ha−1) as a positive control; (ii) a half N dose (52 kg N ha−1); (iii) a half N dose + biofertilizer (4.94 kg ha−1) as a soil mixture; (iv) a half N dose + biofertilizer as a seed inoculation; (v) a full N dose as broadcast + biochar (5 t ha−1) inoculated with biofertilizer; and (vi) a full N dose loaded on biochar + biofertilizer applied as a soil mixture. The half dose N application or biofertilizer addition as soil mix/seed inoculated/biochar inoculation with biofertilizer caused reduced wheat growth and yield compared to the control (conventional N fertilization). However, co-application of chemically enhanced biochar (loaded with a full N dose) and biofertilizer as soil mixture significantly increased the crop growth rate (CGR) and leaf area index (LAI). A significantly higher crop growth and canopy development led to a higher light interception and radiation use efficiency (RUE) for total dry matter (TDM) and grain yield (11% greater than control) production compared to the control. A greater grain yield, observed for the full N dose loaded on biochar + biofertilizer applied as a soil mixture, is attributed to prolonged N availability as indicated by greater plant and soil N content at harvest and different crop growth stages, respectively. The present study has improved our understanding of how the application of nitrogen loaded biochar and biofertilizer as soil mixtures can synergize to positively affect wheat growth and soil-nitrogen retention under arid environmental conditions.

Application of mixed bacteria-loaded biochar to enhance uranium and cadmium immobilization in a co-contaminated soil

This research explored the effect of biochar pyrolyzed from five different materials on U and Cd immobilization in soil. The results showed that all biochars improved the soil properties and microbial metabolic activities, and effectively immobilized U and Cd, especially corn stalk biochar. Subsequently, three strains Bacillus subtilis, Bacillus cereus, and Citrobacter sp. were mixed in a 3:3:2 proportion as a kind of mixed bacteria (MB9) that could adsorb U and Cd effectively. Two types of MB9-loaded biochar were synthesized by physical adsorption and sodium alginate embed method and referred to as AIB and EIB, respectively. MB9-loaded biochar showed superior U and Cd immobilization performance. At 75 d, the highest reduction in the DTPA- extractable U and Cd (69 % and 56 %) was achieved with the 3% AIB amendment. Additionally, compared to the addition of biochar or MB9 alone, AIB was more effective in promoting celery growth and reducing U and Cd accumulation. Finally, the microbial community structure analysis suggested that the relative abundance of Citrobacter genus and Bacillus genus was significantly increased, suggesting that the mixed bacteria MB9 was successfully colonized. These findings may provide a feasible technology for green and cost-effective remediation of heavy metal contamination in farmland soil.

Biochar—A Panacea for Agriculture or Just Carbon?

The sustainable production of food faces formidable challenges. Foremost is the availability of arable soils, which have been ravaged by the overuse of fertilizers and detrimental soil management techniques. The maintenance of soil quality and reclamation of marginal soils are urgent priorities. The use of biochar, a carbon-rich, porous material thought to improve various soil properties, is gaining interest. Biochar (BC) is produced through the thermochemical decomposition of organic matter in a process known as pyrolysis. Importantly, the source of organic material, or ‘feedstock’, used in this process and different parameters of pyrolysis determine the chemical and physical properties of biochar. The incorporation of BC impacts soil–water relations and soil health, and it has been shown to have an overall positive impact on crop yield; however, pre-existing physical, chemical, and biological soil properties influence the outcome. The effects of long-term field application of BC and how it influences the soil microcosm also need to be understood. This literature review, including a focused meta-analysis, summarizes the key outcomes of BC studies and identifies critical research areas for future investigations. This knowledge will facilitate the predictable enhancement of crop productivity and meaningful carbon sequestration.

Biochar and nitrogen application rates effect on phosphorus removal from a mixed grass sward irrigated with reclaimed wastewater

This 2-year (2017 and 2018) field study evaluated biochar and nitrogen application rates effect on herbage phosphorus (P) and nitrogen (N) removal from a mixed-grass sward of tall fescue [Schedonorus arundinaceus (Schreb.) Dumort] and Kentucky bluegrass (Poa pratensis L.) irrigated with treated wastewater. Treatments used in this study carried out at the Main Station Field Laboratory, Reno, NV were three biochar application rates (0, 8.9, and 17.8 Mg/ha), and three N rates (0, 80, and 120 kg N/ha) arranged in a 3 × 3 factorial in a randomized complete block design experiment with four replications of each treatment combination. Responses were considered different P < 0.05. There was a linear increased in soil volumetric water content as biochar rate increased from 0 to 17.9 Mg/ha. However, biochar application rate did not affect the quantity of biomass produced, forage tissue P and N concentrations, P and N removal or interact with the other experimental variables of N rate and year to influence the response variables. There was, however, an N rate effect (P < 0.05) on biomass production and it was greater for the 80 and 120 kg N rate (average = 8.3 Mg DM/ha) relative to the 0 kg N/ha rate (6.0 Mg DM/ha). Further, cumulative P removal for the 80 and 120 kg N rate (average = 48.9 kg/ha) was greater than the 0 kg N/ha rate (38.1 kg/ha), and cumulative N removal was in the order 120 kg N/ha (321.1 kg/ha) > 80 kg N/ha (267.4 kg/ha) > 0 kg N/ha (187.8 kg/ha). There was a trend for a biochar × N rate interaction on soil P concentration and it tended to be greater for the combinations 8.9 and 17.8 Mg/ha biochar rates and 80 and 120 kg N/ha rates compared to the unamended control. Even though our study did not reveal a definitive effect of biochar on the major response parameters (biomass, tissue P and N concentrations) evaluated, the trend for a biochar × N rate interaction on soil P concentration offers hope that biochar-amended soils coupled with appropriate N fertilization will be effective in P retention on agricultural landscapes irrigated with treated wastewater.

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.

Biochar drives microbially-mediated rice production by increasing soil carbon

The effects of an on-site biomass (rice straw) equivalent biochar-returning strategy (RSC) on rice yield, soil nutrients and bacterial community composition were examined in a four-year field trial (2013-2016) conducted in a paddy field in south China. Three treatments were set up including annual on-site biomass return (RS, rice straw at 8 t ha^-1 yr^-1), annual on-site biomass equivalent biochar-return (RSC, rice straw biochar at 2.8 t ha^-1 yr^-1 with a 35 % carbonization rate) and control (CK, no rice straw or biochar added). Results showed that a low rate of biochar application (RSC) could significantly increase rice yield in four successive years. The increase in rice yield was mainly attributed to the increase in soil potassium and magnesium contents resulting from the presence of the unique surface functional groups of biochar. As a result of biochar amendment, soil bacterial cooperative relationships were improved in the RSC, compared to those in the RS treatment. Our study indicated that RSC might be promoted as a promising strategy to enhance rice productivity and soil fertility in a sustainable way.