Reducing ammonia volatilization from paddy field with rice straw derived biochar

Biochar influences nitrogen and phosphorus dynamics in two texturally different soils

Nitrogen (N) and phosphorus (P) are vital for crop growth. However, most agricultural systems have limited inherent ability to supply N and P to crops. Biochars (BCs) are strongly advocated in agrosystems and are known to improve the availability of N and P in crops through different chemical transformations. Herein, a soil-biochar incubation experiment was carried out to investigate the transformations of N and P in two different textured soils, namely clay loam and loamy sand, on mixing with rice straw biochar (RSB) and acacia wood biochar (ACB) at each level (0, 0.5, and 1.0% w/w). Ammonium N (NH4-N) decreased continuously with the increasing incubation period. The ammonium N content disappeared rapidly in both the soils incubated with biochars compared to the unamended soil. RSB increased the nitrate N (NO3-N) content significantly compared to ACB for the entire study period in both texturally divergent soils. The nitrate N content increased with the enhanced biochar addition rate in clay loam soil until 15 days after incubation; however, it was reduced for the biochar addition rate of 1% compared to 0.5% at 30 and 60 days after incubation in loamy sand soil. With ACB, the net increase in nitrate N content with the biochar addition rate of 1% remained higher than the 0.5% rate for 60 days in clay loam and 30 days in loamy sand soil. The phosphorus content remained consistently higher in both the soils amended with two types of biochars till the completion of the experiment.

The increasing risk of ammonia volatilization in farmland from the recovery product of magnesium-modified biochar after nitrogen and phosphorus adsorption

Many studies have shown that magnesium modified biochar (MgBC) can recover nutrients from wastewater and be applied as an excellent slow-release fertilizer in farmland. However, the recovery products (NP-loaden MgBC), represented by struvite or magnesium phosphate, have a high degree of self-alkalinity, which may significantly increase the ammonia (NH3) volatilization in farmland. In this study, the optimal adsorption parameters, self-alkaline regulation process and co-adsorption mechanism of MgBC for ammonium ion (NH4+) and phosphate ion (PO43-) were studied through batch experiments. A field experiment was conducted with three treatments, including local conventional fertilization (N1B0) and the application of 5 t·ha-1 or 10 t·ha-1 NP-loaden MgBC in combination with local conventional fertilization (N1B1 and N1B2, respectively), to determine the impact of NP-loaden MgBC on NH3 volatilization, surface water c(NH4+-N) and pH. The results indicated that the maximum NH4+ and PO43- synergistic recovery of MgBC under the optimal adsorption parameters (dosage of 0.6 g·L-1; initial NH4+ and PO43- concentrations of 120 and 60 mg·L-1 and pH of 8) were 59.96 and 98.60 mg·g-1, respectively. Self-regulating alkaline MgBC maintained pH suitable for struvite, and precipitation mechanism controlled the adsorption. The presence of NP-loaden MgBC raised the pH levels in surface water during the basal fertilization stage and increased c(NH4+-N) in surface water during the topdressing stages. This, in turn, led to a significant increase in NH3 volatilization loss during the entire rice-growing period, with N1B1 and N1B2 experiencing a 23.87 % and 48.91 % increase respectively, compared to N1B0. Therefore, it is imperative to take into account the adverse impact of NP-laden MgBC on NH3 loss in paddy fields when considering its application in future field studies.

Hydrothermal carbonization of biogas slurry and cattle manure into soil conditioner mitigates ammonia volatilization from paddy soil

Hydrothermal carbonization of biogas slurry and animal manure into hydrochar could enhance waste recycling waste and minimize ammonia (NH3) volatilization from paddy fields. In this study, cattle manure-derived hydrochar prepared in the presence of Milli-Q water (CMWH) and biogas slurry (CMBSH), and biogas slurry-based hydrochar embedded with zeolite (ZHC) were applied to rice-paddy soil. The results demonstrated that CMBSH and ZHC treatments could significantly mitigate the cumulative NH3 volatilization and yield-scale NH3 volatilization by 27.9-45.2% and 28.5-45.4%, respectively, compared to the control group (without hydrochar addition), and significantly correlated with pH and ammonium-nitrogen (NH4+-N) concentration in floodwater. Nitrogen (N) loss via NH3 volatilization in the control group accounted for 24.9% of the applied N fertilizer, whereas CMBSH- and ZHC-amended treatments accounted for 13.6-17.9% of N in applied fertilizer. The reduced N loss improved soil N retention and availability for rice; consequently, grain N content significantly increased by 6.5-14.9% and N-use efficiency increased by 6.4-16.0% (P < 0.05), respectively. Based on linear fitting results, NH3 volatilization mitigation resulted from lower pH and NH4+-N concentration in floodwater that resulted from the acidic property and specific surface area of hydrochar treatments. Moreover, NH3-oxidizing archaea abundance in hydrochar-treated soil decreased by 40.9-46.9% in response to CMBSH and ZHC treatments, potentially suppressing NH4+-N transformation into nitrate and improving soil NH4+-N retention capacity. To date, this study applied biogas slurry-based hydrochar into paddy soil for the first time and demonstrated that ZHC significantly mitigated NH3 and increased N content. Overall, this study proposes an environmental-friendly strategy to recycle the wastes, biogas slurry, to the paddy fields to mitigate NH3 volatilization and increase grain yield of rice.

Mitigating ammonia volatilization in rice cultivation: The impact of partial organic fertilizer substitution

Optimizing water and nitrogen management to minimize NH3 volatilization from paddy fields has been extensively studied. However, there is limited research on the combined effect of different rates of organic fertilizer substitution (OFS) and irrigation methods in rice cultivation, exploring an effective water and nitrogen combination is beneficial to mitigate NH3 volatilization. To address this gap, we conducted a two-year field experiment to investigate NH3 volatilization under different OFS rates (0%, 25%, and 50%) combined with continuous flooding irrigation (CF) and alternate wet and dry irrigation (AWD). Our findings revealed that NH3 fluxes exhibited similar emission patterns after each fertilization event and significantly decreased with increasing rates of OFS during the basal stage. Compared to no substitution (ON0), the low (ON25) and high (ON50) rates of OFS reduced cumulative NH3 emissions by 18.9% and 16.6%, and lowed NH3 emission factors (EFs) by 26.7% and 23.3%, respectively. Although OFS resulted in a slight reduction in rice yield, yield-scaled NH3 emissions were significantly reduced by 11.9% and 6.5% under the low and high substitution rates, respectively. This reduction was mainly attributed to the slight yield reduction observed at the low substitution rate. Furthermore, when combined with ON0, AWD irrigation had the potential to increase NH3 volatilization. However, this increase was not observed when combined with ON25 and ON50. During each fertilization stage, floodwater + concentration emerged as the prominent environmental factor influencing NH3 volatilization, showing a stronger and more positive correlation compared to other factors such as floodwater pH, soil pH, and NH4+ concentration. Based on our findings, we recommend implementing effective water and nitrogen management strategies to minimize NH3 volatilization in rice cultivation. This involves applying a lower rate of organic fertilizer substitution during the basal stage, maintaining high water levels during fertilization, and implementing mild AWD irrigation during non-fertilization periods.

Nitrification and urease inhibitors mitigate global warming potential and ammonia volatilization from urea in rice-wheat system in India: A field to lab experiment

The efficacy of alternative nitrogenous fertilizers for mitigating greenhouse gas and ammonia emissions from a rice-wheat cropping system in northern India was addressed in a laboratory incubation experiment using soil from a 10-year residue management field experiment (crop residue removal, CRR, vs. incorporation, CRI). Neem coated urea (NCU), standard urea (U), urea ammonium sulfate (UAS), and two alternative fertilizers, urea + urease inhibitor NBPT (UUI) and urea + urease inhibitor NBPT + nitrification inhibitor DMPSA (UUINI) were compared to non-fertilized controls for four weeks in incubation under anaerobic condition. Effects of fertilizers on global warming potential (GWP) and ammonia volatilization were dependent on residue treatment. Relative to standard urea, NCU reduced GWP by 11 % in CRI but not significantly in CRR; conversely, UAS reduced GWP by 12 % in CRR but not significantly in CRI. UUI and UUINI reduced GWP in both residue treatments and were more effective in CRI (21 % and 26 %) than CRR (15 % and 14 %). Relative to standard urea, NCU increased ammonia volatilization by 8 % in CRI but not significantly in CRR. Ammonia volatilization was reduced most strongly by UUI (40 % in CRI and 37 % in CRR); it was reduced 28-29 % by UUINI and 12-15 % by UAS. Overall, the urease inhibitor, alone and in combination with the nitrification inhibitor, was more effective in mitigating greenhouse gas and ammonia emissions than NCU. However, these products need to be tested in field settings to validate findings from the controlled laboratory experiment.

Reviewing the role of biochar in paddy soils: An agricultural and environmental perspective

The main challenge of the twenty-first century is to find a balance between environmental sustainability and crop productivity in a world with a rapidly growing population. Soil health is the backbone of a resilient environment and stable food production systems. In recent years, the use of biochar to bind nutrients, sorption of pollutants, and increase crop productivity has gained popularity. This article reviews key recent studies on the environmental impacts of biochar and the benefits of its unique physicochemical features in paddy soils. This review provides critical information on the role of biochar properties on environmental pollutants, carbon and nitrogen cycling, plant growth regulation, and microbial activities. Biochar improves the soil properties of paddy soils through increasing microbial activities and nutrient availability, accelerating carbon and nitrogen cycle, and reducing the availability of heavy metals and micropollutants. For example, a study showed that the application of a maximum of 40 t ha-1 of biochar from rice husks prior to cultivation (at high temperature and slow pyrolysis) increases nutrient utilization and rice grain yield by 40%. Biochar can be used to minimize the use of chemical fertilizers to ensure sustainable food production.

Effects of the Combining Straw Return with Urease Inhibitor on Ammonia Volatilization, Nitrogen Use Efficiency, and Rice Yield in Purple Soil Areas

Straw return in rice (Oryza sativa L.) paddy has been heavily criticized for its potential to influence ammonia (NH₃) volatilization loss due to irrational fertilizer N application. Therefore, improving the N fertilization strategies within residue straw systems is necessary to reduce N loss from NH₃ volatilization. This study investigated how the incorporation of oilseed rape straw and the urease inhibitor affected NH₃ volatilization, fertilizer N use efficiency (FNUE), and rice yields over two growing seasons (2018-2019) in the purple soil region. This study arranged eight treatments combined straw (2, 5, 8 ton ha^-1, named 2S, 5S, 8S, respectively), with urea or urease inhibitor (UI, 1% NBPT) with three replicates, which included control (CK), UR (Urea, 150 kg N ha^-1), UR + 2S, UR + 5S, UR + 8S, UR + 2S + UI, UR + 5S + UI, UR + 8S + UI, based on the randomized complete block method. Our results indicated that incorporating oilseed rape straw increased NH₃ losses by 3.2-30.4% in 2018 and 4.3-17.6% in 2019 than the UR treatment, attributing to the higher NH₄⁺-N content and pH value within floodwater. However, the UR + 2S + UI, UR + 5S + UI and UR + 8S + UI treatments reduced NH₃ losses by 3.8%, 30.3%, and 8.1% in 2018 and 19.9%, 39.5%, and 35.8% in 2019, separately compared to their corresponding UR plus straw treatments. According to the findings, adding 1% NBPT significantly decreased NH₃ losses while incorporating 5 ton ha^-1 oilseed rape straw. Furthermore, adding straw, either alone or in conjunction with 1% NBPT, increased rice yield and FNUE by 0.6-18.8% and 0.6-18.8%, respectively. Otherwise, NH₃ losses scaled by yield in the UR + 5S + UI treatment decreased significantly between all treatments in 2018 and 2019. These results suggest that optimizing the oilseed rape straw rate combined with 1% NBPT applied with urea efficiently increased rice yield and reduced NH₃ emissions in the purple soil region of Sichuan Province, China.

Effects of biochar in combination with varied N inputs on grain yield, N uptake, NH3 volatilization, and N2O emission in paddy soil

Biochar application can improve crop yield, reduce ammonia (NH₃) volatilization and nitrous oxide (N₂O) emission from farmland. We here conducted a pot experiment to compare the effects of biochar application on rice yield, nitrogen (N) uptake, NH₃ and N₂O losses in paddy soil with low, medium, and high N inputs at 160 kg/ha, 200 kg/ha and 240 kg/ha, respectively. The results showed that: (1) Biochar significantly increased the rice grain yield at medium (200 kg/ha) and high (240 kg/ha) N inputs by 56.4 and 70.5%, respectively. The way to increase yield was to increase the rice N uptake, rice panicle number per pot and 1,000 grain weight by 78.5-96.5%, 6-16% and 4.4-6.1%, respectively; (2) Under low (160 kg/ha) N input, adding biochar effectively reduced the NH₃ volatilization by 31.6% in rice season. The decreases of pH value and NH₄⁺-N content in surface water, and the increases of the abundance of NH₄⁺-N oxidizing archaea and bacteria (AOA and AOB) communities contributed to the reduction of NH₃ volatilization following the biochar application; (3) Under same N input levels, the total N₂O emission in rice season decreased by 43.3-73.9% after biochar addition. The decreases of nirK and nirS gene abundances but the increases of nosZ gene abundance are the main mechanisms for biochar application to reduce N₂O emissions. Based on the results of the current study, adding biochar at medium (200 kg/ha) N level (N200 + BC) is the best treatment to synchronically reduce NH₃ and N₂O losses, improve grain yield, and reduce fertilizer application in rice production system.

Biochar application can mitigate NH3 volatilization in acidic forest and upland soils but stimulates gaseous N losses in flooded acidic paddy soil

Biochar (BC) has attracted attention for carbon sequestration, a strategy to mitigate climate change and alleviate soil acidification. Most meta-analyses have insufficiently elaborated the effects of BC on soil N transformation so the practical importance of BC could not be assessed. In this study, a ¹⁵N tracing study was conducted to investigate the effects of BC amendment on soil gross N transformations in acidic soils with different land-use types. The results show that the BC amendment accelerated the soil gross mineralization rate of labile organic N to NH₄⁺ (M_Nlab) (3 %-128 %) which was associated with an increase in total nitrogen. BC mitigated NH₃ volatilization (V_NH3) (52 %-99 %) in upland and forest soils due to NH₄⁺/NH₃ adsorption, while it caused higher gaseous N losses (NH₃ and N₂O) in flooded paddy soils. An important function was the effect of BC addition on NH₄⁺ oxidation (O_NH4). While O_NH4 increased (4 %-19 %) in upland soils, it was inhibited (34 %-71 %) in paddy soils and did not show a response in forest soils. Overall, the BC amendment reduced the potential risk of N loss (P_RL), especially in forest soils (82 %-98 %). This study also shows that the BC effect on soil N cycling is land-use specific. The suitability of practices including BC hinges on the effects on gaseous N losses.

Environmental risk of microplastics after field aging: Reduced rice yield without mitigating yield-scale ammonia volatilization from paddy soils

Microplastics (MPs, <5 mm) are enriched in paddy ecosystems as emerging environmental pollutants. Biochar (BC) is a controversial recalcitrant carbon product that poses potential environmental risks. The presence of these two exogenous organic substances has been demonstrated to have impacts on soil nitrogen cycling and crop production. However, the after-effects of MPs and BC on soil ammonia (NH₃) volatilization and rice yield after field aging remain unexplored. In this study, two common MPs, including polyethylene (PE) and polyacrylonitrile (PAN), and BC were selected for rice growing season observations to study the impacts on soil NH₃ volatilization and rice yield after field aging. The results showed that the reduction of cumulative soil NH₃ losses by MPs was around 45% after one-year field aging, which was within the range of 40-57% in the previous rice season. Abatement of NH₃ volatilization by MPs mainly occurred in basal fertilization and was related to floodwater pH. Besides, the reduction rate of NH₃ volatilization by BC and MPs + BC was enhanced after field aging (63% and 50-57%) compared to that in the previous rice season (5% and 11-19%), with the abatement process occurring in the first supplementary fertilization. There was a significant positive correlation between cumulative NH₃ volatilization and soil urease activity. Notably, field aging removed the positive effect of MPs and MPs + BC in reducing yield-scale NH₃ losses in the previous rice season (∼62%). Furthermore, despite BC affecting rice yield insignificantly after field aging, the presence of MPs led to a significant 17-19% reduction in rice yield. Our findings reveal that differences in the after-effects of BC and MPs in field aging emerge, where the negative impacts of MPs on soil NH₃ abatement and crop yield are progressively becoming apparent and should be taken into serious consideration.

Ammonia volatilization mitigation in crop farming: A review of fertilizer amendment technologies and mechanisms

Good practices in controlling ammonia produced from the predominant agricultural contributor, crop farming, are the most direct yet effective approaches for mitigating ammonia emissions and further relieving air pollution. Of all the practices that have been investigated in recent decades, fertilizer amendment technologies are garnering increased attention as the low nitrogen use efficiency in most applied quick-acting fertilizers is the main cause of high ammonia emissions. This paper systematically reviews the fertilizer amendment technologies and associated mechanisms that have been developed for ammonia control, especially the technology development of inorganic additives-based complex fertilizers, coating-based enhanced efficiency fertilizers, organic waste-based resource fertilizers and microbial agent and algae-based biofertilizers, and their corresponding mechanisms in farmland properties shifting towards inhibiting ammonia volatilization and enhancing nitrogen use efficiency. The systematic analysis of the literature shows that both enhanced efficiency fertilizers technique and biofertilizers technique present outstanding ammonia inhibition performance with an average mitigation efficiency of 54% and 50.1%, respectively, which is mainly attributed to the slowing down in release and hydrolysis of nitrogen fertilizer, the enhancement in the adsorption and retention of NH₄⁺/NH₃ in soil, and the promotion in the microbial consumption of NH₄⁺ in soil. Furthermore, a combined physical and chemical means, namely membrane/film-based mulching technology, for ammonia volatilization inhibition is also evaluated, which is capable of increasing the resistance of ammonia volatilization. Finally, the review addresses the challenges of mitigating agricultural ammonia emissions with the aim of providing an outlook for future research.

The Mechanism of Cu2+ Sorption by Rice Straw Biochar and Its Sorption-Desorption Capacity to Cu2+ in Soil

Copper (Cu) pollution in soils has received considerable research attention globally, and biochar has been widely used as an adsorbent for soil pollution of Cu. However, most of the studies focused on the adsorption capacity of biochar, the bioavailability of Cu absorbed by biochar remains unclear. In this work, rice straw biomass was pyrolyzed under oxygen-limited conditions at 400°C (BC400) and 600°C (BC600), their apparent structure, group characteristics, and basic physical and chemical properties were determined. The isothermal and kinetics adsorption of Cu by BC400 and BC600 were analyzed. A pot experiment was used to evaluate the passivation of Cu in the soil by biochar and the bioavailability of Cu adsorbed by biochar in the soil. The smooth surfaces of BC400 evolved into more rough surfaces for BC600, and both types of surfaces may give active sorption sites for Cu, according to SEM pictures. FTIR analysis suggested that BC600 is endowed with more condensed aromatic carbon structures and more available polar functional groups. The adsorption processes of Cu²⁺ by biochar were better fitted Langmuir equation and pseudo-second-order kinetic model. The adsorption isotherms showed monolayer adsorption of Cu²⁺ on biochar. The maximum adsorption capacities of BC600 and BC400 on Cu²⁺ were 43.75 and 30.70 mg g^-1, respectively. Moreover, the pot experiment showed that BC400 and BC600 not only have a strong "passivation" effect on Cu in soil but also prevent the release of adsorbed Cu. Overall, more aromatic carbon structure, more polar functional groups, and higher pH are associated with BC600's increased Cu immobilization ability in soil.

Volatilisations of ammonia from the soils amended with modified and nitrogen-enriched biochars

Biochar's capacity to abate NH₃ emissions from fertilised agricultural soils may be enhanced through both modifications and formulation of slow-release biochar-based N fertilisers but there is a dearth of data in this area. Sulphuric acid (H₂SO₄), hydrogen peroxide (H₂O₂) and potassium hydroxide (KOH) were used to modify biochars which are denoted as BSAD, BHPO and BKOH, respectively. Nitrogen (N) enrichment was performed using urea and ammonium nitrate and the enriched biochars are denoted as BUR and BAN, respectively. The biochars were characterised by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The ammonia abatement potentials of both the modified and N-enriched biochars were assessed in the incubation experiments which lasted for 30 days. Urea was used as a control while non-modified biochar (PrBC) was included for comparison. Compared to the control, PrBC, BKOH, BHPO, BSAD, BUR and BAN attenuated gaseous NH₃ emissions by 57.62%, 63.06%, 73.23% and 74.85%, 79.93% and 82.88%, respectively. Biochar modifications increased the content of oxygen containing surface groups especially carboxyl and sulphoxide in the case of BSAD as depicted from the instrumental analysis data, which most probably increased the sorption of NH₃ and its transformation to nitrates thus, resulting in a higher NH₃ abatement capacity than that of PrBC. XPS data indicated that N-enrichment resulted in reactions of N with the surface groups of biochar which slowed its release, concomitantly lowering NH₃ volatilisation better than even the modified biochars.

Banana, pineapple, cassava and sugarcane residue biochars cannot mitigate ammonia volatilization from latosols in tropical farmland

Ammonia (NH₃) volatilization is a major pathway of soil nitrogen loss in tropical farmland, causing many environmental issues. Biochar can improve soil quality and affect soil NH₃ volatilization. However, little is known about the effects of tropical crop residue biochar on soil NH₃ volatilization in tropical farmland. Therefore, a laboratory incubation study was conducted using four kinds of tropical crop residue biochar (pineapple straw (stem and leaves), banana straw, cassava straw and sugarcane bagasse pyrolyzed at 500 °C) with five addition rates (0.5%, 1%, 2%, 4%, and 6%) to evaluate their impact on NH₃ volatilization from tropical latosols. The results showed that NH₃ volatilization peaked twice under biochar application, once at 1-5 days and again at 12-16 days. The cumulative NH₃ volatilization (0.14-0.47 mg kg^-1) of the 20 biochar treatments was higher than that of the control (0.12 mg kg^-1). With the increase in the biochar addition rate, the soil pH, soil organic matter (SOM), urease activity, nitrate nitrogen content (NO₃^--N), nitrification rate and cumulative NH₃ volatilization increased gradually, and the 6% biochar treatment resulted in the highest NH₃ volatilization loss (0.19-0.47 mg kg^-1). The type of biochar is also a main factor affecting soil NH₃ volatilization. The cumulative NH₃ volatilization was the highest under pineapple straw biochar, as it was 19-43% higher than when the other three biochars were applied. However, sugarcane bagasse biochar had the lowest cumulative NH₃ volatilization due to its low quartz, sylvite and calcite contents, lack of -OH hydroxyl groups and high adsorbability. NH₃ volatilization was positively correlated with the soil pH, SOM, urease activity, NO₃^--N and nitrification rate. In conclusion, four tropical crop residue biochars can increase NH₃ volatilization in tropical latosols, so reducing NH₃ volatilization needs to be further considered in tropical crop residue biochar applications.

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.

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.

Effect of rapeseed straw-derived biochar on soil bacterial community structure at tillering stage of Oryza Sativa

Numerous studies have reported the dynamics of microbes when biochar was applied, whereas the information on the alterations of bacterial community after application of rapeseed straw-derived biochar is limited. A pot experiment with two rapeseed straw-derived biochar application treatments (with biochar application at the rate of 200 g/pot, C1) and (without biochar application, 0 g/pot, C0) was conducted. No significant differences were observed in the number of operational taxonomic units, observed species, Shannon index, Simpson index, Chao1, ACE, and phylogenetic diversity whole tree between the C1 and C0 treatments. Taxonomic analysis at the genus level showed that the abundances of Gracilibacter, Lentimicrobium, unidentified Rikenellaceae, Hydrogenophaga, and Bacillus were higher in the C1 compared to the C0 treatment, while Candidatus Solibacter, Candidatus Koribacter, and Lutispora abundances were found to be higher abundant in the C0 compared to the C1 treatment. Obvious clusters were observed between the C1 and C0 in both principal component analysis and non-metric multidimensional scaling. These results indicate that soil bacterial community was altered after rapeseed straw-derived biochar was applied.

Various quantification methods for estimating ammonia volatilization from wheat-maize cropping system

Ammonia volatilization (AV) dominates the pathway of nitrogen (N) fertilizer losses in crops throughout the world. However, different methods are highly responsible for the different measurements of AV. The existing techniques were separated into static chamber methods (SCM), dynamic chamber methods (DCM), calibrated Dräger-tube method (DTM) and micrometeorological methods (MMM), which were analyzed by a meta-study of 595 observations from 33 published studies. An exponential relationship (P < 0.01) was found between AV and the N fertilizer applied to wheat and maize using all the methods. The amount of AV using SCM was the lowest. The AV monitored by DCM was 24.5%-55.0% (wheat) and 46.9%-65.0% (maize) lower than that for the DTM. Additionally, the AV measured by DTM did not differ significantly in the wheat season but was 58.9% lower (P < 0.05) in the maize season than that in the MMM. To reveal the influencing factors responsible that were for DCM and DTM, a field experiment was conducted during the period of Oct. 2016 to Oct. 2017. The study indicated that the AV was 15.8%-28.3% (wheat, P < 0.05) and 36.7%-44.2% (maize, P < 0.05) lower when monitored by the DCM than when estimated by DTM. The concentration of soil NH4+-N, air temperature, and wind speed positively correlated with the NH3 fluxes. In addition, there was a significant linear correlation (P < 0.01) between the AV measured by DCM and DTM when the wind speed was <1.5 m s-1. This study highlighted the fact that wind speed was the main factor that caused the large difference between DCM and DTM. Herein, DTM or MMM was first recommended, and DCM was accepted when wind speed was <1.5 m s-1 for quantitative estimates of AV. However, only a straight comparison between DCM and DTM under the same field experiment was done, the other comparisons only being based on similar fertilization and environmental conditions. Consequently, the differences between methods have to be treated carefully.

Applying Cassava Stems Biochar Produced from Agronomical Waste to Enhance the Yield and Productivity of Maize in Unfertile Soil

Many agronomical wastes are produced annually in significant amounts after cultivation, especially in agricultural countries. This study applied biochar produced from the pyrolysis of cassava stems to improve soil with low fertility for maize cultivation. The effect of soil biochar incorporation on maize yield and productivity was also investigated. Eight experimental plots, each with four replicates, were applied with cassava stem biochar (CSB) at different rates of 0.5 kg/m2 (TB0.5), 2.5 kg/m2 (TB2.5) and 3.0 kg/m2 (TB3.0), fertilizer at 0.56 kg/m2 (TM), fertilizer at 0.56 kg/m2 mixed with CSB at 0.5 kg/m2 (TMB0.5), 2.5 kg/m2 (TMB2.5), 3.0 kg/m2 (TMB3.0) and untreated soil (TC). Pyrolysis of cassava stems at 450–500 °C produced strongly alkaline CSB with pH 9.6 and increased nutrient contents. Specific surface area and total pore volume increased, and pores were classified as mesoporous, while average pore diameter decreased. CSB had a highly stable carbon content of 58.46%, with high aromaticity and polarity obtained from O/C and H/C ratios. Results indicated that CSB enhanced and supported maize growth by improving soil physicochemical properties to suit cultivation. Applying CSB into the soil gave higher maize yield and productivity than cultivation using fertilizer. The highest yield and nutrition contents were obtained in seed from cultivation using fertilizer mixed with biochar at 3.0 kg/m2. Biochar production from cassava stems generated a useful commodity from waste material.

Efficiency and mechanism of reducing ammonia volatilization in alkaline farmland soil using Bacillus amyloliquefaciens biofertilizer

Ammonia volatilization from the farmland caused by the application of synthetic nitrogen fertilizer is the most important source of anthropogenic ammonia emissions. Biofertilizer application has been considered as an alternative option for agriculture sustainability and soil improvement. In this study, field trials were carried out to investigate the efficiency of Bacillus amyloliquefaciens (BA) biofertilizer on alleviating ammonia volatilization in alkaline farmland soil and increasing crop yield and nitrogen utilization. Potential response mechanisms were investigated from soil enzyme, nitrogen cycle function genes and microbial community levels. Compared with conventional fertilization, BA biofertilizer application reduced the ammonia volatilization by 68%, increased the crop yield and nitrogen recovery by 19% and 19%, respectively. Soil enzyme activity analysis showed that BA biofertilizer inhibited the urease activity and enhanced the potential ammonia oxidation (PAO). In addition, BA biofertilizer application also increased the bacterial amoA gene abundance, while decreased the ureC gene abundance. BA biofertilizer also significantly altered the community structure and composition, and especially raised the abundance of ammonia oxidation bacteria (AOB), while no changes were observed in abundance of nitrite oxidation bacteria (NOB). Briefly, BA biofertilizer was approved to reduce the transformation of fertilizer nitrogen to NH₄⁺-N, simultaneously accelerating NH₄⁺-N into the nitrification process, thus decreasing the NH₄⁺-N content remained in alkaline soil and consequently alleviating the ammonia volatilization. Thus, these results suggested that the application of BA biofertilizer is a feasible strategy to improve crop yields and reduce agricultural ammonia emissions.

Blending urea and slow-release nitrogen fertilizer increases dryland maize yield and nitrogen use efficiency while mitigating ammonia volatilization

Agricultural non-point source pollution has become the main pollution source in China. Ammonia (NH₃) volatilization is one of the main factors of agricultural non-point source pollution. Slow-release nitrogen fertilizer (S) has been widely recognized as an efficient management measure to increase crop yields and mitigate NH₃ volatilization. However, few studies have reported the effects of urea (U) blended with slow-release nitrogen fertilizer (UNS) on maize yield and NH₃ volatilization under dryland farming conditions. A two-season field experiment with U, S and various blending ratios of U and S (UNS) under two N application rates (N1: 180 kg N ha^-1, N2: 240 kg N ha^-1) was conducted to determine their effects on maize yield, NH₃ volatilization and residual soil NO₃^--N. The results showed that UNS substantially reduced NH₃ volatilization compared with U, primarily because of the relatively low soil pH and electrical conductivity, and the relatively high soil organic matter. UNS significantly increased dry matter, grain yield, N uptake and N use efficiency (NUE), but reduced residual soil NO₃^--N compared with U and S. Among UNS treatments, the blending ratio of U and S at 3:7 (UNS2) was most effective in improving maize yield and NUE, while mitigating NH₃ volatilization and soil NO₃^--N leaching. N1 not only reduced N losses, but also increased NUE compared with N2. In conclusion, UNS2N1 is recommended as the best N fertilizer application strategy for the sustainable production of dryland maize in northwest China.

Biochar modulates mineral nitrogen dynamics in soil and terrestrial ecosystems: A critical review

Biochar, over the last two decades, has become the focal point of agro-environmental research given its unique functionality, cost-effectiveness and recyclability potentials. It has been studied intensively as an efficient scavenger for the decontamination of several organic and inorganic pollutants. However, the ability of biochar to modulate nitrogen (N) dynamics in soil and terrestrial ecosystems remains controversial. This work deliberates on the premise that biochar functionality enables maximizing N use efficiency by reducing the potential losses induced by volatilization/emission and runoff/leaching as well as stimulating available N inputs derived from symbiotic and nonsymbiotic biological nitrogen fixation (BNF) and N mineralization/retention. For this purpose, we carried out a critical review on different intriguing dimensions surrounding the potentiality of biochar to modulate the complicated reactions of soil N cycle with emphasis on its pros and cons. Previous studies in the literature have shown contradictory results with a noticeable significant effect of biochar toward stimulating available N inputs and reducing its losses under short-term laboratory experimentations. However, long-term field investigations have indicated minimal or negative effects in this regard. Furthermore, some of the experimentations lack appropriate controls or fail to account for inputs or losses associated with biochar particles. It is thus of great importance to contextualise lab-scale experimentations based on real field data to provide a holistic approach for understanding the complicated reactions responsible for modulating N cycle in the charosphere. Additionally, biochar functionalization should be highlighted in the foreseeable research to develop fit-for-purpose forms tailored in agro-environmental applications.

Biowaste hydrothermal carbonization aqueous product application in rice paddy: Focus on rice growth and ammonia volatilization

Hydrothermal carbonization (HTC) is known as a green biomass conversion technology. However, it often suffers from the issue of disposing hydrothermal carbonization aqueous products (HCAP). Based on the characterization and composition of acidic HCAP, a rice paddy soil column experiment was conducted to observe the effects of HCAP on ammonia (NH₃) volatilization form paddy soil and rice yield. The experiment was designed with five treatments. HCAPs were produced at 220 °C and (SHC220-L) and 260 °C (SHC260-L) derived from poplar sawdust, HCAP produced at 220 °C (WHC220-L) and 260 °C (WHC260-L) derived from wheat straw, and a control group without HCAP application (termed CKU hereafter). The results showed that HCAP treatments increased the rice yield by 4.30%-26.0% compared to CKU. HACPs prepared at lower temperatures (SHC220-L and WHC220-L) mitigated the cumulative NH₃ volatilization by 11.2% and 7.6%, respectively, and mitigated yield-scale NH₃ volatilization (cumulative NH₃ volatilization/total yield) by 14.2% ∼ 22.4%. HCAP significantly improved the N use efficiency of rice. We found that the NH₃ volatilization was related to NH₄⁺-N concentration and pH of surface water, soil TOC and NH₄⁺-N oxidation functional genes. This study implied that HCAP could be potentially used as a liquid fertilizer, which will be a potential substitute for chemical N fertilizers. There is still a long way before HCAP can be applied in full-scale for N fertilizer reduction and waste recycle.

Nitrogen fertilizer reduction in combination with Azolla cover for reducing ammonia volatilization and improving nitrogen use efficiency of rice

Background

Excessive nitrogen (N) application rate with low N use efficiency (NUE) caused a considerable amount of N losses, especially ammonia volatilization (AV). Proper N fertilizer reduction (RN) could significantly reduce AV. However, continuous RN led to a nutrient deficiency in the soil and therefore negatively impacted the NUE and rice yield. Paddy Azolla, a good green manure, is considered as a promising measure to decrease AV and improve NUE and grain yield of rice. However, there is limited information on the integrated effects of RN and Azolla cover on the AV, NUE, and rice yield, especially in the highly fertilized rice-growing systems.

Methods

The experiment was conducted including eight treatments: the control (without N fertilizer and Azolla cover), Azolla cover without N fertilizer (A), farmer’s N application rate (FN), FN + Azolla cover (FNA), 15% RN from FN (RN₁₅), RN₁₅ + Azolla cover (RN₁₅A). 30% RN from FN (RN₃₀), RN₃₀ + Azolla cover (RN₃₀A). The integrated effects of N fertilizer reduction and Azolla cover on AV, NUE, and rice grain was evaluated.

Results

RN₁₅A and RN₃₀A substantially reduced total AV by 50.3 and 66.9% compared with FN, respectively, primarily due to the lower surface water ammonia concentrations and pH. RN improved the efficiency of Azolla cover on reducing AV, with 4.1–9.9% higher than for FN. Compared with the FN, RN₁₅A and RN₃₀A enhanced apparent N recovery efficiency (ANRE) by 46.5 and 39.1%, which might be responsible for the lower NH₃ emission and the increased total N uptake / total chemical N applied. Furthermore, RN₁₅A and RN₃₀A reduced yield-scaled volatilization by 52.3 and 64.3% than for FN, respectively. Thus, combining 15–30% RN with Azolla cover may be a way to reduce AV and improve ANRE without decreasing rice grain yield.

Effects of different types of humic acid isolated from coal on soil NH3 volatilization and CO2 emissions

Humic acid can improve soil nutrients and promote plant growth. Weathered coal and lignite can be used as agricultural resources due to high humic acid content, but their impact on soil NH₃ volatilization and CO₂ emissions are yet to be determined. In this study, a field experiment was carried out to compare the effects of four types of humic acid isolated from coal (pulverized weathered coal (HC), pulverized lignite (HL), alkalized weathered coal (AC) and alkalized lignite (AL)) on NH₃ volatilization, CO₂ emissions, pH, the C/N ratio and enzyme activities in soil cultivated with maize. The effect of biotechnology humic acids (BHA) was also examined for comparison. HL, AC, AL and BHA all increased cumulative NH₃ losses by 147.7, 278.5, 113.9, and 355.3%, respectively, compared with the control (chemical fertilizer only), and notably, BHA caused an increase of 90.71% compared with the humic acids isolated from coal. A significant increase in cumulative CO₂ losses was observed only under AL treatment, by 14.44-24.90% compared with all other treatments. Soil urease activity was positively correlated with cumulative NH₃ losses (P < 0.001), while the soil C/N ratio (P < 0.001) and soil sucrase activity (P < 0.05) were positively correlated with cumulative CO₂ losses. Since humic acid from pulverized weathered coal caused no increase in NH₃ volatilization or CO₂ emissions, it is therefore thought to be the most suitable humic acid for field application.

Combining Azolla and urease inhibitor to reduce ammonia volatilization and increase nitrogen use efficiency and grain yield of rice

Paddy Azolla is considered as a promising technical approach to reduce ammonia (NH₃) volatilization and increase nitrogen use efficiency (NUE). However, it is not effective in highly fertilized paddy fields as the high ammonium N (NH₄⁺-N) concentrations adversely inhibit the growth and N uptake of Azolla. Urease inhibitors could effectively decrease NH₄⁺-N concentrations in surface water and NH₃ volatilization. However, a lack of information still exists regarding the combined effects of Azolla and urease inhibitors on NH₃ volatilization, NUE, and grain yield (GY) of rice. A two-year field experiment was conducted including five treatments (no urea application (control), urea (N), urea + Azolla (NA), urea + urease inhibitor (NUI), and urea + Azolla + urease inhibitor (NAUI)). Results showed that NA treatment (-25.2%) was not effective in reducing NH₃ volatilization compared with NUI treatment (-43.3%). The NAUI treatment substantially reduced NH₃ volatilization (-54.6%) more than that by NA and NUI treatments, primarily because of the lower NH₄⁺-N concentrations, pH, and temperature in surface water. Furthermore, NAUI treatments significantly increased the grain yield (GY) and the apparent N recovery efficiency (ANRE) of rice by 9.0-9.7% and 66.0-71.3%, respectively. The significant increase in GY was mainly from the increased panicle number (4.0%), spikelet number per panicle (15.9%), and total biomass (22.9%), which caused by the enhanced total N uptake (35.8%). NAUI treatment also decreased the yield-scaled NH₃ volatilization by 61.1-63.6%. Overall, the co-application of Azolla and urease inhibitor in the rice field substantially decreased NH₃ volatilization, and increased NUE and rice yield.

Hydrochar did not reduce rice paddy NH3 volatilization compared to pyrochar in a soil column experiment

Pyrochar (PC) is always with high pH value, and improper application might increase rice paddy ammonia volatilization (PAV), which is the main nitrogen loss through air during rice production. Differently, hydrochar (HC) takes the advantages of high productive rate and always with lower pH value compared with PC. However, effect pattern and mechanism of HC on PAV are still unclear. In the present study, soil column experiments were conducted to investigate the effect of PC and HC application on PAV. In total, treatments with four types of biochar (WPC, SPC, WHC and SHC, i.e., PC and HC prepared with wheat straw and sawdust, respectively) and two application rates (0.5% and 1.5%, w/w) were set up and non-biochar application was used as control. Results showed that, application of HC with low pH value could not reduce PAV compared with PC. Total PAV increased significantly as the increase of HC application rate (especially for WHC). The increment of PAV under high rate HC application might be due to the strong buffer capacity of soil, the aging of biochar, the high nitrogen from HC. The results indicated that HC should be pretreatment before utilization in agricultural environment considering PAV reduction.

Bacillus subtilis biofertilizer mitigating agricultural ammonia emission and shifting soil nitrogen cycling microbiomes

Excessive ammonia (NH₃) emitted from nitrogen fertilizer application in farmland have caused serious disturbance to global environment, including reduction of visibility, formation of regional haze, and increase of nitrogen deposition. Application of biofertilizer has been considered as an effective approach for soil improvement and agriculture sustainability. In this study, a field experiment was conducted to evaluate the potential of B. subtilis biofertilizer on mitigating NH₃ volatilization and to investigate the underlying mechanisms. Compared with organic fertilizer, the incorporation of B. subtilis biofertilizer reduced NH₃ volatilization by up to 44%. Moreover, the application of B. subtilis biofertilizer reduced the abundance of ureC gene, and increased the abundance of functional genes (bacterial amoA and comammox amoA) and ammonia-oxidizing bacteria (AOB). This indicated that the conversion of fertilizer nitrogen to NH₄⁺-N was decreased and the nitrification process was increased. In brief, the application of B. subtilis biofertilizer reduced the "source" and increased the "sink" of NH₄⁺-N, thus reducing the retention of NH₄⁺-N in alkaline soil, and mitigating NH₃ volatilization. These results indicated that B. subtilis biofertilizer is an effective control strategy for agricultural NH₃ emission, maintaining high crop yield and mitigating environmental disturbance.

A Critical Review on Advancement and Challenges of Biochar Application in Paddy Fields: Environmental and Life Cycle Cost Analysis

Paddy fields emit considerable amounts of methane (CH4), which is a potent greenhouse gas (GHG) and, thereby, causes significant environmental impacts, even as they generate wealth and jobs directly in the agricultural sector, and indirectly in the food-processing sector. Application of biochar in rice production systems will not just help to truncate their carbon footprints, but also add to the bottom-line. In this work, the authors have reviewed the literature on climate change, human health, and economic impacts of using organic residues to make biochar for the addition to croplands especially to rice paddy fields. Biochar-bioenergy systems range in scale from small household cook-stoves to large industrial pyrolysis plants. Biochar can be purveyed in different forms—raw, mineral-enriched, or blended with compost. The review of published environmental life cycle assessment (E-LCA) studies showed biochar has the potential to mitigate the carbon footprint of farming systems through a range of mechanisms. The most important factors are the stabilization of the carbon in the biochar and the generation of recoverable energy from pyrolysis gases produced as co-products with biochar as well as decreased fertiliser requirement and enhanced crop productivity. The quantitative review of E-LCA studies concluded that the carbon footprint of rice produced in biochar-treated soil was estimated to range from −1.43 to 2.79 kg CO2-eq per kg rice grain, implying a significant reduction relative to rice produced without a biochar soil amendment. The suppression of soil-methane emission due to the biochar addition is the dominant process with a negative contribution of 40–70% in the climate change mitigation of rice production. The review of the life cycle cost studies on biochar use as an additive in farmlands demonstrated that biochar application can be an economically-feasible approach in some conditions. Strategies like the subsidization of the initial biochar capital cost and assignment of a non-trivial price for carbon abatement in future pricing mechanisms will enhance the economic benefits for the rice farmers.

Carbon sequestration, kinetics of ammonia volatilization and nutrient availability in alkaline sandy soil as a function on applying calotropis biochar produced at different pyrolysis temperatures

This incubation study assessed the effects of unpyrolyzed Calotropis procera and its biochar produced at different pyrolysis temperatures as well as incubation periods on carbon (C) emission, ammonia (NH₃) volatilization, soil quality indicators and nutrient availability of alkaline sandy soil. Five treatments were studied in this experiment: unamended soil (CK), unpyrolyzed calotropis (UPC), calotropis biochar at 250 °C (CB250), calotropis biochar at 400 °C (CB400), and calotropis biochar at 650 °C (CB650). These amendments were applied to the soil at level of 4% (w/w). The results of this study showed that applying unpyrolyzed calotropis residues increased cumulative CO₂ emission from the soil by 117.3, 239.4 and 232.0% over CB250, CB400, and CB650, respectively, by the end of incubation. Compared to the unamended soil, applying CB250 reduced cumulative NH₃ volatilization in soil by 71.5%, which attributed to ammonia adsorption because of increased cation exchange capacity and decreased soil pH, but CB650 increased cumulative NH₃ volatilization by 73.3% after the 3-day incubation as a result of high soil pH. The available phosphorus in soil improved significantly (p ≤ 0.01) with adding unpyrolyzed calotropis residues and its biochar produced at different pyrolysis temperatures compared to the unamended soil. The values of available phosphorus in the soil under study influenced significantly by pyrolysis temperatures of produced biochar; this is due to the pyrolysis of feedstocks increases labile phosphorus. Thenceforth, using biochar is an important strategy for enhancing carbon sequestration, decreasing ammonia volatilization and improving soil quality parameters in arid regions.

Effect of rice straw and swine manure biochar on N2O emission from paddy soil

We analyzed the effects of rice straw biochar (RSBC) and swine manure biochar (SMBC) on N₂O emission from paddy soil. The biochars were added to soil at the rates of 1% and 5% (w/w), and N₂O emission, soil properties and soil enzyme activities were determined at the elongation, heading and maturation stages of rice growth. The N₂O flux started within 2 h of adding the biochar, and decreased significantly thereafter during the three growth stages. The cumulative N₂O emission was suppressed by 45.14–73.96% following biochar application, and 5% SMBC resulted in the lowest cumulative emission. In addition, biochar application significantly increased soil pH, soil organic carbon (SOC), NO₃⁻ levels and urease activity, and decreased soil NH₄⁺ and nitrate reductase activity. Regression analysis indicated that cumulative N₂O emission was correlated positively to NH₄⁺, and negatively to soil pH, SOC and NO₃⁻. SEM further revealed that biochar application weakened the denitrification process, and the NH₄⁺ level had the most significant impact on N₂O emission. Taken together, RSBC and SMBC regulated the nitrogen cycle in paddy soil and mitigated N₂O emission by increasing soil pH, decreasing nitrate reductase activity and NH₄⁺ content.

Wheat Straw Biochar as a Specific Sorbent of Cobalt in Soil

There is an urgent need to search for new sorbents of pollutants presently delivered to the environment. Recently biochar has received much attention as a low-cost, highly effective heavy metal adsorbent. Biochar has been identified as an efficient material for cobalt (Co) immobilization from waters; however, little is known about the role of Co immobilization in soil. Hence, in this study, a batch experiment and a long-term incubation experiment with biochar application to multi-contaminated soil with distinct properties (sand, loam) were conducted to provide a brief explanation of the potential mechanisms of Co (II) sorption on wheat straw biochar and to describe additional processes that modify material efficiency for metal sorption in soil. The soil treatments with 5% (v/w) wheat straw biochar proved to be efficient in reducing Co mobility and bioavailability. The mechanism of these processes could be related to direct and indirect effects of biochar incorporation into soil. The FT-IR analysis confirmed that hydroxyl and carboxyl groups present on the biochar surface played a dominant role in Co (II) surface complexation. The combined effect of pH, metal complexation capacity, and the presence of Fe and Mn oxides added to wheat straw biochar resulted in an effective reduction of soluble Co (II), showing high efficiency of this material for cobalt sorption in contaminated soils.

Bentonite hydrochar composites mitigate ammonia volatilization from paddy soil and improve nitrogen use efficiency

Clay-hydrochar composites (CHCs) are of great significance in ammonium (NH₄⁺) adsorption and have the potential to be applied to paddy fields to prevent ammonia (NH₃) volatilization. In this study, three CHCs were produced by infusing different clays to poplar-sawdust-derived hydrochar, including a bentonite hydrochar composite (BTHC), montmorillonite hydrochar composite (MTHC), and kaolinite hydrochar composite (KTHC). These three CHCs were applied to a paddy soil column system growing rice. The temporal variations in NH₃ volatilization and NH₄⁺ loss in floodwater were monitored after three fertilization dates. The results showed that among the three CHCs, only the BTHC significantly reduced cumulative NH₃ volatilization (by 41.8%), compared to that of the unamended control (without addition of hydrochar or clay-hydrochar-composite). In the unamended control, NH₃ volatilization loss accounted for 31.4% of the applied N fertilizer; with the BTHC amendment, NH₃ volatilization loss accounted for 17.4% of the applied N fertilizer. The reduced N loss via the BTHC amendment resulted in an increased N supply and further improved the N use efficiency and yield by 37.36% and 18.8% compared to that of the control, respectively. The inhibited NH₃ volatilization was mainly attributed to the increased soil NH₄⁺ retention as a result of BTHC's larger pore volume and specific surface area. In addition, the BTHC treatment significantly reduced the abundance of archaeal amoA genes (AOA), which possibly inhibited nitrification and increased soil NH₄⁺ retention. This study, for the first time, screened BTHC as an excellent material for mitigating NH₃ volatilization from paddy fields. The reduced NH₃ volatilization loss might contribute to increased soil N retention and plant N use efficiency.

Microalgae-derived hydrochar application on rice paddy soil: Higher rice yield but increased gaseous nitrogen loss

Hydrothermal carbonization represents a promising technique for transforming microalgae into the hydrochar with abundant phytoavailable nutrients. However, the effects of microalgae-derived hydrochars on the gaseous nitrogen (N) loss from agricultural field are still unclear. Chlorella vulgaris powder (CVP) and two Chlorella vulgaris-derived hydrochars that employ water (CVHW) or citrate acid solution (CVHCA) as the reaction medium were applied to a soil column system grown with rice. The temporal variations of nitrous oxide (N₂O) emissions and ammonia (NH₃) volatilization were monitored during the whole rice-growing season. Results showed that CVHW and CVHCA addition significantly increased the grain yield (by 13.5-26.8% and 10.5-23.4%) compared with control and CVP group, while concomitantly increasing the ammonia volatilization (by 53.8% and 72.9%) as well as N₂O emissions (by 2.17- and 2.82-fold) from paddy soil compared to control. The microbial functional genes (AOA, AOB, nirk, nirS, nosZ) in soil indicated that CVHW and CVHCA treatment stimulated the nitrification and denitrification, and inhibited the N₂O oxidation in soil. Notably, CVHW was recommended in the view of improving yield and controlling NH₃ volatilization because no significant difference of the yield-scale NH₃ volatilization was detected between control and CVHW treatment. This study for the first time uncovered that Chlorella vulgaris-derived hydrochars have positive effects on rice N utilization and growth but negative effects on the atmospheric environment.

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.

Insights into the effects of long-term biochar loading on water-soluble organic matter in soil: Implications for the vertical co-migration of heavy metals

Although interest in biochar remediation is growing, the effects of long-term biochar loading on soil environments have not been clearly confirmed. The contents and characteristics of water-soluble organic matter (WSOM) from soils after eight years of biochar remediation were investigated, and the vertical co-migration of heavy metals controlled by interactions between WSOM, soil and contaminants were also analyzed. The results showed that biochar-leaching WSOM featured high aromaticity. Fluorescence excitation-emission matrix (EEM) spectrophotometry was employed, and three primary components, including fulvic-acid-like (FA_-like), tryptophan, and humic-acid-like (HA_-like) compounds, were identified in the EEM spectra via parallel factor analysis models. With increasing biochar loading, FA_-like and HA_-like greatly increased, but tryptophan showed a weak response. Furthermore, the WSOM was freeze dried and analyzed with Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, and the results demonstrated that the BC treatment increased oxygen-containing functional groups and enhanced the complexation capability of the WSOM. Finally, the Cd and Pb concentrations in the WSOM were investigated, and Cd was found to decrease in top-soil WSOM with added BC because of increased complexation, but the Pb content increased because exchangeable and carbonate Pb converted into organic Pb. Further, the Cd and Pb concentrations decreased in sub-soil WSOM. These findings suggest that more efforts should be devoted to studying the effects of long-term biochar loading on soil environments.

Quantitatively ranking the influencing factors of ammonia volatilization from paddy soils by grey relational entropy

Ammonia (NH₃) volatilization from paddy soils is a main source of atmospheric NH₃ and the magnitude is affected by many factors. Because of the complex field condition, it is difficult to identify the relative importance of individual factor on NH₃ volatilization process in different locations and at different times. In this study, the grey relational entropy method was used to evaluate the relative impact of four main factors (i.e., nitrogen fertilizer application rate, NH₄-N concentration, pH, and temperature of the floodwater) on NH₃ volatilization loss from three different field experiments. The results demonstrated that floodwater NH₄-N concentration was the most important factor governing NH₃ volatilization process. Floodwater pH was the second most important factor, followed by temperature of the floodwater and nitrogen fertilizer application rate. We further validated the grey relational entropy method with NH₃ volatilization loss data from other published study and confirmed the order of importance for the four factors. We hope the findings of this study will be helpful for guiding design to reduce paddy soil NH₃ emission.

Treatment of horizontal silage bunker runoff using biochar amended vegetative filter strips

Horizontal silage bunkers produce leachate that contains contaminants that can be detrimental to the environment if released untreated. Vegetated filter strips are used to treat silage bunker runoff to prevent contamination of surface waters via infiltration, however increased infiltration poses risks to groundwater, particularly for nitrate (NO₃^-). Vegetated filter strip plots with a sandy loam soil, half of which are amended with biochar, were investigated to assess the treatment of silage bunker runoff over 20 application events. The subsurface effluent biological oxygen demand (BOD₅), chemical oxygen demand (COD), and total phosphorus (TP) were reduced on average by 40%, 46%, and 75%, respectively, and there was no statistical difference between treatments. The total nitrogen (TN) was reduced by 49 and 64% for control and biochar plots, respectively, which was significantly different between treatments. Biochar significantly reduced nitrate nitrogen (NO₃^--N) leaching by 40% compared to the control, however, the NO₃^--N concentration in leachate was still high ranging from 0.19 to 191.04 mg NO₃^--N L^-1 and 0.18-108.89 mg NO₃^--N L^-1 for control and biochar plots, respectively. A mass balance suggests the primary mechanism for a decrease in TN and NO₃^--N leaching from biochar amended plots was greater retention of NO₃^--N and organic N (ORG-N) within the soil/biochar matrix. The development of oxygenated functional groups and/or formation of organomineral layer on the biochar surface likely enhanced N retention.

Biochar with near-neutral pH reduces ammonia volatilization and improves plant growth in a soil-plant system: A closed chamber experiment

Ammonia (NH₃) volatilization is considered as one of the major mechanisms responsible for the loss of nitrogen (N) from soil-plant systems worldwide. This study investigated the effect of biochar amendment to a calcareous soil (pH 7.8) on NH₃ volatilization and plant N uptake. In particular, the effect of biochar's feedstock and application rate on both NH₃ volatilization and plant growth were quantified using a specially designed closed chamber system. Two well-characterized biochars prepared from poultry manure (PM-BC) and green waste compost (GW-BC) were applied to the soil (0, 0.5, 1, 1.5 and 2% w/w equivalent to 0, 7.5, 15, 22 and 30 t ha^-1) and wheat (Triticum aestivum, variety: Calingiri) was grown for 30 days. Both PM-BC and GW-BC decreased NH₃ volatilization to a similar degree (by 47 and 38%, respectively), in the soil-plant system compared to the unamended control. Higher plant biomass production of up to 70% was obtained in the closed chamber systems with the addition of biochar. The increase in plant biomass was due to the reduction in N loss as NH₃ gas, thereby increasing the N supply to the plants. Plant N uptake was improved by as much as 58% with biochar addition when additional NPK nutrients were supplied to the soil. This study demonstrates that the application of biochars can mitigate NH₃ emission from calcareous agricultural cropping soil and that the retained N is plant-available and can improve wheat biomass yield.

The effect of biochar amendment on N-cycling genes in soils: A meta-analysis

Nitrogen (N) cycling by soil microbes can be estimated by quantifying the abundance of microbial functional genes (MFG) involved in N-transformation processes. In agro-ecosystems, biochars are regularly applied for increasing soil fertility and stability. In turn, it has been shown that biochar amendment can alter soil N cycling by altering MFG abundance and richness. However, the general patterns and mechanisms of how biochar amendment modifies N-cycling gene abundance have not been synthesized to date. Here, we addressed this knowledge gap by performing a meta-analysis of existing literatures up to 2019. We included five main marker genes involved in N cycling: nifH, amoA, nirK, nirS and nosZ. We found that biochar addition significantly increased the abundance of ammonia-oxidizing archaea (AOA), nirK, nirS and nosZ by an average of 25.3%, 32.0%, 14.6% and 17.0%, respectively. Particularly, biochar amendment increased the abundances of most N-cycling genes when soil pH changed from very acidic (pH < 5) to acidic (pH: 5.5-6.5). Experimental conditions, cover plants, biochar pyrolysis temperature and fertilizer application were also important factors regulating the response of most N-cycling genes to biochar amendment. Moreover, soil pH significantly correlated with ammonia-oxidizing bacteria (AOB) abundance, while we found that most genes involved in nitrification and denitrification were not significantly correlated with each other across studies. Our results contribute to developing quantitative models of microbially-mediated N-transforming processes in response to biochar addition, and stimulate research on how to use biochar amendment for reducing reactive N gas emissions and enhancing N bioavailability to crop plants in agro-ecosystems.

Enhanced anaerobic degradation of selected nitrogen heterocyclic compounds with the assistance of carboxymethyl cellulose

Carboxymethyl cellulose (CMC) is a modified cellulose compound that is dispersible in water. Microbial anaerobic degradation of nitrogen heterocyclic compounds (NHCs) in wastewater treatment may be enhanced by CMC addition, but this remains uncertain due to a lack of experimental evidence. In this study, It was demonstrated that CMC is a suitable co-metabolic matrix in an enhanced anaerobic degradation of quinoline and indole in coal gasification wastewater. When the dosage of CMC was 0.5 mg/L, a reactor exhibited a high degradation efficiency on quinoline and indole, with ratios of 95.23 ± 1.99% and 94.33 ± 3.45%. The addition of CMC increased the concentration of extracellular polymeric substances in anaerobic sludge and increased the particle size of the sludge, which improved the microbial stability and sedimentation of anaerobic granular sludge. Analysis of high-throughput sequencing indicated that the addition of CMC improved the richness and diversity of bacterial and archaea communities. Acetic acid metabolism was the primary mechanism to produce methane during anaerobic degradation of NHCs wastewater.

Agro-environmental impacts, carbon sequestration and profit analysis of blended biochar pellet application in the paddy soil-water system

The application of biochar pellet blended with pig manure compost was investigated to estimate its agro-environmental impacts and to evaluate its soil carbon sequestration and profit analysis during rice cultivation. The experiment consisted of four different treatments such as control as pig manure compost only, pig manure compost pellet (PMCP), biochar pellet blended with biochar and pig manure compost (4: 6 ratios, BCP), and slow release fertilizer (SRF). The application of chemical fertilizer and pig manure compost in the whole treatment except the BCP were 90-45-57 kg ha^-1 (N-P-K) and 2600 kg ha^-1, respectively, based on the recommended rates for rice cultivation at National Institute of Agricultural Sciences (NIAS). The BCP and SRF were applied with N 90 kg ha^-1 basis only as basal application before transplanting. The pig manure compost, phosphorous and potassium were applied at basal application while nitrogen fertilizer was applied with three separations as basal and two additional applications. Results showed that concentrations of ammonium nitrogen (NH₄-N) and nitrate nitrogen (NO₃-N) in the BCP at an early stage of rice growth were lowest among the treatments, but their concentrations in the paddy water rapidly decreased at 21 days after transplanting. For paddy soil, NH₄-N concentration in the SRF was continuously high compared to the BCP until 20 days of rice cultivation. For paddy water, phosphate phosphorous (PO₄-P) concentrations in the BCP were three fold lower than the SRF at an early stage of rice growth. Similar pattern between potassium (K) concentrations in paddy water and potassium oxide (K₂O) contents in surface soil was also observed during rice cultivation where their concentrations decreased abruptly 41 days after transplanting. Carbon sequestration and mitigation of carbon dioxide equivalency (CO₂-eq.) emission in the BCP were higher at 1.65 tons ha^-1 and 6.06 tons ha^-1, respectively, than the control while result of its profit analysis was $145.59 (KAU, Korean Allowance Unit) per hectare during rice cultivation. The rice yield were not significantly different (p > 0.05) among all treatments.