Importance of biochar as a key amendment to convert rice paddy into carbon negative.

Biochar being made up of recalcitrant carbon (C) compounds is considered a negative emission technology (NET) due to its indirect removal of atmospheric carbon dioxide (CO2). However, there is no clear report about how biochar remains a NET when organic amendment application in rice paddy results in a huge emission of greenhouse gases (GHG) particularly, methane (CH4). To evaluate the net impact of biochar application on the net global warming potential (GWP) in rice paddy, no organic amendment (control), fresh manure, compost, and biochar treatments were selected during the whole investigation period. Compared to compost, biochar application decreased annual CH4 and N2O emissions by 55 and 31 %, respectively. In comparison to the control, biochar application increased CH4 emission by 163 % but decreased N2O emission by 19 %. Soil organic carbon (SOC) stock would annually deplete by 2.2 Mg C ha-1 under control; however, biochar application could increase the SOC stock by 18.1 Mg C ha-1 which was 63 and 33 % higher than fresh and compost treatments, respectively. As a result, the control had a net GWP of 10 Mg CO2-eq ha-1 however, this impact was increased with fresh manure and compost application by around 319 and 159 %, respectively. Interestingly, biochar application converted rice paddy into a C sink having a net GWP of -0.104 to -0.191 Mg CO2-eq ha-1. Since there was a comparable difference in grain yield among organic amendments, greenhouse gas intensity (GHGI) which is the net GWP per grain yield was significantly high in compost application of approximately 3.1 Mg CO2-eq Mg-1 grain being 127 % higher than control. However, the biochar application had a -0.02 Mg CO2-eq Mg-1 grain which was 1.4 Mg CO2-eq Mg-1 grain lower than the control. Conclusively, biochar application could be a considerable option in maintaining soil quality and productivity without contributing any GHG emissions and their associated impacts.

Critical evaluation of biochar utilization effect on mitigating global warming in whole rice cropping boundary.

Biochar and compost were accepted as a stable organic amendment to increase soil C stock as well as to decrease greenhouse gas (GHG) emissions in rice paddy soils. However, in most studies, their effect on GHG flux was evaluated only within the cropping boundary without considering industrial processes. To compare the net effect of these organic amendment utilizations on global warming within the whole rice cropping system boundary from industrial process to cropping, fresh, compost, and biochar manures were applied at a rate of 12 Mg ha-1 (dry weight) in a rice paddy, and total GHG fluxes were evaluated. Compared with fresh manure, compost utilization decreased net global warming potential (GWP) which summated GHG fluxes and soil C stock change with CO2 equivalent by 43% within rice cropping boundary, via a 25% decrease of CH4 flux and 39% increase of soil C stock. However, 34 Mg CO2-eq. of GHGs were additionally emitted during composting to make 12 Mg of compost and then increased the net GWP by 34% within the whole system boundary. In comparison, biochar changed paddy soil into a GHG sink, via 56% decrease of CH4 flux and 13% increase of soil C stock. However, pyrolysis emitted a total of 0.08 and 19 Mg CO2-eq. of GHGs under with and without syngas recycling system, respectively, to make 12 Mg of biochar. As a result, biochar utilization decreased net GWP by approximately 28-70% over fresh manure within the whole system boundary. Rice grain productivity was not discriminated between biochar and compost manures, but compost considerably increased grain yield over fresh manure. Consequently, biochar utilization significantly decreased GHG intensity which indicates net GWP per grain by 33-72% over fresh manure, but compost increased by 22%. In conclusion, biochar could be a sustainable organic amendment to mitigate GHG emission impact in the rice paddy, but compost should be carefully selected.

Biochar as soil amendment: Syngas recycling system is essential to create positive carbon credit.

Biochar utilization is accepted as the most cost-effective practice to mitigate global warming via increase in soil C stock. However, its utilization effect on greenhouse gas (GHG) fluxes was evaluated only within land application without considering industrial processes. To evaluate the net effect of biochar utilization on global warming within whole system boundary, swine manure-saw dust mixture was pyrolyzed under four different temperatures, and GHG fluxes were characterized under with/without syngas recycling systems. To determine GHG fluxes from biochar amended soil, 40 Mg ha-1 of biochar was mixed with soil and incubated under flooded and dried soil conditions. Finally, the effect of biochar utilization was generalized using net global warming potential (GWP) from industrial process to land application. Under without syngas recycling system, huge amounts of GHGs were emitted during pyrolysis, and GHG fluxes highly increased with increasing pyrolysis temperature, due to direct and indirect GHG emissions from feedstock combustion and electricity, respectively. However, syngas recycling system removed most of GHGs, except for direct N2O and indirect GHG emissions from electricity. Biochar application was very effective to mitigate GHG emissions within soil system boundary, and biochar produced at higher pyrolysis temperature showed higher effectivity in decreasing GHG fluxes. Within the whole system boundary from pyrolysis to soil application, without the installation of syngas recycling system, fresh manure application was more effective than biochar to reduce GHG emissions, regardless of soil water conditions. However, with the installation of syngas recycling system, biochar application was much more effective than fresh manure to decrease GHG fluxes. Biochar produced at higher temperature had higher effectivity to mitigate global warming impacts. In conclusion, to functionally mitigate global warming in soils, biochar should be produced in pyrolysis reactors equipped with syngas recycling system under higher temperature.

Uncertainty of methane emissions coming from the physical volume of plant biomass inside the closed chamber was negligible during cropping period.

In rice paddy, the closed chamber method is broadly used to estimate methane (CH4) emission rate. Since rice plants can significantly affect CH4 production, oxidation and emission, rice plantation inside the chamber is standardized in IPCC guidelines. Methane emission rate is calculated using the increased concentration inside the headspace. Biomass growth might decrease the headspace volume, and thus CH4 emission rates might be overestimated. To evaluate the influence of chamber headspace decreased by rice plant development on CH4 emission rates, five Korean rice cultivars were cultivated in a typical rice paddy, and physical volume changes in rice biomass were assayed using water displacement method. The recommended acrylic closed chambers (H. 1.2 m x W. 0.6 m x L. 0.6 m) were installed, and eight rice plants were transplanted inside the chamber with the same space interval with the outside. Biomass growth significantly decreased the headspace volume of the chamber. However, this volume covered only 0.48-0.55% of the closed chamber volume at the maximum growth stage. During the whole cropping period, mean 0.24-0.28% of chamber headspace was allocated by plant biomass, and thus this level of total CH4 emissions was overestimated. However, this overestimation was much smaller than the errors coming from other investigation processes (i.e., chamber closing hour, temperature recording, inconstant flooding level, different soil environments, etc.) and rice physiological changes. In conclusion, the influence of physical biomass volume inside the closed chamber was negligible to make the error in total CH4 emission assessment in rice paddies.

Steel slag amendment impacts on soil microbial communities and activities of rice (Oryza sativa L.).

With the increase in iron/steel production, the higher volume of by-products (slag) generated necessitates its efficient recycling. Because the Linz-Donawitz (LD) slag is rich in silicon (Si) and other fertilizer components, we aim to evaluate the impact of the LD slag amendment on soil quality (by measuring soil physicochemical and biological properties), plant nutrient uptake, and strengthens correlations between nutrient uptake and soil bacterial communities. We used 16 S rRNA illumine sequencing to study soil bacterial community and APIZYM assay to study soil enzymes involved in C, N, and P cycling. The LD slag was applied at 2 Mg ha-1 to Japonica and Indica rice cultivated under flooded conditions. The LD slag amendment significantly improved soil pH, plant photosynthesis, soil nutrient availability, and the crop yield, irrespective of cultivars. It significantly increased N, P, and Si uptake of rice straw. The slag amendment enhanced soil microbial biomass, soil enzyme activities and enriched certain bacterial taxa featuring copiotrophic lifestyles and having the potential role for ecosystem services provided to the benefit of the plant. The study evidenced that the short-term LD slag amendment in rice cropping systems is useful to improve soil physicochemical and biological status, and the crop yield.

Different response of plastic film mulching on greenhouse gas intensity (GHGI) between chemical and organic fertilization in maize upland soil.

Since plastic film mulching (PFM) can deplete soil organic carbon (SOC) stock but increase greenhouse gas (GHG) emissions, PFM utilization is still debating. To determine the influence of PFM on global warming, PFM and no-mulching treatments were installed under chemical and organic fertilizations. In organic fertilization, cover crop was cultivated during the fallow season, and its biomass was incorporated as green manure. To estimate net global warming potential (GWP), two GHG (N2O and CH4) fluxes and SOC stock changes were determined. SOC stock changes were estimated using net ecosystem carbon budget (NECB), which implies the difference between C input and output. GHG intensity (GHGI) was calculated using net GWP per unit of grain yield. PFM increased maize grain yields by approximately 45-95% over no-mulching under the same fertilization, but it was more effective in organic fertilization, due to higher nutrient input. In contrast, PFM increased seasonal N2O and CH4 emissions by 5-10% and 130-260% over no-mulching under the same amendment, respectively. Compared with no-mulching, PFM decreased more highly SOC stock in organic fertilization (around 140-200%) than chemical fertilization (20-30%), due to higher harvest removal and respired C loss. Irrespective with fertilization and mulching background, net GWP was decided by SOC stock change, which covered about 75-90% of net GWP, and then followed by N2O. PFM differently influenced net GWP and GHGI between chemical and organic fertilizations. In chemical fertilization, PFM increased net GWP by 20-30% over no-mulching, but decreased GHGI by 25-35%. However, in organic amendment, PFM highly increased net GWP and GHGI by approximately 115-160% and 30-80% over no-mulching, respectively. PFM in chemical fertilization could be very powerful tool to decrease GHGI via crop yield increase. However, in organic farming, PFM should be carefully selected to increase crop yields, due to more highly increased global warming impact.

Beneficial effect of compost utilization on reducing greenhouse gas emissions in a rice cultivation system through the overall management chain.

Livestock manure application can stimulate greenhouse gas (GHG) emissions, especially methane (CH4) in rice paddy. The stabilized organic matter (OM) is recommended to suppress CH4 emission without counting the additional GHG emission during the composting process. To evaluate the effect of compost utilization on the net global warming potential (GWP) of a rice cropping system, the fluxes of GHGs from composting to land application were calculated by a life cycle assessment (LCA) method. The model framework was composed of GHG fluxes from industrial activities and biogenic GHG fluxes from the composting and rice cultivation processes. Fresh manure emitted 30MgCO2-eq.ha-1, 90% and 10% of which were contributed by CH4 and nitrous oxide (N2O) fluxes, respectively, during rice cultivation. Compost utilization decreased net GWP by 25% over that of the fresh manure during the whole process. The composting process increased the GWP of the industrial processes by 35%, but the 60% reduction in CH4 emissions from the rice paddy mainly influenced the reduction of GWP during the overall process. Therefore, compost application could be a good management strategy to reduce GHG emissions from rice paddy systems.

Composted Cattle Manure Increases Microbial Activity and Soil Fertility More Than Composted Swine Manure in a Submerged Rice Paddy.

Livestock waste composts with minimum inorganic fertilizer as a soil amendment in low-input intensive farming are a feasible agricultural practice to improve soil fertility and productivity and to mitigate soil degradation. The key benefits of the practice rely on the activities of soil microorganisms. However, the role of different livestock composts [composted cattle manure (CCM) vs. composted swine manure (CSM)] on soil microbes, their activities and the overall impact on soil fertility and productivity in a flooded paddy remains elusive. This study compares the effectiveness of CCM and CSM amendment on bacterial communities, activities, nutrient availability, and crop yield in a flooded rice cropping system. We used deep 16S amplicon sequencing and soil enzyme activities to decipher bacterial communities and activities, respectively. Both CCM and CSM amendment significantly increased soil pH, nutrient availability (C, N, and P), microbial biomass, soil enzyme activities indicative for C and N cycles, aboveground plant biomass and grain yield. And the increase in above-mentioned parameters was more prominent in the CCM treatment compared to the CSM treatment. The CCM amendment increased species richness and stimulated copiotrophic microbial groups (Alphaproteobacteria, Betaproteobacteria, and Firmicutes) which are often involved in degradation of complex organic compounds. Moreover, some dominant species (e.g., Azospirillum zeae, Azospirillum halopraeferens, Azospirillum rugosum, Clostridium alkalicellulosi, Clostridium caenicola, Clostridium termitidis, Clostridium cellulolyticum, Magnetospirillum magnetotacticum, Pleomorphomonas oryzae, Variovorax boronicumulans, Pseudomonas xanthomarina, Pseudomonas stutzeri, and Bacillus niacini) which have key roles in plant growth promotion and/or lignocellulose degradation were enhanced under CCM treatment compared to CSM treatment. Multivariate analysis revealed that soil pH and available carbon (C) and nitrogen (N) were the major, while total organic carbon (TOC), total nitrogen (TN), and available phosphorus (P) were the minor drivers of variation in bacterial communities. Overall, our observations suggest that CCM amendment is better than CSM amendment to improve soil fertility and crop yield in a submerged rice cropping system.