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.

Cover cropping and its biomass incorporation: Not enough to compensate the negative impact of plastic film mulching on global warming.

Plastic film mulching (FM) became a general practice to enhance crop productivity and its net primary production (NPP), but it can increase greenhouse gas (GHG) emissions. The proper addition of organic amendments might effectively decrease the impact of FM on global warming. To evaluate the feasibility of biomass addition on decreasing this negative influence, cover crop biomass as a green manure was incorporated with different recycling levels (0-100% of aboveground biomass) under FM and no-mulching. The net global warming potential (GWP) which integrated with soil C stock change and GHG (N2O and CH4) fluxes with CO2-equivalent was evaluated during maize cultivation. Under the same biomass incorporation, FM significantly enhanced the grain productivity and NPP of maize by 22-61 and 18-58% over no-mulching, respectively. In contrast, FM also highly increased the respired C loss, which was 11-95% higher than NPP increase, over no-mulching. Irrespective with biomass recycling ratio and mulching system, negative NECB which indicates the decrease of soil C stock was observed, mainly due to big harvest removal. FM decreased more soil C stock by 57-158% over no-mulching, but its C stock was clearly increased with increasing biomass addition. FM significantly increased total N2O and CH4 fluxes by 4-61 and 140-600% over no-mulching, respectively. Soil C stock changes mainly decided net GWP scale, but N2O and CH4 fluxes negligibly influenced. As a result, FM highly increased net GWP over no-mulching, while this net GWP was clearly decreased with increasing biomass application. However, cover cropping, and its biomass recycling was not enough to compensate the negative impact of FM on global warming. Therefore, more biomass incorporation might be essential to compensate this negative effect of FM.

A new approach to suppress methane emissions from rice cropping systems using ethephon.

Rice is the main staple food for more than half of the world's population. Yet, rice cultivation is subjected to criticism because of its important role in methane (CH4) emissions. Although several agronomic practices such as controlled irrigation and conservation tillage have been widely adopted to mitigate CH4 emissions from rice cultivation, the benefits gained by these practices are highly dependent on site-specific soil and climate conditions, and often offset by yield reduction. The use of plant growth regulating compounds having the potential to increase the crop yield and mitigate CH4 emissions may be an innovative approach to sustainable agriculture. Ethylene (C2H4), a plant growth regulator is known to have a strong inhibitory effect on methanogenesis. However, due to gaseous form and low water solubility, C2H4 has not been used to suppress methanogenesis in paddy fields. To develop C2H4 as a prospective soil amendment for reducing methane (CH4) emissions, ethephon (2-Chloroethylphosphonic acid), a precursor of C2H4 was tested. We found that ethephon reduced CH4 formation by 43%, similar to other well known methanogenic inhibitors (2-Bromoethanesulfonate, 2-Chlomoethanesulfonate, 2-Mercaptoethanesulfonate). However, ethephon rapidly hydrolyzed to C2H4 and methanogenic activity recuperated completely after C2H4 removal. To slow down the release of C2H4, ethephon was mixed with bio-degradable polymers such as cellulose acetate and applied to paddy soils. We found that compared with the control, the C2H4 release of ethephon slowed down to 90 days, and the CH4 emissions were reduced by 90%. The application of ethephon at lower concentrations did not significantly alter bacterial communities, their relative abundance, and the abundance of methanotrophs, but it significantly reduced archaeal communities and the relative abundance and expression level of methanogens in paddy soils. Results suggest that cellulose acetate-mixed ethephon has great promise to suppress CH4 emissions in rice paddies while ensuring sustainable yields.

Effect of plastic film mulching on maize productivity and nitrogen use efficiency under organic farming in South Korea.

Winter cover crop cultivation and its biomass recycling as green manure (GM) were accepted as an ideal nutrient management practice in temperate organic farming fields. Since its biomass growth was boosted with air temperature rising from late Spring to early Summer, this stage overlapped with cash crops' seeding or transplanting. Thus, organic farmers were suffering from low crop productivity, due to delayed mineralization of incorporated biomass. To accelerate the mineralization of biomass nutrients and control weeds, plastic film mulching (PM) was broadly utilized in organic farming fields of temperate-monsoon climate region. However, the effect of PM on increasing nutrient use efficiency was not properly quantified in GM amended soil. To determine the effect of PM on crop productivity and nutrient use efficiency in GM amended soils, PM and no-mulching treatments were installed under different levels of GM biomass amended soils (0, 25, 50 and 100% of harvested aboveground biomass). The biomass productivity of barley and hairy vetch mixture as cover crop and biomass nitrogen productivity were dramatically increased from the mid May to the early June. PM significantly improved soil temperature and moisture regimes during maize cropping seasons, and then increased inorganic N (NH4+ and NO3-) contents in soils. These improved soil properties under PM highly increased maize productivity and nitrogen use efficiency (NUE). Comparing with no-mulching, as GM application level was increased, the effect of PM on increasing maize productivity became greater, but its effect on increasing NUE became smaller. In conclusion, PM could be very useful tool to improve productivity and NUE of cash crop maize in organic cropping fields, in which the harvesting time of GM biomass might be sustained to increase GM biomass productivity.

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.

Silicate Fertilizer Amendment Alters Fungal Communities and Accelerates Soil Organic Matter Decomposition.

Soil microorganisms play a crucial role in organic matter decomposition and nutrient cycling in cropping systems. Compared to bacteria, fungal community composition and the role of fungi in organic matter decomposition and nutrient cycling in agro-systems are, however, elusive. Silicon (Si) fertilization is essential to improve agronomic performance of rice. The effects of the Si fertilizer application on the soil fungal community composition and their contribution in soil organic matter (SOM) decomposition are not yet studied. We investigated the short-term (120 days) slag silicate fertilizer (SSF) amendment impacts on plant photosynthesis and soil biochemical changes, soil fungal communities (assessed by ITS amplicon illumina sequencing), hydrolytic and oxidase enzyme activities, CO2 emissions, and bacterial and fungal respiration in diverse eco-geographic races of rice (Oryza sativa L.), i.e., Japonica rice (O. sativa japonica) and Indica rice (O. sativa indica). The short-term SSF amendment significantly increased the relative abundance of saprotrophic fungi and accelerated organic matter decomposition. The increase in saprotrophic fungi was mostly attributed to greater labile C availability and Si availability. Higher organic matter decomposition was accompanied by an increase in both hydrolytic and oxidative enzyme activities in response to the SSF amendment. The stimulation of oxidative enzyme activities was explained by an increase in root oxidase activities and iron redox cycling, whereas stimulation of hydrolytic enzyme activities was explained by the greater labile C availability under SSF fertilization. We conclude that the short-term SSF amendment increases saprotrophic fungal communities and soil hydrolytic and oxidative enzyme activities, which in turn stimulates SOM mineralization and thus could have negative feedback impacts on soil C storage in submerged rice paddies.

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.

Source partitioning and emission factor of nitrous oxide during warm and cold cropping seasons from an upland soil in South Korea.

Nitrous oxide (N2O) is a major greenhouse gas (GHG) with high global warming potential. A majority of the N2O flux comes from agricultural sources, mainly due to nitrogen (N) fertilization. The soil N2O flux, induced by N fertilization, mainly originated from two different sources, i.e., fertilizer and soil organic nitrogen (SON). It is essential to know the individual contribution of these two different sources in total N2O flux for planning necessary mitigation strategies. It is also indispensable to know the seasonal difference of emission factors (EF) for having more accurate N2O inventory. Therefore, an experiment was conducted in a South Korean upland soil during summer and winter seasons using 15N labeled urea as an artificial N source and source specific N2O emissions were distinguished under different environmental conditions. To characterize the N2O emissions from urea, 0, 50, 100 and 200% of the Korean N recommendation rate was selected for specified crops. The Korean N recommendation rate for red pepper (Capsicum annuum) and garlic (Allium sativum) was 190 and 250 kg N ha-1, respectively. Direct emissions from urea were estimated from the difference of 15N2O flux emitted from 15N-urea treated soil and the natural abundance of 15N2O. From total N2O fluxes, urea originated N2O flux was 0.87% and 0.13% of the applied N in warm and cold seasons, respectively and the rest comes from SON. Nitrous oxide EF in the warm season was 2.69% of applied N and in the cold season that was 0.25%. Nitrous oxide fluxes showed a significant exponential relationship with soil temperature. The results show the necessity of considering the different N2O EF for warm and cold cropping seasons to reduce uncertainty in N2O inventory. The findings of this research may help better understand N2O source partitioning and the emission budget from warm and cold cropping seasons.