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.

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.

Creating new value of blast furnace slag as soil amendment to mitigate methane emission and improve rice cropping environments.

Blast furnace slag (BFS), a by-product of iron making, has been utilized as silicate fertilizer in Korean and Japanese rice paddy. Silicate fertilizer, which has high contents of active iron and manganese as electron acceptor, was newly known to suppress methane (CH4) emission in flooded rice paddies, but the effect of its long-term application on rice cropping environment is still debatable. To evaluate the effect of silicate fertilization on suppressing CH4 emissions, the changes of CH4 index, indicating the ratio (%) of seasonal CH4 flux at the silicate fertilization treatment to that at the control, were generalized using the global investigation data (42 observations from 8 fields in Bangladesh, China, and Korea). Seasonal CH4 fluxes significantly decreased with increasing silicate fertilization levels. In CH4 index changes, 1.5 Mg ha-1 of silicate fertilizer application (the recommended level of rice cultivation in Korea) decreased by 15% of seasonal CH4 fluxes. Rice grain yield highly increased with increasing silicate fertilization rates and maximized at approximately 4 Mg ha-1 with 18% higher than no-silicate fertilization due to overall improvement of soil properties. To evaluate the long-term silicate fertilization effect on rice cropping environments, silicate (1.5 Mg ha-1 year-1) and non-silicate fertilization treatments were installed in a typical temperate-monsoon climate paddy field in South Korea in 1990. Periodic silicate fertilization significantly increased rice grain productivity by an average of 14% over the control for the last 28 years. This fertilization evidently improved rice quality without changes in chemical quality. Consecutive silicate fertilization effectively improved soil physical and chemical properties but did not increase any acid extractable heavy metal concentration in soil. In conclusion, BFS as silicate fertilizer could be a beneficial amendment to mitigate CH4 emission in the rice paddy and improve soil properties and rice productivity and quality without hazardous material accumulation.

Improving methane mitigating functionality of blast furnace slag by adding electron acceptor.

Blast furnace slag (BFS), a byproduct of iron-producing process, has been applied as silicate fertilizer in rice paddy. Silicate fertilizer contains lime and silicate as main components and iron and manganese as electron acceptors. This amendment improves soil productivity and mitigates methane (CH4) emissions. However, its suppression effect was limited to <20 % at a field level, and its functionality needs improvement to encourage recycling. We hypothesized that the effect of silicate fertilizer on suppressing CH4 emission might improve by increasing electron acceptor concentration. To investigate the feasibility of electron acceptor added silicate fertilizer on increasing CH4 flux suppression, four byproducts of the iron-production process (basic oxygen slag-BOF, ferromanganese slag-FerroMn, iron rust, and Kambara reactor slag-KR) were selected and compared through soil incubation test. Iron rust effectively suppressed CH4 production by 67 %, which is comparable with a 15-30 % reduction of others. To find the optimum mixing ratio of iron rust, it was mixed to BFS with the rate of 0-5 % (wt wt-1), and their effect on CH4 flux was compared. The 3 % mixing ratio highly increased the BFS functionality on suppressing CH4 production. To confirm the field adaptability of the improved BFS, three types of silicate fertilizer (mixing iron rust with the ratios of 0, 2.5, and 5 %) were applied with the recommendation level (1.5 Mg ha-1) before rice transplanting. Seasonal CH4 flux was significantly decreased by the original silicate fertilizer (BFS0) application to 20 % over control. This effectiveness was enhanced by adding 2.5 % iron rust but thereafter, not more increased. Silicate fertilization (BFS0) significantly increased rice grain productivity by 9 % over control, and the improved silicate fertilizer (BFS2.5 & 5.0) more highly increased by 13 %. In conclusion, the BFS's functionality to increase rice productivity and suppress CH4 emission could be improved by adding an effective electron acceptor such as Fe2O3.