Influencing Factors And Potential Estimation Of The Carbon Sequestration And Emission Reduction From Farmland Soil With Biochar Amendment

China's farmland soil has great potential for carbon sequestration.Soil and biochar system carbon sequestration is a new carbon sequestration technology that combines biochar carbon capture and soil carbon storage.It mainly includes three aspects:biochar carbon sequestration during production process,soil greenhouse gas emission reduction and promotion of plant carbon sinks.The impact of biochar on the increase and emission reduction of greenhouse gases in farmland soil is closely related to biochar,soil properties and agricultural processes,and is a major difficulty in the research.So far,there are still controversies over the soil greenhouse gas emission reduction effects of biochar under the impact of different agricultural processes.Therefore,this paper explores the effects of agricultural practices(nitrogen application amount and field aging)on the increase and emission reduction of greenhouse gas in farmland soil induced by biochar;estimates the farmland soil and biochar system carbon sequestration and emission reduction potential in China,and the contribution rate of the biochar induced soil emission increase or reduction to the biochar carbon sequestration potential during production process.The main research results are as follows:(1)The CO₂ and N₂O mitigation potential of biochar in the soils varied significantly with the amount of nitrogen fertilizer applied(P<0.05).When the amount of nitrogen fertilizer was low(50 mg N/kg),biochar significantly increased soil CO₂ and N₂O emissions(P<0.01);when the amount of nitrogen fertilizer was high(450 mg N/kg),biochar weakened the soil CO₂ emission increase effect,and significantly reduced N₂O emissions(P<0.01).When nitrogen input was low,soil N₂O mainly came from nitrification,and the unstable components of biochar promoted microbial nitrification to promote the increase of N₂O,promoted microbial chemical heterotrpphism,etc.to promote the increase of CO₂.When nitrogen input was high,soil N₂O mainly came from denitrification,biochar mainly inhibited denitrification through the adsorption and protection of nitrogen,and promoted N₂O emission reduction.(2)Field aging made the surface of biochar accumulate minerals and form an organic-mineral complex,thereby enhancing the physical stability of the particles(such as mechanical strength)and significantly reducing soil CO₂ and N₂O emissions.The results of SEM-EDS,XRD and FTIR together shown that after field aging,biochar surface mineral element content significantly increased,with some oxygen-containing functional groups increased simultaneously.The physical stability(compressive strength and surface hardness)of the particles were significantly improved compared with those biochars pre-aging(P<0.01).Field aging enhanced the physical stability of biochar,weakened the release of debris components and their effects on soil microorganisms,thereby enhancing the potential for carbon sequestration and emission reduction.(3)The carbon sequestration potential of biochar in farmland soil was mainly determined by the biomass mass that could produce biochar each year,and the impact of biochar on the greenhouse gas emission of farmland soil was small(about 3%on average).It was conservatively estimated that if biochar was produced from five types of biomass(agricultural straw,forestry waste,poultry manure,sludge and algae),China could fix about 290±40 million tons of carbon(equivalent to reducing emissions of 1050±150million tons of CO₂)every year.When these biochar were applied to farmland in China(at the addition amount of 1?2%)for carbon sequestration,the annual carbon dioxide-equivalent(CO_2e)emission from farmland soil(under the macroscopic background)could be reduced by about 35±109 million tons,compensating relatively small proportion of the carbon sequestration potential of biochar.