Different Carbon Addition Effects On Greenhouse Gas Emissions And The Underlying Mechanisms In Semi-arid Northwest China

Agroecosystems has become very important for its role in achieving food security and also in the mitigation of greenhouse gas emissions.Different conservation tillage techniques,carbon sources?biochar and straw?,and N fertilizers have been shown to boost soil and crop productivity.However,an improved understanding of the effects on greenhouse gas emission and the underlying mechanisms associated with the production of greenhouse gases is required to enhance agriculture sustainability in semi-arid areas.A study was carried out to investigate the effect of carbon addition sources and patterns on nitrous oxide?N₂O?,methane?CH₄?and carbon dioxide?CO₂?emissions,soil properties and crop productivity in rainfed spring wheat cropping.The study was conducted at the Rainfed Agricultural Experimental Station?35°28?N,104°44?E,elevation 197-m above-sea-level?of Gansu Agricultural University,Dingxi,Gansu Province in Northwestern China.Experimental work of the carbon addition sources included the application of straw,biochar and N fertilizer?no carbon?.The straw and biochar were applied alone or combined with N fertilizer(50 and 100 kg N ha^-1),whilst the N fertilizer was applied at 0,50and 100 kg N ha^-1.The carbon addition pattern studied included;conventional tillage with straw removed?T?,no till with straw removed?NT?,no till with straw retention?NTS?and conventional tillage with straw incorporated into the soil?TS?.The experiments were arranged in a randomized complete block design with three replications.Main findings are summarized as follows:All the treatments served as sources of atmospheric N₂O,and a sink of atmospheric CH₄throughout the sampling period.The pattern of seasonal N₂O,CH₄ and CO₂ fluxes showed,the higher emissions occurred mostly when soil temperature and soil moisture were high.Overall,CO₂ emissions reduced with increasing N fertilization rates under all treatments;and the greatest N₂O emissions and CH₄ sink were recorded under the highest rate of N fertilization(100 kg N ha^-1).Biochar treated soils produced the lowest N₂O and CO₂ emissions,and the greatest CH₄sink.The mean cumulative N₂O emission of biochar and straw treated soils were averaged at1.83 kg ha^-1 and 2.03 kg ha^-1,representing a decreased of 32.44%and 19.57%compared to no carbon soils,respectively.The cumulative CO₂ emission was lower in biochar treated plots(2538 kg ha^-1),followed by no carbon soils(2805 kg ha^-1)and the greatest by straw treated plots(2928 kg ha^-1).Application of biochar in combination with 100 kg N ha^-1 had the greatest CH₄sink than zero N treatment on soils without carbon treatment(-3.06 kg ha^-1 versus-2.14 kg ha^-1).No tillage treatments resulted in significant decrease in cumulative N₂O and CO₂ emissions,and a significant increase in CH₄ sink compared to conventional tilled plots;but the greatest effect occurred on straw treated plots.The mean cumulative N₂O and CO₂ fluxes were lower under no tillage with straw retained(2.01 kg ha^-1)and(2696 kg ha^-1)compared to conventional tillage with straw removed(2.64 kg ha^-1)and(3257 kg ha^-1)soils,respectively.No till with straw retention increased cumulative CH₄ sink by 25.71%compared to conventional tillage with straw removed.The study indicated that biochar amended soils with 100 kg N ha^-1 and no tilled soils with straw retention had lower global warming potential.Higher soil organic C,saturated hydraulic conductivity and lower bulk density and temperature occurred with the application of 100 kg N ha^-1 under all carbon sources.Both biochar and straw treated soils produced the lowest bulk density?0-5 cm?at 1.15 g cm^-3,and no carbon soils the highest at 1.20 g cm^-3.Soil organic C and total N were lower with no carbon soils,followed by straw treated soils and the greatest in biochar treated soils.At a depth of 0-10cm,it was found that biochar amended soils contained higher soil C(13.01 g kg^-1),representing an increase of 26.87%(10.25 g kg^-1)and 17.14%(11.10 g kg^-1)compared to soils without carbon amendment and straw treated soils,respectively.Soil bulk density?0-10 cm?was lower in straw amended soils(1.19 g cm^-3)than that of non-straw amended soils(1.30 g cm^-3),but the effect was higher on no tillage plots.In addition,the contribution to increased microbial biomass carbon and soil organic carbon was much higher under straw retention treatments.No tillage with straw retained and conventional tillage with straw incorporated treatments significantly increased soil organic C by 20.81%and 10.60%,respectively,at a depth of 0-10 cm,compared to conventional tillage with straw removed treatments.Cumulative N₂O,CH₄ and CO₂ emissions positively correlated with soil temperature but negatively correlated with microbial biomass and soil organic C.These data indicate that saturated hydraulic conductivity,microbial biomass C,soil organic C,and carbon-to-nitrogen ratio are crucial drivers of greenhouse gas emissions.The greatest yields were found with 100 kg N ha^-1 fertilization under biochar,straw and soils without carbon,but the greatest effect occurred on the biochar amended soils.Biochar treated soils produced the greatest grain yield at 1906 kg ha^-1,followed by straw treated soils at1643 kg ha^-1,and soils without carbon gave the least at 1553 kg ha^-1.Biochar amended plots(100 kg N ha^-1)and no tillage with straw retained plots had lower emission intensities.Increased grain yield in biochar plots translated into greater C inputs(7764 kg C ha^-1 yr^-1),followed by straw amended plots(5678 kg C ha^-1 yr^-1),and no carbon plots the lowest at 1068 kg C ha^-1 yr^-1.Soils treated with biochar also had positive C balance and to a lesser extent soils treated with straw and negative soil C balance was recorded in no carbon amended soils.No tillage with straw retained treatments produced the greatest grain yield of 1809 kg ha^-1 on average than that of conventional tillage with straw removed(1280 kg ha^-1)and no till with straw removed(1337kg ha^-1).No tillage with straw retained and conventional tillage with straw incorporated had positive soil C balance,but the effect was greater on no tillage plots.The lower C inputs under treatments without straw translated into negative soil C balance.Application of biochar with 100 kg N ha^-1 and no tillage with straw retained decreased soil bulk density and increased soil hydraulic conductivity and soil organic C;the increased carbon inputs,in terms of biochar and straw application and the less soil disturbances explained the favorable soils conditions achieved under those treatments.Biochar treated soils with zero N application and no tillage with straw retained soils decreased cumulative N₂O emissions.Application of biochar with 100 kg N ha^-1 decreased total C output and increased CH₄ uptake due to improved soil physical and chemical properties.Improved soil quality in biochar amended plots and no tillage with straw retained plots translated into higher biomass production and therefore grain yield in those treatments.Reduced overall emissions in these treatments translated into a dual benefit of lower emission intensities and global warming potential.In general,combined application of biochar with 100 kg N ha^-1 and no tillage with straw retention consistently increased crop productivity,improved soil quality and reduced greenhouse gas intensity in rainfed spring wheat cropping and was a suitable agricultural practice in this semi-arid region.