Effects Of Agricultural Practices On Soil Carbon Stocks And Greenhouse Gas Emissions

In order to realize the sustainable development of agriculture,the primary goals of China's government are to increase soil carbon sequestration and reduce the N fertilizer input rate on the premise of ensuring food security.We use the meta-analysis to integrate the results of 70 long-term experiment sites,and to examine the responses of SOC and crop yield to straw incorporation under different influencing factor levels.North China Plain is a typical area of intensive agriculture,in which the high N fertilizer input rate(?600 kg N ha-1 yr-1),frequent irrigation would lead to a higher environment risk and shortage of water resource.Two experiments were established to evaluate the effects of optimized agriculture management practices on crop yield,soil C stock,N2O and CH4 emission and the Net GWP.Experiment 1 including seven treatments:conventional(CON:600 kg N ha-1 yr1 +maize straw removed/burned+rotary tillage+winter wheat-summer maize),optimum(OPT:balanced N fertilization+dereased irrigation+straw incorporation+deep plowing+winter wheat-summer maize),no tillage(NT:balanced N fertilization+dereased irrigation+straw incorporation+no tillage+winter wheat-summer maize),water saving I(WS I:balanced N fertilization+dereased irrigation+straw incorporation+rotary tillage+winter wheat-summer maize-spring maize),water saving ?(WS ?:balanced N fertilization+dereased irrigation+straw incorporation+rotary tillage+winter wheat-summer soybean-spring maize),water restoration(WR:balanced N fertilization+dereased irrigation+straw incorporation+rotary tillage+spring maize-fallow),organic farming(ORG:organic fertilizer application+dereased irrigation+straw incorporation+rotary tillage+winter wheat-alfalfa-summer maize-alfalfa).Experiment 2 including six treatments:no N fertilizer(CK),urea only(U),urea with nitrification inhibitor(NI),urea with urease inhibitor(UI),urea with combined nitrification inhibitor and urea inhibitor(NIUI).The main findings are as follows:Meta-analysis showed that straw incorporation significantly sequestered SOC(0-20 cm depth)at the rate of 0.33 Mg C ha-1 yr-1,increased crop grain yield by 9.8%and had a conversion efficiency of the incorporated straw C of 11%in NC;the combined straw incorporation at the rate of 3 Mg C ha-1 yr-1 with mineral fertilizer of 200-400 kg N ha-1 yr-1 was demonstrated to be the best farming practice with crop yield increased by 32.7%(17.9%-56.4%)and SOC sequestrated by the rate of 0.85(0.54-1.15)Mg Cha-1 yr-1;the SOC responses were highest in the initial starting phase of straw incorporation then subsequently declined and finally became negligible after 28-62 yr,and crop yield responses were initially low and then increased,reaching their highest level at 11-15 yr after straw incorporation.Field experiments showed that,comparing with the CON,the reducing of 22.2%and 42.7%N fertilizer input in OPT and NT didn't significantly decrease the crop yields,the crop yield of NT tended to increase with the experiment duration;the N fertilizer input rate and irrigation volume of other optimized agricultural management practices could be reduced by 58.3%-79.7%and 26.5%-37.1%,while the crop yield were also decreased by 20.5%-44%.The averaged SIC stock in the 0-200 cm soil profile was 363.92 Mg C ha-1,which is three times of SOC stock(120.39 Mg C ha-1),this highlights the importance of SIC in the estimation of soil C sequestration in arable land of Northern China;Comparing with the initial level,the SOC stocks of CON,OPT,NT,WS ?,WS ? and ORG were improved by 38%?89%,133%,136%,121%,27%and 36%,respectively,SIC stocks were changed by-3.4%?0.9%?4.8%?-3.6%?-0.3%?4.6%?0.8%,respectively.The accumulated N2O emissions of CON was 3.3 kg N2O-N ha-1 yr-1,the accumulated N2O emissions of OPT,NT,WS ?,ORG were 2.3,2.2,1.9 and 1.6 kg N2O-N ha-1 yr-1,which could be reduced by 30.2%,33.6%,41%,51.2%,comparing with CON.Field study displayed that 83%-87%of the total emission were derived from the indirect emissions,which means it should be pay more attention on the N fertilizer production and electricity generation in the policy making.Comparing with U treatment,NI,UI,NIUI and the CRP treatments could significantly reduce the N2O emission by 25%-57%;NI and CRF treatment could annually decrease the GWP by 17%and 19%,respectively,without reducing the crop yield.Overall,our results highlights that,comparing with conventional agricultural practices,the implement of "100%straw incorporation+optimized N fertilizer+non-tillage" could achieve a high level of crop yield,and improve the SOC and SIC stock by 68%and 8.5%,reduce the N fertilizer and irrigation volume input by42.7%and 16.2%,decrease the accumulative N2O emission and GWP by 32%and 80%,respectively;there is a larger potential for reducing the N2O emission,if the urea fertilizer could be combined with nitrification inhibitor or replaced by controlled-release fertilizer.