| Agriculture is a major source of the emissions of reactive nitrogen(Nr).As the increase in nitrogen(N)fertilizer used in crop production,partial N cannot be absorbed by crops and released into the environment in the form of NH3,N2O,NH4+and NO3-,causing environmental problems such as global warming,eutrophication of water,and soil acidification.The middle and lower reaches of the Yangtze River is an important crops production region with widely planting area and diverse cropping systems in China.However,little attention has been paid to the emissions of Nr in different cropping systems in this region.In this study,DNDC model was accordingly implemented to simulate the Nr emissions of rice-wheat(RW),rice-rape(RR),double rice(DR),double rice-rape(DR-R)and middle rice-fallow(MR-F)cropping systems in this region,and to explore the environmental factors affected Nr emissions of different cropping systems.The current study was conducted to identify approaches to optimize crop production,reduce Nr emissions in farmland with the goal of achieving sustainable crops production in China.The main results revealed as follows(1)From 2005 to 2017,there were significant differences in total planting area and total Nr emissions of the rice-based cropping systems among years in study region.The interannual variation range of total planting area of rice-based cropping systems is 10.4≤106～11.2×106 ha.The fitting results showed that it increases significantly with years with an average annual increase of 7.0×104 ha.In which,planting area of RW and DR-R markedly increased with years with the range of 3.07×106～3.61×106 and 0.10～0.12×106 ha;RR,DR and MR-F shows an insignificant trend,with the range of 3.07×106～3.61×106,2.11×106～2.31×106,3.47×106～3.37×106 and 1.69×106～1.79×106 ha,respectively;in addition,planting area of RR and MR showed a downward trend,but planting of MR-F showed a upward trend from 2013-2017.The total Nr emissions was 1.06 Tg N year-1,with the range of 0.95～1.18 Tg N year-1,and showed an increasing trend,with an average annual increase of 0.02 Tg N,in which the total Nr emissions of RW markedly increased with years,but not in other cropping systems.After 2015,total Nr emissions of RR,DR and DR-R revealed a downward trend,but shown an upward trend in DR-F.Significant differences were observed in components of total Nr emissions between different Nr emissions and cropping systems.The contribution of Nr forms to total Nr emissions is shown as NH3(75.0%)＞leaching N(18.2%)＞runoff N(4.1%)＞ N2O(2.7%);the contribution of cropping systems to total Nr emissions is shown as DR(43.2%)＞RW(24.8%)＞RR(18.2%)＞MR-F(11.8%)>DR-R(1.8%).(2)Significant differences in rice equivalent yield per area,N rates,N partial factor productivity(PFPN)among years were observed.The rice equivalent yield per area and N rates showed a significant upward trend in RW,RR,DR and DR-R from 2005 to 2017,but not in MR-F.Additionally,The PFPN of RR significantly decreased with years,but not in other cropping systems.And the range of rice equivalent yield per area were 11.1～12.4,10.5～10.8,11.2～12.1,12.9～14.2 and 7.3～7.4 t ha-1,the range of N rates were 446.8～503.0,310.9～340.9,347.9～378.5,464.2～497.6,212.9～231.6 kg N ha-1;and the range of PFPN were 23.4～26.6,31.9～34.1,31.1-～33.8,27.0～30.4,31.8～35.5 kg kg-1 in RW,RR,DR,DR-R and MR-F cropping system from 2005-2017,respectively(3)There were significant differences in the annual Nr emissions per area of farmland between cropping systems.Nr emission per area were revealed as DR-R(162.0 kg N ha-1 year-1)＞ DR(134.4 kg N ha-1 year-1)＞ RR(84.5 kg N ha-1 year-1)＞RW(80.2 kg N ha-1 year-1)＞MR-F(61.4 kg N ha-1 year-1).In addition,between 2005 and 2017,The Nr emissions per area of the RW and RR increased significantly with the years,and the average annual increase was 2.93 and 0.86 kg N ha-1,respectively,but the Nr emissions per area of DR,DR-R and MR-F were unsignificant with the years(4)There were significant differences in INr(reactive nitrogen intensity)between cropping systems.The INr followed the order of DR-R(11.9 g kg-1 year-1 year-1)＞DR(11.5 g kg-1 year-1)＞MR-F(10.5 g kg-1 year-1)＞RR(7.9 g kg-1 year-1)＞ RW(6.7 g kg-1 year-1).From 2005 to 2017,The INr of RW and MR-F increased with the years,with an average increase of 0.14 g kg-1 and 0.15 g kg-1,however,the INr of DR-R decreased with the years,with an average annual decrease of 0.25 g kg-1,the INr of RR,DR,and MR-F were unsignificant with the years(5)There were significant differences in factors affected the emissions of Nr between cropping systems.Sensitivity analysis shows that N rates,soil pH,and soil organic carbon(SOC)(sensitive index＞0.2)are common factors affected the Nr emissions of farmland in all cropping systems.Addionally,the Nr emissions of RW was impacted by precipitation with the sensitive index of 0.45,the Nr emissions of RR and MR-F was impacted by air temperature and soil bulk density with the sensitive index of 0.50,0.21 and 0.33,0.28,respectively.The Nr emissions of DR was affected by precipitation and air temperature with the sensitive index of 0.41and 0.36,the Nr emissions of DR-R was impacted by soul bulk density and the sensitive index were 0.23.Based on annual yield,DR-R and DR produced higher production,followed by RW and RR,and the production of MR-F is the lowest.Based on Nr emissions,DR-R and DR produced higher Nr emissions,followed by RR and RW,and the Nr emissions of MR-F is the lowest.These results suggested that RW and RR can maintain or increase annual yield and reduce Nr emissions,it should be widely developed in suitable planting regions;in addition,DR produced higher annual yield and Nr emissions,it is therefore urgent to reduce the emissions of Nr of DR system Our results suggested that it is important to reduce Nr emissions in farmland in China by optimizing crops planting patterns and decreasing N rates to mitigate the negative environmental impacts caused by crop production,enhance annual yield,and ensure the sustainable development of agriculture. |
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