Transformation Of Soil Carbon And Nitrogen And Their Simulation On Dryland Under Different Mulching

There are a lot of problems on Eum-Orthric Anthrosols in Guanzhong region of Shaanxi Province,such as low water use efficiency and low soil productivity.This study is based on the irrigation experiment station of the Key Laboratory of Agricultural Soil and Water Engineering,Northwest A&F University,Yangling,Shaanxi,China.Field trial of different mulching modes was conducted as no mulching(NM),straw mulching(SM),half plastic film(HWF)and full plastic mulching(WF)under the rotation of winter wheat and summer maize to discover the effects of different mulching on topsoil organic carbon(SOC),total nitrogen(TN),microbial biomass carbon(MBC)and nitrogen(MBN),soil temperature,and the distribution and accumulation of water and nitrate in soil profiles.DNDC model was verified and evaluated by these data from the trial,and the long-term dynamics of soil organic carbon was simulated under different mulching in the region.The results showed that:1)Compared with the first season(2013~2014 winter wheat),the SOC and TN storage in topsoil(0-20 cm)under SM after the harvest of the fifth season(2015~2016 winter wheat)were increased by 20.8% and 30.2% on rainfed dryland,respectively,and were increased by5.5% and 10.1% on irrigated dryland,respectively.SM increased the soil minimum temperature of 15 cm and 30 cm in the early stage of winter wheat growth on rainfed dryland,while reduced the soil mean temperature of 15 cm and 30 cm soil in the middle and later stage.SM reduced the soil mean temperature of 15 cm and 30 cm in the growth period of wheat and maize on irrigated dryland.On both of rainfed and irrigated dryland,SM increased soil organic carbon and total nitrogen,and the content and accumulation of NO3--N in topsoil,also increased the soil water storage in 0-20 cm and 0-100 cm soil layer,thus uprised the soil depth of the cumulative peak of NO3--N in the 0-200 cm profile,while decreased the accumulation of NO3--N in the 0~200 cm profile.Straw mulching was beneficial for soil fertility,meanwhile to reduce the risk of nitrate leaching to deeper soil.2)With the extension of trial time,the plastic film mulching(HWF and WF)reduced thecontent and storage of soil organic carbon and total nitrogen in topsoil on both of rain-fed and irrigated condition.Compared with the harvest of the first season,the content and storage of SOC and TN after the fifth season were reduced by 4.0% and 3.2% under WF,and were reduced by 8.0% and 0.7% under HWF on rainfed dryland.While they reduced by 2.5% and7.2% under WF,and were reduced by 5.2% and 7.6% under HWF on irrigated dryland.Plastic film mulching will deplete the soil fertility,and is harmful to the improvement of soil fertility.The plastic film mulching increased the content and accumulation of NO3--N on topsoil,and uprised the soil depth of the cumulative peak of NO3--N in the 0-200 cm profile.With the increase of mulching period,the accumulation of NO3--N under the plastic film mulching was significantly lower than that under SM in 0-200 cm profile.In term of soil water storage of topsoil,WF was better than SM and HWF.While in term of the increasing of the minimum temperature and mean temperature of 15 cm and 30 cm soil in the early stage of winter wheat growth on rainfed dryland,HWF was better than WF and SM.Regardless of the rainfed or irrigated condition,in term of the increasing of the soil max temperature and mean temperature of 15 cm and 30 cm soil in the late stage of summer maize growth,WF was better than HWF.Both of MBC and MBN under WF and HWF were all lower than that under SM at the crop harvest stage on rainfed dryland.3)The correlation coefficient was relatively high between the actual observation of SOC and crop yield under different mulching and the simulation of them by DNDC model on rainfed dryland,and the most of the NRMSE of actual observation and simulation were acceptable,which means that DNDC model could be used to predict the crop yield and SOC dynamics on dryland under long-term different mulching on Eum-Orthric Anthrosols in Guanzhong region of Shaanxi Province.The results of 50 years' simulation showed that plastic film mulching had a tendency to increase SOC in topsoil(0-20 cm),while straw mulching did not significantly improve SOC in a long run,which is different from the results of short-term field monitoring,and the reason is worthy to be analyzed further.In conclusion,straw mulching was beneficial to improve soil fertility,to increase the soil water storage of 0-20 cm and 0-100 cm,while to reduce the risk of nitrate leaching to deeper soil.The plastic mulching could increase the capacity of soil water storage,the content and accumulation of NO3--N on topsoil,while reduce the accumulation of NO3--N in profile and the risk of leaching to deeper soil.Meanwhile it would deplete soil fertility,and was harmful to soil fertility.Thus,straw mulching is a kind of cultivation mode that was worthy to be recommended in Northwest China.There were no a large amount of actual observation data to test the long-term simulation effect of DNDC model because of the relatively short time oftrial,therefore,the long-term effect of film mulching on SOC sequestration deserves the further research and attention.