Response And Simulation Of Soil Nitrogen Dynamics To Different Fertilization In Typical Farmland Of Northern China

Soil total nitrogen（TN）is a central contributor for soil quality and crop productivity.N fertilizer application as an important strategy for improving crop production has been used worldwide,while excessive N application and low nitrogen efficiency is a hot issue in China.Moreover,climate change has brought great challenge to agriculture system.So,it is essential to indicate the response of soil nitrogen to long-term fertilization and quantify the effects of future climate change on soil nitrogen,aiming at finding an effective fertilization to maintain soil fertility,keep a sustainable agroecosystem and reduce future climate risk in China.However,studies on soil N dynamics and nitrogen apparent use efficiency（NAUE）are mostly focused on case or field characteristics,and the temporal and spatial evolution of soil nitrogen dynamics under different fertilizations and the future fate under climate change are rarely reported.Thus,based on well managed long-term experiments across China with different climate,soil types and cropping systems,the purposes of current study were:1)to quantify the effects of fertilizations on TN content and 2)soil available nitrogen during historical experimental period,3)to analyze the SMBN characteristics under different fertilizations and climate zones by Meta,4)to analyze the nitrogen apparent use efficiency of wheat and maize system in Northern China.5)to evaluate SPACSYS model on modelling TN stocks and to predict future changes of TN stocks and N balance under different fertilizations with different climate conditions.The main results and conclusions were as following:1.Manure amendment（hNPKM and NPKM）improved soil N fertility furthest with soil TN content and soil nitrogen fixation efficiency（NFE）of Northeastern China higher than North and Northwestern China,and the magnitude of the increment for later period was higher than former period during the whole experiment compared with no fertilizer（CK）.The responses of TN content to long-term fertilization in typical farmland in Northern China were obviously different among different sites and fertilizations and ranged as high amount manure combined with chemical NPK fertilizer（hNPKM）>manure combined with chemical NPK fertilizer（NPKM）>NPK chemical fertilizer plus straw（NPKS）>NPK chemical fertilizer（NPK）.Manure amendment had the best benefits for improving soil nitrogen fertility with a significant improvement of 0.025-0.054 g kg^-1 yr^-1 for hNPKM and0.006-0.042 g kg^-1 yr^-1 for NPKM.STN content for hNPKM and NPKM were 50.4-107.8%and29.9-113.2%higher than that of CK,and 43.5-75.8%and 18.9-55.9%higher than that of NPK,respectively.The STN content with manure amendment treatments in different sites was followed as:GZL>YL>UM>CP>ZZ（hNPKM）and GZL>YL>XZ>UM>YL>XZ>YL>PL>CP>ZZ（NPKM）,which generally indicated that Northeastern China was higher than other regions.The NFE of NPKM and hNPKM was higher than other treatments with the NEF of 12.0～74.2%and 10.8～54.8%,respectively.It should note that soil nitrogen fertility improvement effects of hNPKM and NPKM during the first decade and second decade for all sites increased STN content by 27.7-59.8%and12.7-52.1%,73.6-161.8%and 30.7-112.2%,respectively,compared to CK.Generally,the increasing of soil N fertility showed that STN content in 11-20 years was higher than that in the first decade for hNPKM and NPKM.2.Similarly,manure combined with chemical NPK fertilizer also improved soil N available fertility furthest with soil available nitrogen content（AN）of Northeastern China higher than North and Northwestern China,and the magnitude of the increment for later period was higher than former period during the whole experiment compared with CK.The responses of AN content to long-term fertilization ranged as hNPKM>NPKM>NPKS>NPK.hNPKM improved soil AN content significantly in GZL,ZZ and UM（P<0.01）with a rate of 2.69-4.698 mg kg^-1 yr^-1.AN content with hNPKM increased by28.8-145.2%in GZL,CP,ZZ and UM compared to CK,and increased by 29.0-93.6%in GZL,ZZ and UM.NPKM increased AN content significantly in GZL,ZZ,UM and ZY with rates of 2.15-2.90 mg kg^-1 yr^-1,and AN content with NPKM significantly increased by 29.4-84.1%in GZL,ZZ,PL,WM and ZY compared to CK,and increased by 16.1-45.3%in GZL,ZZ,PL and UM.The AN content of manure amendment treatments in different sites was followed as:GZL>UM>ZZ>CP（hNPKM）and GZL>XZ>UM>ZZ>PL>ZY（NPKM）,which generally indicated that Northeastern China was higher than other regions;In addition,the same as STN content,hNPKM and NPKM increased AN content significantly by 23.3-60.8%and 27.7-37.7%in first decade and 33.1-212.8%and 42.3-121.3%in second decade compared to CK.Generally,the increasing of soil AN fertility showed that AN content in11-20 years was higher than that in the first decade for hNPKM and NPKM.3.Manure application（NPKM and M）can promote soil microorganisms growth and improve soil fertility.Manure application increased SMBN by 97.5%and 57.3%,respectively,across all the observations compared to CK and NPK.The increment of SMBN contents improved by manure in Northeast region was the largest（67.2%）compared with NPK.The increments of SMBN contents improved by manure in temperate continental climate and temperate monsoon climate were similarly（54.4%and 60.7%,respectively）compared with NPK.For the acidic soils（pH<6）,the increment of SMBN contents by 111.62%in the treatment with manure applied were higher than those in the alkaline（pH>8,53.6%）and neutral soils（pH 6-8,48.8%）,compared with NPK,respectively.The increase in SMBN under manure addition was higher in soils with low TN and SOC content than in soils with high TN and SOC content.There was a significant positive correlation between TN and SMBN.The InRR of SMBN between manure and NPK has a significant positive correlation with manure nitrogen input per year（P<0.01）.4.Manure amendment can increase nitrogen apparent use efficiency and nitrogen uptake.The NAUE in NPKS was（88.1%）lower than in other treatments（93.7-95.8%）in GZL,and the average of NAUE was no significant difference in different treatments（with NAUE:38.5-51.0%）.In XZ,the NAUE in NPK（69.3%）was larger than that in NPKM（53.8%）.In ZZ,the NAUE in NPKM（61.5%）was larger than that in NPKS（51.8%）.There was no significant difference in different treatments in PL,and the NAUE of wheat and maize was 56.9-67.3%and 59.1-78.4%,respectively.In UM,the NAUE in NPK was the largest（51.9%in wheat and 44.4%in maize）.Overall,the annual increase rate of nitrogen uptake and NAUE in NPKM,was significantly larger than in other treatments.5.Future climate change with elevated temperature,increasing precipitation and high CO₂ concentration would lead to a decrease of soil TN stocks in Northeastern China,while would increase TN stocks in North and Northwestern China,and manure amendment had the most potential to face future climate risk as soil-crop system in Northern China with manure amendment could gain the highest N sink in future.The SPACSYS model is used for simulation and Representative Concentration Pathway（RCP）scenarios of Intergovernmental Panel on Climate Change（IPCC）were used for prediction.SPACSYS is able to simulate soil total nitrogen content in Northern China accurately with the R² of 0.78 and 0.70,RMSE of 5.33%and 6.98%for calibration and validation,which showed a better model performance in statistic test.Without considering the impact of cultivar change,the soil total nitrogen stocks under future climate change would decrease by 0.1-14.4%in Northeast China,increase by 2.2-26.2%in North China and increase by 4.7-34.5%in Northwestern China,respectively.For each climate scenario,manure amendment could have the most positive effect on TN stocks in Northern China,and TN reached to 3.67-4.30 t N ha^-1,3.32-5.68 t N ha^-11 and 3.40-5.58 t N ha^-1 for Northeastern China,North China and Northwestern China.In addition,future climate change would decline the N sink level of crop-soil system with NPKM in Northeastern China(8.75 kg N ha^-1 yr^-1-17.78 kg N ha^-1 yr^-1),which ranged as Baseline≥RCP2.6,RCP4.5>RCP8.5.However,future climate increase N sink level in North China with no difference among three RCP scenarios(91.14-174.82 kg N ha^-1 yr^-1).Specially,future climate had no influence on N sink level in Northwestern China(4.67-49.24 kg N ha^-1 yr^-1)with current application.Among all treatments,only NPKM gain a N sink in soil-crop system in Northeastern China,and NPKM gained the highest N sink gain in crop-soil system North(120.84-174.82 kg N ha^-1 yr^-1)and Northwestern China(31.62-49.24 kg N ha^-1 yr^-1).Thus,manure amendment is the most appropriate fertilizer strategy in Northern China as it is beneficial to the improvement of soil TN,NFE,AN,SMBN and nitrogen apparent use efficiency;Under future climate change,North China and Northwestern China have stronger nitrogen fertility potential than Northeastern China,and the soil-crop system with manure would increase the nitrogen sink level furthest,which indicated manure combined with NPK would be the reasonable fertilizer strategy to remit future climate risk.