| Long-term carbon and nitrogen inputs is one of the main solutions to ensure the food security on North China Plain (NCP), which is also deeply effect the carbon and nitrogen biogeochemical cycles in this area. How to ensure food security simultaneously achieve environmental friendly and improve soil quality is always the important scientific problem in this area. We set up a long-term field experiment, which includes eight treatments with four nitrogen managements (zero-N control, improved Nmin, calculated N balance and conventional N) and two straw managements (straw removed and straw returned). Firstly, we systematic researched the effects of long-term carbon and nitrogen inputs on crop yield, N utilization, nitrate leaching and N2O emissions by the measurements in situ. Secondly, we systematic researched the effects of long-term carbon and nitrogen inputs on quantity of soil organic carbon and nitrogen, soil aggregates, soil organic carbon fractions, soil basal N2O emissions and potential nitrification and denitrification rates by the determinations in laboratory. At last, we calculate the effects of long-term carbon and nitrogen inputs on the net global warming potential (Net-GWP) and greenhouse gas intensity (GHGI) after integrated consider all carbon resource by all agricultural managements.Compared to conventional farming practice (Ncon and C+Ncon)(11.3and11.6t ha-1yr-1), the improved Nmin tested (Nopt and C+Nopt) could reduce52-57%fertilizer N inputs, but not obviously decreased the crop yield (10.8and11.3t ha-1yr-1). However, compared to conventional farming practice, the calculated N balance (C+M and C+W) could not only decrease45-85%fertilizer N inputs, but also slightly increase the crop yield (12.8and12.4t ha-1yr-1). The long-term conventional farming practice will be decrease the straw C:N ratio of wheat or maize since the excessive nitrogen input, which also make the plants to become more susceptible to insect attack and fungal diseases and can be prone to lodging. Compared to the control, the carbon and nitrogen inputs could enhance the yield stability. The organic fertilizer inputs from the calculated N balance could more benefit for the yield stability.Compared to the zero N treatments, the conventional farming practices have a large number of soil NO3-N accumulation (320-896and449-699kg N ha-1at0-1m depth, respectively;265-873and323-911kg N ha-1at1-2m depth, respectively), the vast apparent N loss (average281and272kg N ha-1yr-1in the whole7years, respectively) and nitrate leaching (average33.7and43.9kg N ha-1yr-1in the3years, respectively) since the N fertilizer overuse. Compared to conventional farming practice, the improved Nmin tested and calculated N balance could significantly reduce soil NO3-N accumulation at0-1m and1-2m depth, the apparent N loss, the nitrate leaching (P<0.05). On the condition of N fertilizer overuse, straw returning could stimulate nitrate leaching. On contrast, straw returning could decrease nitrate leaching under the optimum N input. Organic fertilizer input didn’t increase nitrate leaching. There is a good linear relationship between the soil profile NO3-N accumulation and nitrate leaching. The nitrate leaching is4.2%and4.3%of soil NO3-N accumulation at0-1m and1-2m, respectively, in this kind of soil climate condition on NCP.Compared to the conventional farming practice (3.4-4.5kg N20-N ha-1yr-1), the N2O emissions from the improved Nmin tested could reduce46.1-46.6%. Compared to the straw removed, straw returned could increase N2O emission by26-36%, which also could promote0-20cm SOC content (26.1vs22.4g kg-1). The Net-GWP from the conventional farming practice is4121-3041kg CO2-eq ha-1yr-1. The improved Nmin tested could decrease the Net-GWP by reducing the N fertilizer inputs, which is2594-2409kg CO2-eq ha-1yr-1. The calculated N balance could decrease the Net-GWP not only by reducing the N fertilizer inputs but also increasing the SOC content, which is-1939to-3330kg CO2-eq ha-1yr-1. The GHGI from the conventional farming practice is0.24-0.35kg CO2-eq kg-1grain. The improved Nmin tested and calculated N balance could decrease by27%and141%, respectively. The main emission sources are the power consumption for irrigation, fertilizer N production, and N2O emissions, with SOC sequestration providing the main emission sink.Compared to the initial SOC content (7.7g kg-1), all treatments increased by8.4-40.6%except No (6.9g kg-1) after6years later. On the condition of straw removed, compared to zero N treatment, the improved Nmin tested and conventional farming practice didn’t significantly increase the0-20cm SOC stocks (P>0.05), but significantly increase the0-20cm total nitrogen (TN) stocks (P<0.05). On the condition of straw returned, compared to the zero N treatment, the improved Nmin tested didn’t significantly increase the0-20cm SOC and TN stocks (P>0.05); the conventional farming practice only significantly increase the0-20cm SOC stocks (P<0.05), didn’t increase the0-20cm TN stocks (P>0.05); but the calculated N balance could significantly increase the0-20cm SOC and TN stocks (P<0.05). Compared to the zero N treatment, all treatments didn’t significantly increase the20-40and40-60cm SOC and TN stocks (P>0.05). Compared to the zero N treatment, the improved Nmin tested didn’t significantly increase the proportion of>250f.im soil aggregate and organic carbon (OC) content of all soil aggregates (P>0.05), but it significantly increase the OC content of all carbon fractions (P<0.05). Compared to the zero N treatment, the conventional farming practice didn’t significantly increase the organic carbon (OC) content of all soil aggregates (P>0.05), but it significantly increase the proportion of>250p.m soil aggregate and also significantly increase the OC content of all carbon fractions (P<0.05). However, compared to the zero N treatment, the calculated N balance not only significantly increase the proportion of>250μm soil aggregate (9.4-9.5%vs6.8-7.3%)(P<0.05) and OC content of2000-250μm soil aggregate (31.4-39.9g C kg-1vs21.4-22.2g C kg-1)(P<0.05), but also increase the OC content of iPOM_m and s+c_m by12.2-40.7%and8.5-30.8%, respectively, since a large number of organic matter input. The results of OC content of different carbon fractions support the concept of aggregate hierarchy, which also means that the organic matter is the main binding agents on the formation of soil aggregates. Long-term organic fertilizer inputs from the calculated N balance could not only increase the SOC and TN stocks, but also promote the quality of SOC.Long-term different carbon and nitrogen inputs didn’t significantly influence the soil N2O basal emissions except the C+M. The soil N2O basal emissions are not only from the nitrification but also the denitrification. It is a more precise indicator to reflect the potential nitrification rate by NO3-N generation on this kind of soil. There is a significant positive correlation between the soil potential nitrification rate and soil NH4-N content (P<0.05). Using the gaseous (N2O and N2) production to reflect the potential denitrification rate is a more accurate method on this kind of soil. There are significant positive correlations between the soil potential denitrification rate and SOC and TN content (P<0.05).In conclusion, judging from saving N fertilizer, ensuring crop yield, reducing nitrate leaching and Net-GWP, the improved Nmin tested is an optimum management. However, the improved Nmin tested not only didn’t increase SOC and TN stocks, but also risking decrease the quality of SOC. The organic fertilizer inputs from the calculated N balance is a more optimum management on NCP after integrated consider saving N fertilizer, stabling crop yield, reducing environmental effects and promoting quantity and quality of SOC and TN. Effects of optimizing nitrogen management and straw returning simultaneously would slightly on increasing crop yield and reducing environmental impacts. It is also very important to pay attention to optimum water management and concomitantly optimize carbon and nitrogen management. Nonetheless, these optimum carbon, nitrogen and water managements need to further research in the long-term field experiment. |
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