Nitrogen Transfodrmation In Soil Of Loess Plateau During Summmer Fallow

The arid and semiarid regions are large in our country, accounting for 52.5% of the total arable land, mainly in the north of China. The Loess Plateau is the typical rainfed agriculture area; and water and nutrients are two key factors limited crop yield. Because of the limitation of water resources, after harvesting winter wheat, famers always leave the fields fallow to restore soil fertility. The area of summer fallow lands is more than 2 million hectares, accounting for about 40% of cultivated area in the region. In recent years, the large application of nitrogen fertilizer in dry farmland increased the crop yields significantly. At the same time, N fertilizer application also increased the residual nitrogen in the soil profile after harvesting crop. About more than 60% of annual rainfall in Loess Plateau occurred in summer fallow period from July to September. Whether the high temperature and abundance of water during summer fallow can influence soil nitrogen mineralization? Will the larger rainfall lead to soil nitrogen loss? These are important issues worth to study.Therefore, we studied the changes of topsoil nutrients, changes of soil nitrogen mineralization and nitrogen accumulation and leaching in 0-200 cm soil profile changes during the summer fallow in Yangling, and Changwu, Shaanxi, to evaluate the effects of summer fallow on soil nitrate nitrogen accumulation and leaching, and provide a theoretical basis for the management of soil nitrogen in dryland. The main results are as follows:1. Two years’ study showed that soil organic matter, total nitrogen and available phosphorus, potassium contents had no significant change after summer fallow. On the contrary, the content of mineral N in topsoil significantly increased after summer fallow. It may be due to the high temperature and humidity, which promote the mineralization of soil organic matter. The effect of summer fallow on soil fertility is mainly reflected by the increase of mineral nitrogen contents.2. After harvesting winter wheat, the amount of nitrate accumulation in 0-200 cm soil profile in Changwu farmlands were 173.3 kg/ha(n=5) and 186.0 kg/ha(n=10) in 2013 and 2014, respectively. And NO3--N accumulation increased with the increase of nitrogen application in Yangling; and it reached 498.77 kg/ha, while the nitrogen application was 240 kg/ha.3. The Stanford and Smith aerobic incubation method was used to study the changes of nitrogen(N) mineralization from soils of different farmer’s fields and in different nitrogen application treatments before and after summer fallow in the Loess Plateau. Compared to the N mineralization from soils before summer fallow, the N mineralization from soils after summer fallow in Changwu and Yangling decreased by 29.52% and 7.15%, respectively. The cumulative mineralized N also was decreased significantly in long-term no N fertilizer treatment after summer fallow; however, for the two different N fertilized treatments, the differences were significant. The correlation analysis showed that there were significant positive correlations between the cumulative mineralized N and soil organic matter and total N before summer fallow. Two pools of first-order kinetic equation was used to model the curves of N mineralization from different soils. The results indicated that the easily mineralized N potential(N1) in soils of Changwu and Yangling after summer fallow were decreased by 85% and 66%, respectively. Therefore, summer fallow significantly reduced the easily mineralized N in soil.4. The leaching of nitrate in soil profile was closely related to the precipitation during the summer fallow. When rain fall was low, NO3--N accumulation peak moved up in soil profile after summer fallow, and NO3--N mainly accumulated in the 0-100 cm soil layer. In high rainfall years, the leaching of soil NO3--N was serious. In our study, we found that NO3--N accumulation peak in the soil profile in the N240 treatment moved down about 80 cm after summer fallow when the rainfall reached 356.5 mm; and soil NO3--N accumulation was mainly concentrated in the 100-200 cm profile. Therefore, soil NO3--N leaching in the soil profile during the summer fallow is worth to study.