Nitrogen Transformation Characteristics From Soils Under Different Cultivation Patterns And Fertilizer

In the highly-intensified agriculture, high amount of chemical fertilizer applied continuously, especially the nitrogen fertilizer, can not only influence the crop yield but also decrease the fertilizer use efficiency. And this way is not favourable to maintain and increase soil quality, even lead to the serious ecological environment problem. So it is the key section to solve the agricultural production issues that using the different ways in the regulation of nitrogen supply. Numerous studies show that changing the cultivation model, and appling different types of organic materials can adjust the soil C, N relations, to coordinate the supply of soil nitrogen, thereby improve nitrogen use efficiency. However, there was little studies about the different cutilvation patterns and fertilizer (especially the straw-returned) on the soil organic nitrogen fractions, nitrogen mineralization potential, soil microbial biomass carbon, nitrogen and the mutual relationship. In this study, we used the long-term field experiment located at the south edge of the Loess Plateau in Shaanxi and the six year long-term positioning experiment of wheat-maize rotation in Yangling as the main objective, and further integrated the ratio-design experiment of legumes straw (soybean) and gramineae straw (maize) to study the effects of different straw-returned patterns on the nitrogen transformation characteristics. The main results were as follows:1. The six year long-term positioning experiment of wheat-maize rotation was used to compare the effects of different cultivation patterns and nitrogen application rates on the N supplying ability of soil. The results showed that compared with the conventional control treatment , the contents of total hydrolysable N , microbial biomass nitrogen (SMBN) and mineralized nitrogen potential (N0) of soil under straw mulching treatment (SM) were significantly increased, and the fractions of different forms of soil organic nitrogen were enhanced. The effect of furrow cultivation pattern (FM) also increased the contents of total hydrolysable N and N0, the increasing rates were lower than SM pattern, and FM pattern decreased the contents of SMBN. The addition of nitrogen fertilizer increased the contents of total hydrolysable N and N0 in the soil; and the contents of amino acid N, amino sugar N, ammonium N were also increased. However, the application of N fertilizer decreased the content of SMBN, especially for the 240 kg?hm-2 N rate. The correlation analysis indicated that there was significantly positive correlation between the N0 and SMBN and amino sugar N, hydrolysable unidentified N (P<0.05), implying these three fractions maybe the major contributors to the soil mineralized nitrogen.2. The long-term field experiment located at the south edge of the Loess Plateau in Shaanxi was used to study the effects of different fertilizer treatments on the forms of organic nitrogen and N0 in the soils. The results showed that the range of organic nitrogen forms in soil was in the following order, amino acid N>non-hydrolysable N>hydrolysable unidentified N>ammonium N>amino sugar N. Compared to the un-fertilization (CK) treatment, the addition of chemical fertilizers (NPK treatment) increased the different forms of organic N in soil, but the increasing rate was low. Compared with the CK treatment, the SNPK (NPK plus straw) and MNPK (NPK plus manure) treatments significantly increased the contents of different fractions of organic nitrogen in soil, especially the content of amino acid N, whereas they decreased the proportion of total hydrolysable N accounted for total nitrogen. Application of MNPK significantly increased N0 and the rate of N mineralization. The results showed that long-term application of NPK with manures or crop straw is an efficient way to enhance the nitrogen supplying capacity of soil. The significantly negative correlation was found between amino sugar N and N0 (P<0.05); and the negative correlationships between hydrolysable unidentified N and non-hydrolysable N and soil N0 were not statistically significant (P>0.05), implying amino sugar N is the major contributor to the soil mineralized nitrogen.3. To further research the effects of straw-returned pattern on the soil nitrogen mineralization and remineralization characteristics, we use the centroid design with differents of nitrogen source including Gramineae (Maize) and Legume (Soybean) crop straw and fertilizer (Urea). The results showed: the F treatment significantly increased the minerilized-N content, the following was F-M,F-S,F-M-S, and the straw-adding lowest. In the whole incubation days, the carbon mineralization rate presented the fastest in the F-M-S treatment. At the same time, the F-M-S increased the soil nitrogen microbial biomass content (SMBN) and the remineralized N rate by SMB, wherease the SMBN decreased in the F treatment and mineralized N rate showed the negative value in the early incubation days. The experiment demonstrated that using the different organic and inorganic nitrogen resource can control the soil nitrogen mineralization and remineralization, in order to coordinate the nitrogen supply and maintenance and it is necessary to do the field experiment.