| Straw incorporation is an important measure to maintain and enhance soil fertility. It is essencial to study the effects of long-term straw returning on soil physical, chemical, and biological properties and soil nitrogen(N) cycle. In this study, soil was collected from three long-term experimental stations(Wangcheng, Beibei, and Jinxian), with different long-term treatment(i.e., no fertilization(CK), chemical fertilization only(NPK), and chemical fertilization with straw incorporation(NPKS)). Indoor simulation experiment was taken to exam soil N adsorption-desorption characteristics among top soil(0-20 cm) and sub-top soil(20~40 cm). We used classic soil physical and chemical analysis and 16 S RNA sequencing technology to find the long-term straw incorporation effects on soil physical, chemical, and biological properties among different sites, aiming to learn the impact factors of soil N fixation and release. Indoor incubation experiment was taken to exam the effects of adding different N sources on soil N transformation under the conditions of sterilization and non-sterilization. The main results were as followed:1. Compared with the CK treatment, soil p H(0-20 cm) values were reduced in the NPK and NPKS treatments at Wangcheng and Beibei. Long-term straw incorporation with chemical fertilization significantly increased the levels of soil organic matter, total nitrogen, ammonium N, nitrate N, available P and available K contents for both soil depths(0-20 cm and 20-40 cm). Especially for the sites under double rice cropping rotations(Wangcheng and Jinxian), soil nutrients were enhanced by long-term straw returning. Nitrospira is the dominant bacteria in different treatment among the sites. Long-term straw returning increased the proportion of dominant species. The proportion of Nitrospira in soil was significantly higher at Wangcheng and Beibei than that at Jinxian. Principal component analysis(PCA) showed that there was significant difference in soil microbial community structure between the double rice cropping rotation and rice-wheat cropping rotation. However, little difference was found between Wangcheng and Jinxian which were both under double rice cropping rotation.2 The effect of long-term straw incorporation on soil N adsorption-desorption characters among different sites. For the top soil(0-20 cm), soil ammonium N adsorption capacity was showed as CK>NPK>NPKS at Wangcheng and Beibei, while little difference was found among the treatments at Jinxian. Ammonium N adsorption and desorption capacity was higher in the top soil than that in the sub-top soil. And there was no difference of the adsorption and desorption capacity among the treatment in the sub-top soil. The amount of N adsorption was highest at Beibei, where N desorption lagged as compared with N adsorption, with lower desorption amount(reducing 386.79 ~ 1291.78 mg/kg, 41.65%~ 63.50%).3 Using the top soil of NPK and NPKS treatment from Wangcheng site, indoor cubation experiment was taken under the conditions of sterilization and non-sterilization. The results showed that adding urea, straw and straw with urea to the soil would significantly increase soil ammonium N and nitrate N contents, expressed as Urea>Urea+Straw>Straw>control. Soil ammonium N maintained at a high level under sterilization incubation(30~130 days). However, soil Nitrate N, microbial N and soluble organic N content were signifant low as compared with soil under no sterilization conditions. The results indicated that the process of conversion to nitrate N was blocked under sterilization conditions. For the treatment under no sterilization, soil microbial N and soluble organic N reached the maximum value after cultured for 20 days and 30 days in all treatments. Among which, soluble organic N level was highest in the Urea+Straw treatment. Compared with the long-term application of chemical fertilizer, long-term application of straw with chemical fertilizer significantly enhanced soil ammonium N, nitrate N, microbial N and dissolved organic N significantly. Compared with the Urea treatment, 15 N ammonium N proportion was reduced to 16.01%-20.85% in the Urea+Straw treatment. And 15 N ammonium N proportion was high under sterilization during the 0-30 days after incubation. The results show that adding different N resouces especially straw N could significantly increase the adsorption capacity to the ammonium N of the soil on the basis of long-term location experiment. And soil sterilization would reduced N adsorption and desorption capacity to ammonium N. |
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