[Effect of Straw Application on the Dynamics of Exogenous Nitrogen and Microbial Activity in Paddy Soil].

Returning straw to the field provides an important source of fertilizer that can increase soil fertility. However, the rate of straw carbon utilization is low and large amounts of greenhouse gases are emitted due to the high carbon to nitrogen ratio of the straw mass. In this regard, the application of inorganic nitrogen and phosphate fertilizers can control the ratio of elements in the soil, increase the activity of microorganisms and their utilization of elements, and promote the improvement of soil fertility. In this study, straw application conditions were simulated, and inorganic nitrogen fertilizer labeled with 15N was added to examine the effects of different nutrient fertilizer additions on the transformation and distribution of exogenous nitrogen in the soil, and also the characteristics of the microbial response. The results showed that application of straw increased the contents of ammonia nitrogen and total nitrogen in the soil and soil solution. When both straw and inorganic nitrogen fertilizer were applied, the 15N-TN in the soil remained at 28 to 33 µg during the 100-day culture phase. In contrast, 15N-NH4+ increased gradually during the initial 30 days of the culture phase, but subsequently decreased gradually. Application of phosphate increased the contents of 15N-TN and 15N-NH4+ in the soil, but decreased the content of 15N in the soil solution by 28%. The distribution of inorganic nitrogen in the soil showed that the proportion of 15N in the soil remained at 52%-61%. Addition of phosphate fertilizer increased the distribution ratio of 15N in the soil by up to 16.5%, whereas the proportion of 15N in the soil solution decreased from 36% on the fifth day to 30% on the 100th day, thereby the loss amount of 15N reduced by 1.2-fold. Addition of straw promoted microbial activity and significantly increased the microbial biomass nitrogen (MBN) content of the soil. Addition of inorganic fertilizer further promoted the microbial activity of the soil. After the 100-day culture experiment, the addition of straw, inorganic nitrogen, and phosphate fertilizer increased MBN to between 2.0-fold and 2.2-fold that of the control treatments. Addition of phosphate fertilizer increased the utilization of 15N by microorganisms, so that the amount of 15N-MBN was 1.5-fold higher than that of treatments where only straw and nitrogen fertilizer were added. Examination of soil enzyme activity showed that nitrogen fertilizer reduced soil enzyme activity and substrate affinity. When both nitrogen and phosphate fertilizers were added, the enzyme activity was 48.1% higher than that when only straw was added. The findings of this study thus provide a theoretical basis for furthering our understanding on the nitrogen cycle of the paddy soil ecosystem, the improvement of soil fertility, and the reduction of greenhouse gas emissions.

[Vertical Distribution Characteristics of Typical Forest Soil Organic Nitrogen in Dawei Mountain].

To clarify altitudinal gradient of subtropical forest soil total nitrogen and organic nitrogen, soil samples were collected per 10 cm on soil profile (0-100 cm) in Dawei Mountain, researched the variation of soil organic nitrogen and correlation with soil physical and chemical properties. The results showed that: (1) Total nitrogen, acid hydrolysable organic nitrogen and soluble organic nitrogen decreased with the increase of depth, content of each component in mountain granite yellow-brown soils was much higher affected by altitude; (2) The average percentage of soil organic nitrogen to total nitrogen was 97.39% ± 1.17%, and soil acid hydrolysable organic nitrogen was 64.38% ± 10.68%, each component decreased with the increase of soil depth; (3) Soil soluble organic nitrogen content was 9.92- 23.45 mg x kg(-1), free amino acids (1.62 - 12.02 mg x kg(-1)) accounted for about 27.36% ± 9.95% of soluble organic nitrogen; (4) Soil acid hydrolysable organic nitrogen and soluble organic nitrogen were significantly positively correlated with total nitrogen, total soluble nitrogen and inorganic nitrogen (P < 0.05), were highly significantly correlated with soil bulk density, organic carbon, and total phosphorus (P < 0.01). Organic nitrogen was the main body of soil nitrogen in typical subtropical forest, each component showed a downward trend increase with soil depth affected by altitude and soil physical and chemical properties. There was a close conversion relationship between soil organic nitrogen and other nitrogen forms, the characteristics of soil organic nitrogen will have profound impact on nitrogen cycling of forest ecological system.