[Effects of long-term application of fertilizers on soil microbial biomass nitrogen and organic nitrogen components in subtropical paddy soils].

The effect of long-term fertilization on soil organic nitrogen components and microbial biomass nitrogen (B(N)) in paddy soils from two experiment sites in Hunan province were studied. Soil samples were collected from the plough layers of different fertilizer treatments. Soil B(N) was measured by the fumigation-extraction method, and soil organic N was fractionated by acid hydrolysis-distillation method according to the scheme of Bremner (1965). Results showed that the soil N increased 40 mg x kg(-1) every year at Ningxiang site (low N level) for 17 years under the application combined of fertilizers and manure, while that at Nanxian site (high N level) was 55 mg x kg(-1). Soil total nitrogen (T(N)), total hydrolysable nitrogen (THN) and microbial biomass nitrogen(B(N)) were increased by long-term combined application of chemical fertilizer and manure (NPKM). NPKM significantly increased the content of T(N), B(N), total hydrolysable nitrogen (THN), ammonia acid nitrogen (AAN), hydrolysable unidentified nitrogen (HUN) and the percentage of B(N) to T(N). Besides, NPKM increased the easily mineralizable B(N), AAN, and low decomposed HUN. There was positive correlated relationship between B(N) and THN and different THN components, and the effect of AAN and HUN on B(N) was biggest. It is obvious that NPKM increased soil fertility and enhanc the nitrogen-supplying capability of paddy soils. NPKM had the effect on increasing soil nitrogen capability of paddy soils, both easily decomposed fractions and difficultly decomposed ones.