[Effects of long-term fertilization on paddy soils organic nitrogen, microbial biomass, and microbial functional diversity].

Soil samples were collected from the plow layers at two long-term experiment sites in Xinhua and Ningxiang counties of Hunan Province, China to study the effects of long-term fertilization on organic nitrogen, microbial biomass, and microbial functional diversity of paddy soils. Long-term fertilization showed great effects on the soil N content. Compared with CK, treatments NPK plus manure or straw increased the contents of soil total acid-hydrolysable N and its fractions amino sugar N, amino acid N, and ammonium N. Treatment NPK had no significant effects on soil microbial biomass C and N, but treatments NPK plus manure increased the contents of soil microbial biomass C and N significantly. BIOLOG test showed that treatments NPK plus manure enhanced the carbon utilization efficiency of soil microbes, and improved the functional diversity of soil microbial communities, compared with CK. Long-term different fertilizer treatments resulted in the differences of carbon substrate utilization patterns of soil microbial communities.

[Effect of long-term fertilization on the diversity of nitrite reductase genes (nirK and nirS) in paddy soil].

In order to investigate the effects of long-term application of nitrogen fertilizer on soil denitrifying communities, the diversities of nir genes (nirK and nirS) were studied using molecular approaches in the long-term paddy field experiment (started in 1990) located in Taoyuan. Analysis of clone sequences indicated that the nirK fragments from paddy soil showed close similarity (90.7%) to the nirK sequences registered in GenBank database, but were not related to any known strain. Whereas, most of the airS clones showed low similarity (74.7%) to the nirS gene fragments registered in GenBank. The Chao1 estimates showed that the diversity of nirK gene 13) OTUs] than in N treatment [(49 +/- 9) OTUs], but the difference was not significant. However, application of nitrogen fertilizer resulted in significant difference of nirS-community compared to CK. Nitrogen fertilizer had obvious effect on tbe community structure of nirK-denitrifiers (p < 0.022), but the nirS-containing community was not affected. Based on phylogenetic analysis, nirK clones grouped into three clusters with aggregations of some OTUs cloned from N treatment. Although nirS clones grouped into four clusters, the majority of the clones were attributed in one cluster. The results suggested that application of nitrogen fertilizer had a greater influence on the diversity of nirS-containing bacterial community than that of the nirK. However, the community structure of nirK-containing denitrifiers was more sensitive to nitrogen fertilization than that of the mrS.