Effects of water condition and nitrogen level on soil dissolved nitrogen compounds

A ventilated incubation experiment was conducted to investigate the dynamic trends of the soil NH+4-N, NO3--N and soluble organic nitrogen(SON)contents under different water conditions and nitrogen levels in two paddy soils, Alluvial soil and Purplish clayey soil. During incubating period, the content of NH+4-N in the Purplish clayey soil is significantly higher under the waterlogged incubation than that under the watercontrolled incubation, whether nitrogen fertilizer is added or not. However, the content of NH+4-N in the Alluvial soil is not obviously enhanced under the waterlogged incubation or nitrogen fertilization, except for being distinctly elevated at the 7th day of the waterlogged incubation with nitrogen fertilizer addition. The NO3--N contents of the two soils are significantly increased under the watercontrolled incubation with or without nitrogen fertilization. Whereas, aforesaid phenomena exhibits during whole incubation period in the alluvial soil but starts from the 21st day of incubation in the Purplish clayey soil. The NO3--N content is significantly increased in the Alluvial soil with nitrogen fertilization under both the watercontrolled condition and waterlogged condition. While, the NO3--N content of the Purplish clayey soil with nitrogen addition is not noticeably increased under the waterlogged condition. The contents of SON in the two soils are gradually increased and reach the highest level during the period of 21th day to 35th day of incubation, and then are rapidly declined to their original levels. On the peak period, the ratio of SON to TSN can reach to 80% in the Alluvial soil and 60% in the Purplish clayey soil, respectively. These results indicate that the nitrification activity in the Alluvial soil is very strong not only under the watercontrolled condition but also under the waterlogged condition. It is, therefore, important to pay much more attention to the NO3--N leaching loss in the Alluvial soil, and the risk of soluble organic nitrogen losses in the two paddy soils during its peak period could not be neglected either.