Abstract: Soil water content is an important factor driving microbial activities related to soil nitrogen (N) transformation. In this study, ¹⁵N pair tracing technique combined with the numerical model FLUAZ was used to investigate the gross N mineralization, immobilization, nitrification, and denitrification rates in grassland and forest land soils from Beian City, Heilongjiang Province, China under laboratory condition [60% or 100% water holding capacity (WHC)]. The responses of soil gross N transformation rates to soil water content changes, and the mechanisms of N production, consumption, and conservation in soil under different water conditions and its environmental effects were elucidated. The results showed that changes of soil water content did not affect gross rates of N mineralization and NH₄⁺ immobilization in the forest land and grassland soils. Increasing soil water content from 60% WHC to 100% WHC significantly increased soil gross nitrification rate in forest land soil, but not affect that in grassland soil. Gross denitrification rates in grassland and forest land soils were close to zero under 60% WHC, and significantly increased under 100% WHC. The rate in grassland soil was significantly lower than that in forest land soil. In forest land soil, the ratio of gross nitrification to NH₄⁺ immobilization rates (gn/ia) and N₂O emissions under 100% WHC were significantly higher than that under 60% WHC. N₂O emission from grassland soil under 100% WHC was significantly higher than that under 60% WHC, but without significant difference in gn/ia between the two water conditions. Our results indicated that increasing soil water content in the short-term may increase the negative environmental effects of nitrogen cycling in grassland and forest land soils, especially in forest land soil.

Key words: water content, forest land, grassland, ¹⁵N trace, nitrogen transformation