This study explored the responses of gross N transformation rates in soil to moisture content changes in a semi-arid area of Heilongjiang Province, in order to improve knowledge regarding the processes of N production, consumption, and loss, and provide a scientific basis for rational application of N fertilizer in farmland soils. Sandy loam soil was collected to study the gross rates of N transformation in soil under 60% water holding capacity(WHC), 100% WHC, and flooding conditions, using a 15N labeled technique and the numerical FLUAZ model under laboratory incubation conditions. The results showed that the gross N mineralization, gross N immobilization, gross nitrification, and gross denitrification rates in soil under 60% WHC were 1.87, 1.16, 2.84, and 0.01 mg·kg-1·d-1, respectively. Increasing soil moisture content to 100% WHC had no significant effect on gross N transformation rates in soil. Under flooding conditions, gross N mineralization and immobilization rates were increased to 2.45 and 2.15 mg · kg-1 ·d-1, gross nitrification rate was decreased to 1.13 mg · kg-1 ·d-1, and gross denitrification rate was increased to 0.65 mg · kg-1 ·d-1, and all differences were statistically significant as compared to the 60% WHC treatment. The ratios of gross nitrification to ammonium immobilization rate(gn/ia)and gross N mineralization to immobilization(gm/gi)were all higher than 1 in 60% WHC and 100% WHC treatments, whereas the gn/ia ratio(0.55) was lower than 1 and the gm/gi ratio(1.14)was close to 1 under flooding conditions. The N supply and conservation capacity was low in sandy loam soil under unsaturated conditions and was vulnerable to the accumulation and leaching of NO3-. Flooding promoted denitrification in sandy loam soil, but the processes of N mineralization and immobilization were tightly coupled in soil, thus increasing the supply and turnover of N. Moreover, the nitrification was greatly inhibited, decreasing the risk of NO3- leaching.
