Responses of soil nitrogen mineralization during growing season to vegetation and slope position on the northern Loess Plateau of China

Nitrogen (N) mineralization is critical for nitrogen cycle in terrestrial ecosystems and significantly influences the availability of soil N. In this paper, we studied the changes in soil mineral N and N mineralization rates in slope lands in the northern Loess Plateau region in relation to vegetation types, sampling sites and slope positions during vegetation growing season. The objectives of the study were to determine the dynamics of N mineralization during growing season and the influencing factors. Slope lands with Chinese pine (Pinus tabulaeformis) and korshinsk peashrub (Caragana korshinskii) were selected in the Liudaogou catchment and an in situ mineralization method was used to measure soil N mineralization for the period from April through October. The measurements were conducted in upper, middle and lower positions of the slope with under-and non-under-canopy at the 0-10 cm and 10-20 cm soil depths. Soil mineral N in the growing season was dominated by ammonium, which accounted for 61% and 70% of total mineral N at the 0-10 cm and 10-20 cm soil depths, respectively. The proportion of ammonium to total mineral N increased during the growing season. Soil ammonium in the upper and middle slope positions was significantly higher than that in the lower slope position for Chinese pine, but not affected by slope positions for korshinsk peashrub. Furthermore, soil ammonium was not affected by sampling site in both Chinese pine and korshinsk peashrub plantations. Soil nitrate and total mineral N were affected by vegetation type and sampling site, rather than by slope position. Under-canopy soil nitrate was similar to that of non-under-canopy in Chinese pine vegetation, but it was greater than that for non-under-canopy in porshinsk peashrub vegetation. Soil N mineralization during growing season resulted mainly from nitrification at the 0-10 cm soil depth, but also influenced by both nitrification and ammonification at the 10-20 cm soil depth. Ammonification rate was significantly high during the early growing season and low during the mid growing season. Moreover, ammonification rate was affected by slope position, vegetation type and sampling site. Nitrification and net N mineralization rates in under-canopy soils were similar to those in non-under-canopy soils in Chinese pine vegetation, but it was greater than that in non-under-canopy soils in korshinsk peashrub vegetation. Additionally, when compared with other slope positions, lower slope position had highest nitrification and net mineralization rates of soil N in under-canopy conditions, but it was the lowest in lower slope position in both Chinese pine and korshinsk peashrub plantations. The results suggested that korshinsk peashrub enhanced nitrification and mineralization of N in soils in under-canopy conditions, while the effects of Chinese pine on soil mineral N and N mineralization were not related to sampling site.