黄土高原北部生长季土壤氮素矿化对植被和地形的响应

氮素矿化是陆地生态系统氮循环的重要过程,对氮素有效性有着重要影响。本文在黄土高原北部六道沟小流域选取退耕年限相近的油松和柠条坡地,用原位培养法测定生长季节(4—10月)不同坡位冠层下和冠层外0~10 cm和10~20 cm土层土壤氮素矿化速率,以确定该区氮素矿化的季节动态特征和主要影响因素。结果表明,研究区生长季土壤矿质氮以铵态氮为主,其含量在0~10 cm和10~20 cm土层分别占矿质氮总量的61%和70%,并随生长季的推移而升高。油松林上坡位和中坡位土壤铵态氮显著高于下坡位土壤,柠条林不同坡位铵态氮差异不显著。土壤硝态氮和矿质氮不受坡位的影响,但与林型和采样位置有关,冠层下硝态氮在油松林与冠层外相近,在柠条林则高于冠层外。生长季土壤氮素矿化在0~10 cm土层由硝化作用引起,在10~20 cm土层则由硝化和铵化作用共同引起。铵化速率在生长季初期较高,中期较低,并受坡位、林型和采样位置的影响。土壤硝化和矿化速率在油松林不受采样位置影响,但是在柠条林则以冠层下较高。硝化和矿化速率在冠层下以下坡位土壤最高,在冠层外则以下坡位土壤最低。柠条林促进了冠层下土壤氮素的硝化和矿化过程,有利于矿质氮的积累;油松林对矿质氮和氮素矿化的影响不受采样位置影响。

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.