Effects of aspect–vegetation complex on soil nitrogen mineralization and microbial activity on the Tibetan Plateau

Abstract

Little is known about the effects of geomorphologic features on the in situ mineralization of soil nitrogen (N) in high-altitude region. We evaluated the effects of aspect–vegetation complex on the in situ (net and gross) mineralization of soil N in 2012 and 2013. Two hill sites were randomly selected, both with vegetation naturally divided by a ridge into south-facing meadows and north-facing shrublands. Despite lower total soil organic carbon and N contents, the gross and net rates of soil N mineralization were greater in meadow than in shrubland at both sites. Over the growing season, the total cumulative net N mineralization across both sites averaged 114% greater under the meadow (7.9gm−2) than under the shrubland (3.7gm−2) in the upper 15cm soil layer, and 155% greater under the meadow (22.2gm−2) than under the shrubland (8.7gm−2) in the upper 30cm soil layer. The faster gross and net soil N mineralization in the meadow was primarily ascribed to 2–9°C higher daily soil temperature compared to the shrubland, measured at 7.5cm soil depth from June to October 2013. Despite the slower mineralization of soil N in the shrubland than in the meadow, there was an abundant inorganic N pool in the topsoil under the former compared to the latter. This was possibly linked to a lower uptake of N by the plants as a result of the slower growth of shrubs than herbaceous grasses. We concluded that the aspect–vegetation complex profoundly affected the cycling of soil N in mountainous areas of the Tibetan Plateau, highlighting the complexity in predicting soil N dynamics in future climatic and land management scenarios.