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

- North-facing shrublands had a notably lower soil temperature than south-facing meadows.
- Gross and net soil nitrogen mineralization was faster in meadows than in shrublands.
- Soil inorganic N pool was larger in shrublands than in meadows.
- We illustrate the complexity of predicting soil N dynamics in mountainous regions.

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 (7.9 g m− 2) than under the shrubland (3.7 g m− 2) in the upper 15 cm soil layer, and 155% greater under the meadow (22.2 g m− 2) than under the shrubland (8.7 g m− 2) in the upper 30 cm 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 7.5 cm 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.