Patterns of soil microbial nutrient limitations and their roles in the variation of soil organic carbon across a precipitation gradient in an arid and semi-arid region.

The effects of precipitation patterns on the metabolism of soil microbes are poorly understood, especially in water-limited ecosystems where soil microorganisms play crucial roles in the turnover of soil organic carbon (SOC) and nutrients. We investigated the influence of the gradient levels of mean annual precipitation (MAP from 300 to 900 mm) on soil microbial metabolism in an arid and semi-arid grassland region located in Loess Plateau, China and identified relationships between microbial metabolic limitations and the variation of soil organic matter (SOM). Microbial metabolism in this arid and semi-arid region was limited by soil C and phosphorus (P) or nitrogen (N). Microbial C and P limitations decreased with the increase of MAP. Microbial C and P limitations were lowest in the areas with MAPs of 700-900 mm, whereas N limitation was observed in the areas with MAPs >700 mm. The results of a variation-partitioning analysis and partial least squares path modeling indicated that the microbial C and N/P limitations on regional scales were mainly determined by climate factors (MAP and mean annual temperature (MAT)), followed by vegetation biomass and soil properties. The extents of soil drying-rewetting processing caused by different MAPs directly affected microbial nutrient limitation. Our results suggested that the influence of precipitation variation on microbial metabolic limitation strongly governed SOM stability and that an increase in the rate of SOM decomposition with increasing precipitation could be caused by increased microbial nutrient limitation. SOM may be most stable at a MAP of 700 mm in the arid and semi-arid regions (300-900 mm MAP) where microbial nutrient limitation was lowest. This study provided novel insights into the responses of soil microbial metabolism to precipitation change and is an important step toward understanding the mechanisms of SOM stability in an arid and semi-arid grassland ecosystem under scenarios of precipitation variation.