Patterns and drivers of soil pH on the Xinjiang temperate steppe

Soil pH is one of the most important factors in regulating plant growth and soil nutrient availability in grasslands. A large number of studies have revealed that soil pH is determined by elevation, climate, and soil physicochemical properties. However, previous studies have focused on the effects of these variables on surface soil pH, while our knowledge about their effects on pH among different soil layers is still limited. To address this knowledge gap, we collected soil pH and related soil physicochemical properties soil organic matter (SOM), soil total nitrogen (STN) content, soil bulk density (SBD), and volume ratio of rock to soil (RRS) of the different soil layers at 0－5, 5－10, 10－20, 20－30, 30－50, 50－70, and 70－100 cm among 86 sites on the Xinjiang temperate steppe during 2011－2013. We also surveyed the surface microenvironment (standing litter, litter, sand covering, and gravel, wind, and water erosion), elevation, and climate mean annual temperature (MAT) and mean annual precipitation (MAP) for each site. Finally, we explored the effects of surface microenvironment on soil pH as well as the relationships between soil pH and elevation, climate, and soil physicochemical properties. Our results showed that on the Xinjiang temperate steppe, soil pH gradually increases, while SOM and STN content gradually decrease with increasing soil depth. Litter, gravel, and wind erosion had no significant effects on pH (P > 0.05). However, standing litter decreased pH at the 0－50 cm layer, while water erosion only increased pH at the 0－20 cm layer and sand covering decreased pH at the 70－100 cm layer. Besides, pH in all soil layers was positively associated with elevation. Soil pH at the 0－10 cm layer also gradually increased with increasing MAT. With increasing MAP, pH at the 0－30 cm layer gradually decreased. In addition, pH at most soil layers was negatively correlated with SOM and STN content, while pH was tightly associated with SBD and RRS in only a few soil layers. Taking all these factors together, we found that on the Xinjiang temperate steppe, SOM, STN content, RRS and MAT play a more important role in regulating pH variation than other factors. These results showed that the changes of soil pH on the Xinjiang temperate steppe were simultaneously affected by surface microenvironment, elevation, climate, and soil physicochemical properties, but their relative importance differed among different factors and soil layers.