Changes in soil C:N:P stoichiometry along an aridity gradient in drylands of northern China

Changes in soil carbon (C): nitrogen (N): phosphorus (P) stoichiometric ratios have significant influences on biogeochemical cycles and ecosystem functions. Soil C, N, and P dynamics are closely related to climatic aridity and soil texture, two factors that are not easily separated at large spatial scale due to their geographic co-variation. While it is well understood that soil C, N, and P cycling are decoupled and soil C:N:P ratios change across climatic gradients, we know little about the role of soil texture in mediating such stoichiometric variation. Here, we examined the regional variation of soil C:N:P ratios with changes in aridity and soil texture based on observations from 57 sites along a 3000-km aridity transect in drylands of northern China. Across the aridity gradient, C:N ratios of bulk soil first decreased and then increased with increasing aridity with a threshold at the aridity value of 0.8 (aridity defined here as 1-precipitation/evapotranspiration), whereas C:P and N:P ratios of bulk soil significantly decreased with increasing aridity. Across the transect, C:N ratios in bulk soils were positively related to sand contents and negatively related to silt and clay contents, while C:P and N:P ratios in bulk soils were negatively related to sand contents and positively related to silt and clay contents. Aridity, soil pH, and silt contents could best predict the variations in bulk soil C:N and N:P ratios, while aridity, soil pH, and clay contents were the most significant variables driving the bulk soil C:P ratio. Aridity could play a more important role than soil texture in driving variations of soil C:N:P ratios, where the predicted increases in aridity in drylands could cause imbalances in the biogeochemical cycles of C, N and P. Our results further highlight the role of soil texture in driving stoichiometric flexibility of soil C:N:P ratios across the aridity gradient, in that element-specific correlations with soil texture played an important role in regulating the decoupled soil C:N:P stoichiometry in response to increasing aridity.