Coupling and Decoupling of Soil Carbon and Nutrient Cycles Across an Aridity Gradient in the Drylands of Northern China: Evidence From Ecoenzymatic Stoichiometry

Drylands are characterized by stressful conditions with the limitation of both carbon (C) and nutrients, particularly nitrogen (N) and phosphorus (P). Biological C, N, and P releases from soil organic matter by enzymes are essential components for biogeochemical cycles and are sensitive to the climate in drylands. However, how the ecoenzymatic C:N:P stoichiometry responds to environmental factors (i.e., climatic and edaphic factors) over broad geographical scales remains largely unclear. We examined the patterns of ecoenzymatic C:N:P ratios across a 3,700‐km aridity gradient (0.43 < aridity < 0.97) in northern China. In wetter sites (aridity < 0.70), the relative C:N:P acquisition ratios via enzymes remained relatively constant with increasing aridity. In contrast, in drier sites (aridity > 0.70), the enzymatic C:nutrient (N and P) ratios declined as the aridity increased, while the enzymatic P:N ratios were mostly lower than those in the wetter sites. In drier sites with low C availability, the increasing carbon use efficiency and the increasing proportion of C converted to biomass (than the proportion of respiration) contributed to the declines of the enzymatic C:nutrient ratios as the aridity increased. The overall lower enzymatic P:N ratios were related to the higher soil P availability compared with N availability (higher organic P and lower soil NH 4 + :available P ratios) in drier sites. Overall, our findings indicate that intrinsic linkages of biological C, N, and P acquisitions and cycles were broken at the aridity threshold of 0.70, with higher acquisition efforts for N and P (particularly for N) with increasing aridity in drier sites with aridity > 0.70.