Soil C : N : P Stoichiometry as Related to Nitrogen Addition in a Meadow Steppe of Northern China

The concentrations and stoichiometry of soil carbon (C), nitrogen (N), and phosphorus (P) have critical implications for nutrient cycling and ecosystem function. While their high sensitivity to atmospheric N deposition is well known, it remains unclear for the soil depth-dependence of such responses to N deposition. Here, we examined the responses of soil C : N : P stoichiometry at three soil depths in the upper humus horizon (0-5, 5-10, and 10-20 cm) of Haplic Chernozem (Loamic) across a gradient of urea addition rates (0, 2, 10, 20, and 50 g N m(-2) year(-1)) after five years treatments in a hay-harvest meadow steppe of northern China. We found that the effects of increasing N addition rates on the concentrations and stoichiometry of soil C, N and P did not depend on soil depth, though those parameters varied greatly across different soil layers. Across all soil depths, the concentrations of soil C and N increased with increasing N addition rates, but soil P concentration was not affected by N addition. The higher sensitivity of soil N than soil C to N enrichment resulted in decreasing soil C : N ratio across the N addition gradient, especially for the surface soil layer. Soil N : P ratio showed a positive response to the increases of N addition rates. The unbalanced responses of soil C, N, and P concentrations to N enrichment, as indicated by the decreases of soil C : N ratio and the increases of soil N : P ratio across the N addition gradient, highlighted the role of N enrichment in decoupling soil nutrient cycling in the temperate steppe.