Depth-dependent soil C-N-P stoichiometry in a mature subtropical broadleaf forest

As the balance of multiple chemical substances in ecological interactions, stoichiometry of soil carbon (C), nitrogen (N) and phosphorus (P) is critical for forest sustainability. Soil depth is critical in regulating soil C-N-P stoichiometry; however, the vertical pattern of soil C-N-P stoichiometry remains unclear at the local scale in mature subtropical forests. Here, we sampled 555 soil columns from 185 grids at three soil depths (i.e., 0-20, 20-40, and 40-60 cm) in a mature subtropical evergreen forest of eastern China. We found the C:N, N:P, and C:P ratios decreased with increasing soil depth. However, the strength of bivariate correlations among C, N, and P converged from the top to the deepest soil layer. Such vertical convergence was jointly driven by decreasing C-N (R-2 as 0.84, 0.80, and 0.76) and increasing C-P (R-2 as 0.11, 0.26, and 0.31) correlations from 0-20 to 40-60 cm depths. Further analyses with a structural equation model showed that the spatial variations in C, N, and P were influenced by different environmental factors. For example, the spatial variations in soil organic C and total N in the top soil layer were largely influenced by soil pH, whereas the spatial variation in total P was jointly influenced by topographical, biotic, and soil factors. Our results validate the important impact of soil depth on soil C-N-P stoichiometry at the landscape scale. The converging bivariate correlations between C, N and P along the increasing soil depth indicate the depth-dependent roles of different nutrient elements in soil C cycling.