Depth-dependent patterns in the C:N:P stoichiometry of different soil components with reclamation time in coastal poplar plantations

Reclamation changes the ecological processes and environmental conditions in coastal wetlands, and greatly affects the biogeochemical cycling of the ecosystem. However, how afforestation influences C, N, and P stoichiometry in plant-soil systems with reclamation time remains unclear. We studied the variations in C, N, and P concentrations and stoichiometries in different poplar compartments (leaves, roots, and litter) and soil components (bulk soil, microbial biomass, and enzymes) in the soil profiles of one tideland (natural wetland) and three poplar plantations at different stages of reclamation (i.e., reclaimed 24, 44, and 64 years prior to data collection) in East China. The C:N:P stoichiometry of the poplar compartments did not vary with coastal reclamation stage despite there was a weak correlation in C:N:P stoichiometry between the poplar compartments and soil components. Afforestation greatly altered the C:N:P stoichiometry of the different soil components with soil depth and reclamation time, especially the microbial biomass (e.g., N:P ratio), which was mainly ascribed to a decrease in soil P availability. Moreover, the magnitude of variation in C:N:P stoichiometry reduced with soil depth. On the basis of a model of ecoenzymatic stoichiometry, we found that, compared with the tideland, microbial C limitation decreased in the 0-20 cm soil layer and increased in soil layers below 20 cm in the reclaimed lands, whereas the nutrient limitation shifted from P in the topsoil (0-40 cm depth) to N in the subsoil (40-100 cm depth). The microbial metabolism in the soil profile was mainly dominated by abiotic factors (e.g., soil C:N:P ratios and salinity). Additionally, reclamation time had a relatively small effect on the microbial resource use limitations in the soil profile. Overall, afforestation in coastal reclaimed lands reshaped the vertical pattern of the C:N:P stoichiometry of different soil components and aggravated microbial C, N, and P limitations. However, the variation in microbial nutrient requirements among the different investigated reclamation stages was less marked. Our results have practical implications for soil nutrient management in poplar plantations on coastal reclaimed land.