Soil C:N:P stoichiometry in plantations of N-fixing black locust and indigenous pine, and secondary oak forests in Northwest China

Soil nutrient concentrations and stoichiometry are important indicators of plant growth, terrestrial productivity, and ecosystem functioning. Nevertheless, little is known about the vertical distribution and the environmental factors influencing the spatial patterns of different forest types under the "Grain for Green" program and the "Natural Forest Resources Protection" project in Northwest of China. We collected 114 soil profile samples within a 0-100-cm depth from black locust and Chinese pine plantations, and secondary oak forests. We determined the vertical distributions of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and their ratios along environmental gradients in Shaanxi Province, Northwest China. The results showed that both SOC and TN concentrations decreased exponentially within the soil profiles of the three forest types, but there was minimal change in TP. Significant differences in SOC, TN, and TP were found in the surface soil layers among the forest types. Both SOC and TN were relatively low in the N-fixing black locust plantations and TP was comparatively low in the Chinese pine plantations. The C:N:P ratios decreased with increasing soil depth for the three forest types. These ratios were comparatively high in the Chinese pine plantations, relatively low in the black locust plantations, and moderate in the oak forests. The differences in the ratios among the three forest types were more significant in surface soil than in deep soil. Precipitation was positively correlated with the concentrations of SOC and TN and the ratios of C:N:P. Temperature was negatively correlated with concentrations of SOC and TN and the ratios of C:N:P across all soil depths. A log-transformed linear C-N relationship was found for all three forest types, suggesting a well-constrained coupling between the levels of the two elements. Our results demonstrated the effect of different tree species on soil C:N:P ratios and their controlling factors within soil profiles along environmental gradients. The secondary oak forest accumulated soil C and N more effectively than the plantations. The Chinese pine plantations were relatively more susceptible to P limitation. Therefore, the mechanism of different plant species on soil biogeochemical processes at the whole soil profile level must be considered when developing forest management strategies and implementing vegetation restoration projects