Dynamics of carbon, nitrogen, and phosphorus stocks and stoichiometry resulting from conversion of primary broadleaf forest to plantation and secondary forest in subtropical China

Large-scale primary native broadleaf forests (BF) have been converted to secondary forests (SF) and plantation forests (PF) in subtropical China over the past decades. However, how and what magnitude of plant and soil carbon (C), nitrogen (N), and phosphorus (P) stocks and stoichiometry are affected by forest conversion is still vague. Here, we addressed this issue by systematically measuring tree biomass and the C, N, and P concentrations in tree organs and soils (0–100 cm) collected from 300 plots in Fujian province. With forest conversion of BF to PF, the tree C, N, and P stocks declined by 43.8, 47.9, and 63.1%, respectively, and the soil C and N stocks across whole soil depth decreased by 19.1% and 13.0%, respectively, and these decreases were more evident after conversion of BF to PF than SF. However, soil P stock showed a tendency of decreasing at 0–20 cm soil depth but increasing at 20–100 cm soil depth following conversion of BF to SF and PF. This unconformity of the vertical pattern of P stock in contrast to C and N stocks, was perhaps due to higher C and N inputs and greater P uptake from the subsoil and its redistribution to the topsoil in BF than in SF and PF. The tree and soil C, N, and P stoichiometry was strongly related to tree biomass, indicating that tree biomass was a vital factor driving soil inputs and retention of nutrients, and thus affecting their stoichiometry. The leaf N:P ratios ranging from 16.7 to 17.2 at our study sites suggested that co-limitations of N and P for forest growth could occur in the studied region. Our results provided insights into the C, N, and P linkages between soils and trees as affected by forest conversion, and advised that predicting these linkages could be an effective approach to identify the impacts of forest conversion, thereby implementing targeted conservation and rehabilitation actions.