Temporal patterns of net soil N mineralization and nitrification through secondary succession in the subtropical forests of eastern China

Linking temporal trends of soil nitrogen (N) transformation with shifting patterns of plants and consequently changes of litter quality during succession is important for understanding developmental patterns of ecosystem function. However, the successional direction of soil N mineralization and nitrification in relation to species shifts in the subtropical regions remains little studied. In this study, successional patterns of net soil N mineralization and nitrification rates, litter-fall, forest floor litter, fine root and soil properties were quantified through a successional sequence in the subtropical forests of eastern China. Net N mineralization rate was 'U-shaped' through succession: highest in climax evergreen broad-leaved forest (CE: 1.6±0.2 mg-N kg^-1 yr^-1) and secondary shrubs (SS: 1.4±0.2 mg-N kg^-1 yr^-1), lowest in conifer and evergreen broad-leaved mixed forest (MF: 1.1±0.1 mg-N kg^-1 yr ^-1) and intermediate in conifer forest (CF: 1.2±0.1 mg-N kg^-1 yr^-1) and sub-climax forest (SE: 1.2±0.2 mg-N kg^-1 yr^-1). Soil nitrification increased with time (0.02±0.1, 0.2±0.1, 0.5±0.1, 0.2±0.1, and 0.6±0.1 mg-N kg^-1 yr^-1 in SS, CF, MF, SE and CE, respectively). Annual production of litter-fall increased through succession. Fine root stocks and total N concentration, soil total N, total carbon (C) and microbial biomass C also followed 'U-shaped' temporal trends in succession. Soil bulk density was highest in MF, lowest in CE, and intermediate in SS, CF and SE. Soil pH was significantly lowest in CE. Temporal patterns of soil N mineralization and nitrification were significant related to the growth of conifers (i.e. Pinus massoniana) and associated successional changes of litter-fall, forest floor, fine roots and soil properties. We concluded that, due to lower litter quality, the position of Pinus massoniana along the succession pathway played an important role in controlling temporal trends of soil N transformation.