Subtropical region has dramatically experienced land use and coverage change. Historical forest management practice has persistent effects on many ecosystem processes such as soil nitrogen (N) mineralization. Soil N dynamics during forest conversion from zonal forest to secondary forest can provide substantial insight into the impacts of forest management on soil. So far, however, few studies have conducted to investigate the effects of forest conversion on soil N transformation in subtropical region of China.
Based on a space-time substitution method, we selected four vegetation types including evergreen broad-leaved forest (EBF, zonal forest), Pinus massoniana forest (PMF, natural regeneration forest), Cunninghamia lanceolata plantation (CLP, artificial forest), Liriodendron chinense plantation (LCP, artificial forest) in Dagangshan National Ecological Station, Jiangxi province. For each forest type, we randomly set two 20 m×20 m plots separated by at least 30 m buffer zones. The concentration of NH₄⁺-N and NO₃^--N, and the rate of N ammonification and nitrification were measured through in situ incubations using the PVC method. Soil properties (total carbon, total N, C:N ratio, and pH value, 0-15 cm in depth), forest fine root (living root biomass, dead root storage) and the rate of N absorption of vegetation were also quantified. Least significant difference (LSD) in One-way ANOVA was used to determine the effects of forest type on the soil variables.
The results showed as follows: (1) Inorganic N pools and N mineralization rates in all forests performed seasonal dynamics, NH₄⁺-N being higher in winter and spring (11.64-15.55 kg/hm²) than in summer and autumn (3.89-8.36 kg/hm²), NO₃^--N being higher in summer and autumn (0.42-8.60 kg/hm²) than in winter and spring (0.24-6.10 kg/hm²). The pool of NH₄⁺-N was considerably larger than NO₃^--N pool, and thus NH₄⁺-N was the main component of inorganic N in these forests. The rates of ammonification and nitrification were faster in summer and autumn than in spring and winter. (2) The forest conversion from EBF to PMF, CLP and LCP, soil N mineralization patterns had changed, and annual ammonification rates decreased by 110.67%, 100.76%, 96.20%, however annual nitrification rates increased by 54.92%, 24.19%, 24.46% respectively. (3) Total N mineralization rates were lower 24.68%, 26.01% in CLP and LCP, respectively than in EBF, and insignificant difference between PMF and EBF. Meanwhile, the N supply amount with soil net N mineralization rate in PMF, CLP and LCP (111.85, 89.58, 88.00 kg·hm^-2·a^-1) were lower than their needs for plant N uptake (137.83, 92.76, 99.28 kg·hm^-2·a^-1, respectively). (4) The size of inorganic N pool depended on mineralization rate and vegetation N uptake rate. Ammonification rate positively correlated with storage of dead root, N uptake positively correlated with biomass of live root, and nitrification rate positively correlated with NH₄⁺-N concentration and negatively correlated with NH₄⁺-N uptake rate by vegetation. These findings indicated that the forest conversion from EBF to other secondary forests would lead to N leaching, thus N deficiency might become a growth-limiting factor for these forests.