[Variations in topsoil carbon and nitrogen contents of five temperate plantations in Northeast China.]

Afforestation is an effective way for carbon (C) sequestration, which also profoundly influences soil nitrogen (N) dynamics in the forest ecosystem. The impacts of tree species on soil C and N budgets and the underlying mechanism remain uncertain. In this study, we used a common garden experiment and measured the soil organic C (C_soil) and total N contents (N_soil) of the topsoil (0-10 cm) and related vegetative and soil microbial properties in 2007 and 2015 (3 and 11 years after afforestation), respectively. Our aim was to explore the effects of five major tree species (i.e., Fraxinus mandshurica, Juglans mandshurica, Betula platyphylla, Larix gmelinii and Pinus sylvestris var. mongolica) in the temperate forests in Northeast China on soil C and N contents and their dri-vers. The results showed that both C_soil and N_soil of the five stands decreased as the stand ages increased, the change rates of which were significantly correlated with each other. The rate of change in C_soil(2.6%·a^-1-4.8%·a^-1) was significantly greater than that in N_soil(0.8%·a^-1-2.8%·a^-1). The decrements of the C_soil and N_soil for the broadleaved stands were significantly lower than those of the coniferous stands. The tree-species traits and microbial properties together explained 68.5% and 90.9% of the variability of the change rates of C_soil and N_soil, respectively. The change rates of C_soil and N_soil decreased with the increases in leaf litter C/N and microbial biomass C/N, but increased with the increases of fine root biomass, microbial biomass C, and the ratio of the C-acquisition to the N-acquisition enzyme activity. Additionally, the change rate of N_soil decreased with the increases of the leaf litter production and the microbial metabolic activity. Our findings indicated that C and N contents in the topsoil of these temperate plantations decreased significantly 11 years after afforestation, while the different change rates mainly resulted from different properties of tree species and soil microbes.