Variability of soil carbon and nitrogen stocks after conversion of natural forest to plantations in Eastern China.

Forest plantation, either through afforestation or reforestation, has been suggested to reverse and mitigate the process of deforestation. However, uncertainties remain in the potential of plantation forest (PF) to sequestrate carbon (C) and nitrogen (N) compared to natural forest (NF). Soil C and N stocks require a critical and updated look at what is happening especially in the context of increasing rate of land use change and climate change. The current study was conducted in China's Eastern forest to estimate soil C and N stocks in six depth layers (0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm) and two forest types (NF and PF) at four sites along climate factors gradient. The results showed that the overall mean soil C and N amounts to a depth of 20 cm ranged from 2.6 ± 1.1 Mg ha to 38.6 ± 23.1 Mg ha, and soil nitrogen stock ranged from 0.2 ± 0.1 Mg ha to 3.3 ± 1.5 Mg ha. Moreover, a loss of C stock was observed at Qingyuan (QY) by -7%, Dinghushan (DH) by -26%, Jianfengling (JF) by -13% while that of N stock was observed at QY (-8%), DH (-19%) and JF (-12%) at both depth layers. These results indicate that NFs have a better capacity to accumulate soil C and N. The soil C and N decreased from the southeast to the northeast and increased from tropical to temperate mixed forests zone in the eastern part of the study area. The C and N stock mainly occurred in the topsoil and decreased significantly with depth. Moreover, soil C and N stocks increased with age of plantation. This study provides an overview of the current spatial distribution and soil stocks of C and N, as well as the effects of environmental factors on soil C and N stocks. It also indicated that, although mean annual temperature and mean annual precipitation are the key factors affecting the variations in soil C and N, their vertical and horizontal distribution differed in various aspects.