Carbon dynamics in subtropical forest soil: effects of atmospheric carbon dioxide enrichment and nitrogen addition

The levels of atmospheric carbon dioxide concentration ([CO2]) are rapidly increasing. Understanding carbon (C) dynamics in soil is important for assessing the soil C sequestration potential under elevated [CO2]. Nitrogen (N) is often regarded as a limiting factor in the soil C sequestration under future CO2 enrichment environment. However, few studies have been carried out to examine what would happen in the subtropical or tropical areas where the ambient N deposition is high. In this study, we used open-top chambers to study the effect of elevated atmospheric [CO2] alone and together with N addition on the soil C dynamics in the first 4 years of the treatments applied in southern China. Above- and below-ground C input (tree biomass) into soil, soil respiration, soil organic C, and total N as well as dissolved organic C (DOC) were measured periodically in each of the open-top chambers. Soil samples were collected randomly in each chamber from each of the soil layers (0-20, 20-40, and 40-60 cm) using a standard soil sampling tube (2.5-cm inside diameter). Soil leachates were collected at the bottom of the chamber below-ground walls in stainless steel boxes. The highest above- and below-ground C input into soil was found in the high CO2 and high N treatment (CN), followed by the only high N treatment (N+), the only high CO2 treatment (C+), and then the control (CK) without any CO2 enrichment or N addition. DOC in the leachates was small for all the treatments. Export of DOC played a minor role in C cycling in our experiment. Generally, soil respiration rate in the chambers followed the order: CN treatment > C + treatment > N + treatment > the control. Except for the C+ treatment, there were no significant differences in soil total N among the CN treatment, N + treatment, and the control. Overall, soil organic C (SOC) was significantly affected by the treatments (p < 0.0001). SOC for all the soil layers in the treatments followed the order: CN treatment > N + treatment > C + treatment = CK treatment. Compared with the control, the higher SOC in the CN and N+ treatment was due to the greater above- and below-ground C input. The increased soil respiration in the C+ treatment led to the lower SOC. Elevated atmospheric [CO2] in the subtropical China accelerated soil C sequestration in this area; however, this increase would still need additional N input. The increased soil C pool was due to the enhanced tree growth. Special climatic condition in this area and the high density of tree planting might further accelerate soil C sequestration in this area.