Effects of nitrogen and phosphorus addition on soil organic carbon: review and prospects

Abstract

Soil organic carbon (SOC) pool is the largest carbon pool in terrestrial ecosystems and plays an important role in regulating the global carbon cycle and climate change. The inputs of nitrogen (N) and phosphorus (P) induced by anthropogenic activities and atmospheric deposition of N and P increase the availabilities of N and P in terrestrial ecosystems, which in turn will have important impacts on SOC dynamics via regulating plant growth and microbial activity. At present, many field-manipulation experiments regarding the effects of N addition and/or P addition on the dynamics of SOC have been conducted worldwide, and some breakthroughs and progress have been made, but a systematic and comprehensive review and summary of them is still lacking. By taking the effects of N addition and/or P addition on the inputs and outputs of soil carbon as the starting point, we systematically reviewed the effects of N addition and/or P addition on SOC and the potential mechanisms from three aspects: the size, fraction and molecular composition of SOC. According to the results of previous studies, N addition, P addition, and combined N and P (N+P) addition generally stimulate the size of SOC pool. The stimulation effect of N is caused by the decreased carbon outputs from microbial decomposition and/or the enhanced carbon inputs of plant above- and/or below-ground under N addition. However, the stimulation effect of P may be dominated by the enhanced carbon inputs of plant above- and/or below-ground under P addition. As for the fractions of SOC separated by particle-size or density fractionation, N addition promotes both labile fractions (particulate organic carbon or light fraction carbon) and stable fractions (mineral-associated organic carbon or heavy fraction carbon) of SOC, but reduces the proportion of stable carbon fractions to total SOC. In addition, the effects of N addition on the molecular composition of SOC are complex and diverse, and are regulated by environmental and experimental factors such as soil N availability, N addition rate, and N fertilizer form. Compared with N addition, studies on the effects of P addition and N+P addition on the fraction and molecular composition of SOC are very limited, and the associated mechanisms for the effects of P addition and N+P addition on these variables are still unclear. To improve our understanding, we propose four aspects of studies that need to be strengthened in the future, including the effects of P addition on SOC in different types of ecosystems (especially tropical forests), the role and relative contribution of plants and microorganisms in regulating the changes of SOC and its fractions under N addition and/or P addition, the effects of long-term N addition and/or P addition and their interactions on SOC, and the effects of N addition and/or P addition on SOC in deep soils (below 20 cm).