Abstract:

【Objective】The aim of the present study is to simulate the effects of long-term nitrogen (N) deposition on soil organic carbon (SOC) and organic carbon storage in subtropical forests with phosphorus (P) limitation to investigate the effects of active SOC and soil organic nitrogen (SON) components on organic carbon storage under long-term N addition, as well as to provide the basis for soil carbon (C) sequestration capacity and sustainable management of subtropical forest ecosystems.【Method】In an evergreen broad-leaved forest of Zhawan Nature Reserve, Qimen County, Anhui Province, two sites were selected, the middle slope and flat ridge, for the present study. Three different N and P addition treatments were designed at each site as follows: control (CK, N and P addition of 0 kg/hm² per year), N addition (N, N addition of 100 kg/hm² per year), and N+P addition (N+P, N addition of 100 kg/hm² and P addition of 50 kg/hm² per year). There were three replicate plots of 30 m × 15 m for each treatment, with a total of 18 sampling plots. In October 2020, soil samples were collected from the 0-40 cm layer in each plot to determine SOC content, SOC storage, active SOC fraction, SON fraction, and other basic physicochemical properties. The collected soil samples were divided into two parts. One part of the soil sample was naturally air-dried, crushed, and passed through a 0.25 mm sieve for the determination of soil SOC and total nitrogen (TN) content. The other part of the soil sample was treated differently to determine soil SOC, SON fraction, and other physicochemical properties to investigate the effect of long-term simulated N deposition on the organic carbon storage capacity of subtropical forest soils, as well as the effects of SOC and SON fractions on organic carbon storage. 【Result】Compared to the CK treatment, N and P additions did not significantly change the SOC content and storage, but it significantly reduced the soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) contents. Correlation analysis showed that SOC storage was significantly and positively correlated with most soil physical and chemical properties but significantly and negatively correlated with pH and bulk density. Linear regression analysis demonstrated that particulate organic carbon (POC) and particulate organic nitrogen (PON) explained 75.4% and 71.7% of the effects on SOC storage, respectively, while MBC and MBN explained 26.0% and 49.3% of the effects on SOC storage, respectively. There was no significant relationship between dissolved organic carbon (DOC) and SOC storage, and DOC explained 11.4% of the effects on SOC storage.【Conclusion】Long-term N and N+P additions significantly reduce the C and N contents of soil microbial biomass and may be detrimental to SOC stocks, with POC and PON having the greatest impact on SOC storage.

Key words: nitrogen and phosphorus addition, soil organic carbon storage, soil organic carbon component, soil organic nitrogen component, subtropical forest