[Effects of Simulated Nitrogen Deposition on Soil Organic Carbon Fractions and Carbon Pool Management Indicators in Mid-subtropical Eucalyptus Plantations].

Abstract

To study the characteristics of soil organic carbon （SOC） components and carbon pool management indicators of a Eucalyptus plantation after continuously simulating nitrogen deposition for five years and to explore the relationship between soil organic carbon components and soil environmental factors, it is helpful to understand the influence of nitrogen deposition on soil quality and SOC pool stability and provide a scientific basis for sustainable management of subtropical forest ecosystems and protection of the soil environment. In a long-term positioning test of simulating nitrogen deposition from 2018, four different treatments were selected： CK [0 kg·（hm²·a）^-1], low nitrogen LN [50 kg·（hm²·a）^-1], middle nitrogen MN [100 kg·（hm²·a）^-1], and high nitrogen HN [150 kg·（hm²·a）^-1]. The contents of SOC, light fraction organic carbon （LFOC）, heavy fraction organic carbon （HFOC）, easily oxidized organic carbon （EOC）, dissolved organic carbon （DOC）, particulate organic carbon （POC）, microbial biomass carbon （MBC）, soil physicochemical properties, carbon cycling-related enzyme activity, and carbon pool management indicators following the different treatments were measured. The results showed that： ① Compared to those in the control, the contents of SOC, DOC, EOC, POC, and MBC increased significantly with the increase in nitrogen deposition. By contrast, HFOC tended to decrease under the higher nitrogen deposition treatment. ② The input of nitrogen improved the carbon pool activity index and carbon pool management indicators significantly with the increase in nitrogen deposition levels. Nitrogen deposition enhanced the activities of sucrase, amylase, and dehydrogenase related to carbon cycling and improved soil carbon pool quality effectively. ③ The correlation and redundancy analysis between organic carbon components and soil environmental factors showed that soil pH, organic carbon content, and activated carbon components were negatively related. Physical and chemical indicators such as TN, AN, TK, AK, exchangeable ions, etc., all showed a significant or extremely significant positive correlation with organic carbon and its components, respectively. The soil physicochemical properties and carbon cycle-related enzymes promoted the increase in soil organic carbon and its component content to a certain extent. Soil exchange hydrogen, available potassium, total potassium, and amylase were the main environmental factors driving the changes in soil organic carbon composition and carbon pool management indicators. In conclusion, the continuous simulated nitrogen deposition for five years improved soil organic carbon, active organic carbon components, and carbon pool management indicators. This was beneficial to maintaining or improving the soil fertility of Eucalyptus plantations in mid-subtropical regions. However, there was a potential decrease in the content of heavy fractions of organic carbon and carbon storage under high nitrogen input levels [150 kg·（hm²·a）^-1]. Long-term high nitrogen deposition may reduce the stability of soil organic carbon.