Long-Term Nitrogen Addition Decreases Soil Carbon Mineralization in an N-Rich Primary Tropical Forest

Abstract

Anthropogenic elevated nitrogen (N) deposition has an accelerated terrestrial N cycle, shaping soil carbon dynamics and storage through altering soil organic carbon mineralization processes. However, it remains unclear how long-term high N deposition affects soil carbon mineralization in tropical forests. To address this question, we established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China with N additions such as NH4NO3 of 0 (Control), 50 (Low-N), 100 (Medium-N) and 150 (High-N) kg N ha−1 yr−1, and laboratory incubation experiment, used to explore the response of soil carbon mineralization to the N additions therein. The results showed that 15 years of N additions significantly decreased soil carbon mineralization rates. During the incubation period from the 14th day to 56th day, the average decreases in soil CO2 emission rates were 18%, 33% and 47% in the low-N, medium-N and high-N treatments, respectively, compared with the Control. These negative effects were primarily aroused by the reduced soil microbial biomass and modified microbial functions (e.g., a decrease in bacteria relative abundance), which could be attributed to N-addition-induced soil acidification and potential phosphorus limitation in this forest. We further found that N additions greatly increased soil-dissolved organic carbon (DOC), and there were significantly negative relationships between microbial biomass and soil DOC, indicating that microbial consumption on soil-soluble carbon pool may decrease. These results suggests that long-term N deposition can increase soil carbon stability and benefit carbon sequestration through decreased carbon mineralization in N-rich tropical forests. This study can help us understand how microbes control soil carbon cycling and carbon sink in the tropics under both elevated N deposition and carbon dioxide in the future.

Deforestation for Agriculture Temporarily Improved Soil Quality and Soil Organic Carbon Stocks

“…The soil C:N ratio (10.82-20.70) in the P and P-W systems indicated that the soil microbial biomass increased rapidly, mineralized soil nitrogen was released, and the organic matter decayed [59]. Compared with cropland, the soil C:N ratio after afforestation and deforestation was significantly higher and indicated that the unstable components made up a more significant proportion of the organic matter, which would increase the soil carbon mineralization rates [60]. Although the soil TN contents increased after deforestation, the proportion of C:N in the P-W system increased and indicated that the available nitrogen in the system was limited and more nitrogen was fixed by SOC; the low N:P ratio confirmed this viewpoint.…”

“…According to the ecological stoichiometry and correlation analysis, our data showed that nitrogen was the limiting element in this study area. Therefore, long-term N addition is necessary in the P-W system, which will decrease the soil carbon mineralization rates and further improve the stability of soil organic carbon [60]. This can be an essential theoretical basis for local farmers to formulate and modify land management measures after deforestation.…”

Nitrogen addition enhances nitrogen but not carbon mineralization in aggregate size fractions of soils in a Pinus massonia plantation

“…In this study, high levels of N addition (N90) led to substantial decreases in soil microbial biomass (Table 1). The decrease in soil microbial biomass may explain the reduced mineralization of SOC in aggregates due to N addition (Lu et al, 2021), considering that soil microbes are the primary regulators of SOC decomposition rates . Secondly, soil N levels strongly influence SOC mineralization.…”

Vertical differences in carbon metabolic diversity and dominant flora of soil bacterial communities in farmlands

“…A small amount of organic matter is difficult to decompose and eventually persists in the soil in the form of stable organic matter (mainly humus). A high carbon-to-nitrogen ratio (C/N) may be more beneficial for SOC sequestration 5 , but litter input generally favours soil nitrogen retention, reduces soil C/N, and increases microbial activity 6 , which is a major factor in multiple SOC changes due to increased nitrogen. The diverse conditions found within soil particles encourage the growth of different types and amounts of soil microorganisms, and changes in these factors can impact the soil quality 7 , 8 .…”