Studying the impact of typical vegetation types in forest conversion zones on soil organic carbon (SOC) structure and stability is crucial for developing terrestrial ecosystem carbon sequestration strategies. In this study, we selected three typical forest stands in the Nanling National Nature Reserve: a primary evergreen broad-leaved forest (BL), a secondary mixed coniferous and broad-leaved forest (ML), and a Chinese fir plantation (CL). Soil samples were collected to examine the SOC fractions and carbon pool management index (CPMI) in three forest stands. The influence of soil property factors on SOC fractions was also analyzed. The results showed that the transformation process from a BL to an ML or a CL changed the structure and stability of organic carbon by reducing the labile SOC fractions and increasing the recalcitrant carbon fraction in the soil. The higher lability index (LI) and CPMI of soils in the BL indicated better carbon accumulation and activity, making this treatment more advantageous for management strategies aimed at promoting natural forest renewal and ecological restoration. Correlation and RDA analysis revealed that the availability of soil P was a key factor limiting the variation in organic C fractions in the acidic soils of tropical forests in South China.

Improving our understanding of how soil microbial community composition and enzyme activities vary with elevation will elucidate the impact of climate change on ecosystem function. We collected soil samples at three elevations (1000 m, 1200 m, 1400 m) from two soil depths in a subtropical forest in the Nanling Nature Reserve to analyze soil nutrient availability and the Gram-positive (GP) to Gram-negative (GN) bacteria ratio. We conducted a vector analysis of soil enzymatic stoichiometry to examine the spatial distribution of soil microbial C, N, and P limitations. The soil C:N ratio decreased with increasing elevation. The GP:GN ratio and vector length (read-outs of relative C versus nutrient limitation) were the highest at 1400 m due to lower C availability. At an elevation of 1200 m, lower P availability was reflected in higher soil C:P and N:P ratios and lower GP:GN ratios, as lower P availability suppressed microbial C decomposition. Furthermore, the GP:GN ratio and vector length showed contrasting responses to variations in soil depth. The validation of enzyme vector analysis to capture the responses of microbial community composition to soil properties is dependent on environmental conditions and should be considered in the development of future soil organic C (SOC) dynamics models.

Rapid urbanization causes the accumulation of large amounts of pollutants, including heavy metals, organic pollutants, and black carbon (BC). BC is the carbonaceous residue generated from the incomplete combustion of fossil fuels and biomass. It plays an important role on the migration of heavy metals and organic pollutants, as well as soil carbon sequestration. BC accumulation due to human activities greatly affects the global carbon budget, helps to drive climate change, and damages human health. To date, few studies have examined how the intensity of urbanization affects the distribution of BC in soils in urban areas. Therefore, the objective of this study is to determine the effects of urbanization intensity on the spatial distribution and content of BC in urban surface soil. We collected samples from 55 sites in South China and used a multi-scale geographical regression model to evaluate the impact of the interference intensity of urbanization on the amount and distribution of BC. Our results showed that the BC content was significantly higher in urban areas (9.74 ± 1.18 g kg⁻¹) than in rural areas (2.94 ± 0.89 g kg⁻¹) and that several urban parks with a higher interference intensity were hotspots of BC accumulation, suggesting that urbanization promoted BC accumulation. Our model revealed that road density was significantly and positively correlated with BC accumulation. Because there are more cars driving in areas with high road density, vehicle emissions may be one of the causes of BC accumulation. Our results also indicated that the impact of urbanization intensity on the BC distribution was sensitive to sampling density.