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Soil organic carbon (SOC), nitrogen (N), and phosphorus (P) are three essential soil nutrients for plant growth, and their stoichiometric ratios are already important indices of elemental balance and the soil fertility status in soil ecosystems. The evolution mechanism of the SOC, Total Nitrogen (TN), Total Phosphorus (TP), and stoichiometry following the "conversion of cropland to forest program" (CCFP) in southwest China is not yet clear. Seven different CCFP restoration models, including Zenia insignis (RD), Toona sinensis (XC), Castanea mollissima (BL), Citrus reticulate (GJ), Zenia insignis and Guimu-1 elephant grass (RG), Guimu-1 elephant grass (GM), and abandoned cropland (LH), were chosen to explore changes in the concentration and stoichiometry of the SOC, TN, and TP, and their recovery times, at a depth of 0–100 cm. The results indicate that the SOC and TN concentrations in different restoration models all increased with restoration years in the topsoil, whereas the soil TP concentration remained relatively stable. The soil C:N and C:P ratios increased with increasing restoration years in the topsoil, whereas the N: P ratio was relatively stable over time. After ten years of reforestation, the SOC and TN concentrations decreased as the soil layer increased. The effects of the restoration model on the C: N ratios were greater in shallow soils. Our results suggest a complex reaction of SOC, soil TN, and soil TP concentrations and stoichiometry to the vegetation restoration mode, particularly in the topsoil. This research further improves the understanding of SOC, N, and P interactions and restricted nutrition, and provides relevant theoretical support for vegetation restoration in the southwest karst region. [ABSTRACT FROM AUTHOR]

The forest ecosystem is an important part of the terrestrial ecosystem carbon sink, and its rate of biomass accumulation influences its carbon sink potential. Therefore, it is particularly important to understand the biomass and productivity of forest ecosystems, and their driving factors, especially in karst areas with a fragile ecological environment. We established a 2 ha plot in karst forest in southwest China, and investigated species composition, community structure, topography and soil nutrients in the years 2007 and 2017. In this analysis, the correlations between tree diversity and each factor were evaluated using a Pearson correlation analysis. In addition, the relationships between soil nutrients and topographies and their effects on productivity and biomass were further evaluated, either directly or indirectly, through species and structural diversity by using a structural equation model (SEM). The results showed that the number of individuals in each species decreased, and productivity was 1.76 Mg ha−1 yr−1, from 2007 to 2017. Species diversity was negatively correlated with biomass and positively correlated with productivity; structural diversity was negatively correlated with biomass and productivity, while structural diversity was negatively correlated with biomass and positively correlated with productivity. In addition, the effects of soil factors on biomass and productivity were significantly different: TN had a significant positive effect on productivity, while all soil factors except total nitrogen (TN) had significant positive effects on biomass. The structural equation results also showed that topographic and soil factors can directly affect productivity; structural diversity has a strong direct negative impact on biomass, while species diversity, structural diversity and biomass have similar direct positive impacts on productivity. Structural diversity was better than species diversity when explaining biomass accumulation. In conclusion, biotic and abiotic factors both influence forest productivity in karst forests in southwest China, and improving species diversity and community structure complexity is of great significance for forest management and productivity promotion. The research further improve the understanding of biomass and productivity in karst forest ecosystems, and their driving factors, which will provide relevant theoretical support for sustainable forest development in southwest karst. [ABSTRACT FROM AUTHOR]

Vegetation and soil have spatial distributions at different scales, while the spatial distribution of soil microorganisms and factors driving their distribution are still unclear. We aimed to reveal the spatial pattern of microbial functional diversity and to identify its drivers in forest soils at a regional scale. Here, we performed an investigation of microbes across several forest types covering an area of 236,700 km2 in Guangxi, southwest China. We examined a total of 185 samples for soil microbial functional diversity using Biolog EcoPlates. The soil microbial functional diversity had strong spatial heterogeneity across the Guangxi region. The distribution of microorganisms in forest soils was mainly determined by total nitrogen, available N, and C:N ratio, and stand age. We found that coniferous forests, especially pine forest, exhibited lower functional diversity, but the reverse was true for deciduous broadleaf forest/mixed evergreen and deciduous broadleaf forest. Our findings suggested that a heterogeneous distribution of microbial functional diversity in forest soils is related to forest types in Guangxi, China. In conclusion, high soil microbial functional diversity is favored in subtropical forests with looser soil structure, lower soil C:N ratio, greater total soil nitrogen and available nitrogen concentration, and broad-leaved tree species. [ABSTRACT FROM AUTHOR]

Understanding the driving factors of forest biomass are critical for further understanding the forest carbon cycle and carbon storage management in karst forests. This study aimed to investigate the distribution of forest aboveground biomass (AGB) and the effects of stand structural and abiotic factors on AGB in karst forests in Southwest China. We established a 25 ha plot and sampled all trees (≥1 cm diameter) in a subtropical mixed evergreen–deciduous broadleaf forest. We mapped the forest biomass distribution and applied a variation of partitioning analysis to examine the topographic, stand structural, and spatial factors. Furthermore, we used structural equation models (SEM) to test how these variables directly and/or indirectly affect AGB. The average AGB of the 25 ha plot was 73.92 Mg/ha, but that varied from 3.22 to 198.11 Mg/ha in the 20 m × 20 m quadrats. Topographic, stand structural, and spatial factors together explained 67.7% of the variation in AGB distribution. The structural variables (including tree density and the diameter at breast height (DBH) diversity) and topographic factors (including elevation, VDCN (vertical distance to channel network), convexity, and slope) were the most crucial driving factors of AGB in the karst forests. Structural equation models indicated that elevation, tree density, and DBH diversity directly affected AGB, and elevation also indirectly affected AGB through tree density and DBH diversity. Meanwhile, AGB was indirectly influenced by VDCN, convexity, and slope. The evaluation of stand structural and abiotic drivers of AGB provides better insights into the mechanisms that play a role in carbon storage in karst forests, which may assist in improving forest carbon management. [ABSTRACT FROM AUTHOR]