Microbial moribunds after microbial biomass turnover (microbial residues) contribute to the formation and stabilization of soil carbon pools; however, the factors influencing their accumulation on a global scale remain unclear. Here, we synthesized data for 268 amino sugar concentrations (biomarkers of microbial residues) in grassland and forest ecosystems for meta-analysis. We found that soil organic carbon, soil carbon-to-nitrogen ratio, and aridity index were key factors that predicted microbial residual carbon accumulation. Threshold aridity index and soil carbon-to-nitrogen ratios were identified (~0.768 and ~9.583, respectively), above which microbial residues decreased sharply. The aridity index threshold was associated with the humid climate range. We suggest that the soil carbon-to-nitrogen ratio threshold may coincide with a sharp decrease in fungal abundance. Although dominant factors vary between ecosystem and climate zone, with soil organic carbon and aridity index being important throughout, our findings suggest that climate and soil environment may govern microbial residue accumulation.
Forests can accumulate large quantities of SOC, but the trend in SOC accumulation with increasing stand age is inconclusive. In this study, we selected five plots in northwestern China: four stands of artificially planted Robinia pseudoacacia of different ages (5, 20, 40, and 56 years old), and a plot of wasteland as the control. The results showed that the SOC contents decreased, whereas δ13C values increased, with soil depth. The soil total nitrogen (TN) content and the carbon/phosphorus (C/P) ratio increased significantly with increasing stand age. The SOC storage in the 0–30 cm soil layer did not differ significantly among stands of different ages. However, SOC storage in the 0–100 and 30–100 cm soil layers was significantly higher in the 40- and 56-year-old stands than in 5-year-old stands. The SOC turnover rate decreased gradually over the 40 years after planting and then rapidly increased between 40 and 56 years after planting. The SOC stocks were most strongly correlated with TN and the C/P ratio, and SOC turnover was most closely related to soil porosity. Afforestation significantly improved soil properties to enhance SOC sequestration, but it took a long time for stored SOC to accumulate in this study area.
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