Ecological restoration has increasingly been employed to reverse land degradation and increase carbon (C) sink, especially in ecologically fragile karst areas. Microbial necromass carbon (MNC) constitutes a critical pool within soil organic carbon (SOC), contributing substantially to long-term C sequestration through mineral stabilization. However, its distribution patterns across soil profiles and grassland restoration stages in karst areas remain unclear. To address this knowledge gap, the contents of bacterial necromass C (BNC), fungal necromass C (FNC), and their contributions to SOC were estimated based on glucosamine and muramic acid contents across the soil profile (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm) for four subalpine restoration stages (grazing enclosure for 5, 11, 17, and 25 years) in the karst region. Our findings demonstrated that both soil depth and grassland restoration stages effectively influenced the BNC and FNC contents. On average, the soil BNC, FNC, and total MNC at the depth of 80–100 cm reduced by 70.50%, 59.70%, and 62.18% compared with in topsoil (0–20 cm), respectively. However, the FNC/BNC ratio gradually increased with the increase in soil depth, which was 43.15% higher at 80–100 cm soil depth than in topsoil, suggesting that the accumulation efficiency of FNC was higher compared to BNC in the deep soil. The BNC, FNC, and MNC were positively correlated with the grassland restoration stage, while FNC/BNC ratio had a negative relationship with the restoration stage (R² = 0.45, p < 0.001). FNC contributed significantly more to SOC (28.6–36.4%) compared to BNC (7.7–9.9%) at all soil depths, indicating that soil fungal necromass has an essential effect on SOC sequestration. The results of the random forest model and distance-based redundancy analysis identified that pH, soil water content, and dissolved organic carbon were the three most essential predictors for the contribution of MNC to SOC. Our study highlights the importance of microbial necromass to SOC accumulation, providing significant scientific implications for the C pool management during the restoration of degraded grasslands in karst regions.

Establishing a suitable and useful soil quality (SQ) assessment tool is imperative for the accurate evaluation of the effect of environmental changes on SQ. This study constructed four soil quality indexes (SQIs) based on different minimum data sets and weighted additive models to evaluate the influence of grassland degradation on SQ in northwest Guizhou, China. A total of 19 soil properties, including six physical properties, six chemical properties, and seven microbial properties, were measured at soil depths 0–20 cm to construct the SQIs. Results showed that 18 soil indicators were selected as the potential SQ indicators in the total data set. Based on the principal component analysis, four indicators, soil organic carbon (SOC), mean weight diameter, α-glucosidase, and β-acetylglucosaminidase, were selected in the minimum data set (MDS). However, six indicators, SOC, pH, β-1,4-xylosidase, β-acetylglucosaminidase, Clay, and Bulk Density, were selected for the selective MDS. Despite the notable inter-correlation among the four established SQIs, the SQI derived from the selective MDS and weighted additive model demonstrated heightened sensitivity and capacity for differentiation with respect to grassland degradation because of the high values of F and CV. Grassland degradation significantly reduced the SQ, and the value of SQ under severely degraded grassland was reduced by 51% compared with that under non-degraded grassland. Under the lightly degraded grassland, the reduction in soil physical quality was the primary reason for the total SQ decline, while the reduction in soil microbial and chemical reduction resulted in a significant decline in total SQ under the severely degraded grassland. In conclusion, greater attention should be paid to the SQ reduction resulting from grassland degradation in the study area, and the SQI established by selective MDS and weighted additive model should be used as a suitable and useful SQ assessment tool to evaluate the influence of environmental changes on SQ in Southwest China and other similar areas.

The biomass allocation patterns between aboveground and belowground are an essential functional trait for plant survival under a changing environment. The effects of arbuscular mycorrhizal fungi (AMF) communities on plant biomass allocation, particularly in degraded Festuca ovina grasslands in ecologically fragile karst areas, remain unclear. Therefore, we conducted a field investigation combined with a greenhouse experiment to explore the importance of AMF compared to bacteria and fungi for plant biomass allocation. The results showed that plant biomass in degraded grasslands exhibited allometric biomass allocation, contrasting with isometric partitioning in non-degraded grasslands. AMF, not bacteria or fungi, were the primary microbial mediators of grassland degradation effects on plant biomass allocation based on structural equation modeling. The greenhouse experiment demonstrated that the selected AMF keystone species from the field study performed according to ecological network analysis, particularly multi-species combinations, enhanced the belowground biomass allocation of F. ovina under rocky desertification stress compared to single-species inoculations, through decreasing soil pH, enhancing alkaline phosphatase (ALP) activity, and increasing the expression level of AMF-inducible phosphate transporter (PT4). This study highlights the critical role of the AMF community, rather than individual species, in mediating plant survival strategies under rocky desertification stress.