Phosphorus (P) is an essential nutrient element that is critical for plant growth and ecosystem functionality. The soil P cycle plays multiple roles, such as sustaining plant growth and productivity, regulating nutrient balance within ecosystems, and enhancing ecosystem adaptability and resilience. This cycle is influenced by factors such as the restoration approach and microbial community dynamics. However, the extent to which the restoration approach alters the P cycle in karst ecosystems and the underlying microbial mechanisms remain poorly understood. The P-cycle multifunctionality index (P-cycle MFI) serves as a comprehensive indicator for evaluating soil P cycle function, and it provides insights into changes in the P cycle between different restoration approaches. To investigate the shifts in soil P-cycle MFI and microbial mechanisms between different restoration approaches, we analyzed soil available P (AP), total P (TP), microbial biomass P (MBP), and the activities of acid phosphatase (ACP) and alkaline phosphatase (ALP). These data were used to calculate the P-cycle MFI by averaging the Z-scores between two restoration approaches (artificial restoration of forest (AF) and natural restoration of forest (NF)) and a control (cropland, CP) at six subtropical karst ecosystem sites in China. We also determined the soil organic carbon (SOC), exchangeable calcium (Ca) and magnesium (Mg), pH, bulk density (BD), microbial biomass C (MBC), and microbial biomass nitrogen (MBN), as well as the community structure, relative abundance, diversity indices, and co-occurrence networks of phoD-harboring bacteria. The results showed that the community structure of phoD-harboring bacteria varied significantly among AF, NF, and CP and across different temperature gradients. These bacteria exhibited increasing complexity and tightness in co-occurrence networks from CP to AF and then to NF, along with the ACP and ALP activities, but not the TP and AP contents. The P-cycle MFI values were significantly higher in NF compared to AF and CP, and the variation was significantly explained by restoration approach, temperature, MBC, MBN, SOC, exchangeable Ca, BD, community structure of phoD-harboring bacteria, and exchangeable Mg. Furthermore, natural restoration had a more substantial impact on the P-cycle MFI than temperature by enhancing SOC, microbial biomass, the complexity and co-occurrence network tightness of the phoD-harboring bacterial community structure, and ACP and ALP activities, but it reduced soil BD. The rare genera of phoD-harboring bacteria significantly influenced the variation of soil P-cycle MFI compared to the dominant genera. This study highlights the importance of rare genera of phoD-harboring bacteria in driving soil P-cycle multifunctionality in karst ecosystems, with natural restoration being more effective than artificial methods for enhancing soil organic matter and microbial community complexity.

Understanding the spatial distributions and corresponding variation mechanisms of key soil nutrients in fragile karst ecosystems can assist in promoting sustainable development.  However, due to the implementation of ecological restoration initiatives such as land-use conversions, novel changes in the spatial characteristics of soil nutrients remain unknown.  To address this gap, we explored nutrient variations and the drivers of the variation in the 0–15 cm topsoil layer using a regional-scale sampling method in a typical karst area in northwest Guangxi Zhuang Autonomous Region, Southwest China.  Descriptive statistics, geostatistics, and spatial analysis were used to assess the soil nutrient variability.  The results indicated that soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) concentrations showed moderate variations, with coefficients of variance being 0.60, 0.60, 0.71, and 0.72, respectively.  Moreover, they demonstrated positive spatial autocorrelations, with global Moran’s indices being 0.68, 0.77, 0.64, and 0.68, respectively.  However, local Moran’s index values were low, indicating large spatial variations in soil nutrients.  The best-fitting semi-variogram models for SOC, TN, TP, and TK concentrations were spherical, Gaussian, exponential, and exponential, respectively.  According to the classification criteria of the Second National Soil Census in China, SOC and TN concentrations were relatively sufficient, with the proportions of rich and very rich levels being up to 90.9 and 96.0%, respectively.  TP concentration was in the medium-deficient level, with the areas of medium and deficient levels accounting for 33.7 and 30.1% of the total, respectively.  TK concentration was deficient, with the cumulative area of extremely deficient, very deficient, and deficient levels accounting for 87.6% of the total area.  Consequently, the terrestrial ecosystems in the study area were more vulnerable to soil P and K than soil N deficiencies.  Furthermore, variance partitioning analysis of the influencing factors showed that, except for the interactions, the single effect of other soil properties accounted more for soil nutrient variations than spatial and environmental variables.  These results will aid in the future management of terrestrial ecosystems.