The phosphorus (P) availability in soils is influenced by microbes, particularly those containing the gene responsible for phosphate solubilization. The present study investigated the community structure, diversity, and co-occurrence networks of phoD-harboring bacteria in karst and non-karst citrus orchard soils across a planting duration gradient, natural forests, and abandoned land, as well as the soil total P (TP) and available P (AP) contents and enzyme activities. The soil AP contents were lower in the karst regions than in the non-karst regions, while the soil organic carbon (C; SOC), exchangeable calcium, and microbial biomass nitrogen (N) contents; alkaline phosphatase (ALP) and β-Glucuronidase activities; and pH had the opposite trends. In addition, the soil AP and SOC contents and the ALP and acid phosphatase (ACP) activities in the karst regions decreased with an increase in the planting years, whereas the AP, TP, and microbial biomass P contents and ACP activities in the non-karst regions increased. The diversity indices and network complexity of phoD-harboring bacteria were higher in the karst regions than in the non-karst regions, with marked community differences between different planting years in the non-karst regions. The soil AP was significantly and positively correlated with the rare genera Pelagicola, Methylobacter, Streptomyces, and Micromonospora in the karst regions and Roseivivax, Collimonas, Methylobacterium, Ralstonia, and Phyllobacterium in the non-karst regions. Structural Equation Modeling showed that citrus cultivation altered the soil pH, SOC, and total N, and, in turn, the phoD-harboring bacterial community structure and diversity, which led to changes in the ALP activity and P availability. Thus, the rare genera of the phoD-harboring bacteria, influenced by the pH and SOC, highly regulated the availability of P in the karst and non-karst citrus orchard soils.

Biochar has garnered considerable attention as a soil amendment due to its unique physicochemical properties. Its application not only enhances soil carbon sequestration but also improves nutrient availability. Incorporating biochar into soil is regarded as a promising strategy for mitigating global climate change while delivering substantial environmental and agricultural benefits. In this study, biochar was extracted from Siraitia grosvenorii and subsequently modified through alkali treatment. A laboratory incubation experiment was conducted to assess the effects of unmodified (JMC) and modified (GXC) biochar, applied at different rates (1%, 2%, and 4%), on organic carbon mineralization and soil nutrient dynamics. Results indicated that, at equivalent application rates, JMC-treated soils exhibited lower CO₂ emissions than those treated with GXC, with emissions increasing alongside biochar dosage. After the incubation, the 1% JMC treatment exhibited a mineralization rate of 17.3 mg·kg⁻¹·d⁻¹, which was lower than that of the control (CK, 18.8 mg·kg⁻¹·d⁻¹), suggesting that JMC effectively inhibited organic carbon mineralization and reduced CO₂ emissions, thereby contributing positively to carbon sequestration in Siraitia grosvenorii farmland. In contrast, GXC application significantly enhanced soil nutrient levels, particularly increasing available phosphorus (AP) by 14.33% to 157.99%. Furthermore, partial least squares structural equation modeling (PLS-SEM) identified application rate and pH as the key direct factors influencing soil nutrient availability.

The soil of Siraitia grosvenorii (LHG) farmland often suffers from acidification, compaction, and declining organic matter content. As biochar helps improve soil quality and enhance soil carbon sequestration capacity, an increasing number of studies are utilizing biochar for soil quality improvement. To address the soil degradation problem in LHG farmland and achieve the goals of soil organic carbon (SOC) sequestration and nutrient increase, we conducted a 100-day indoor constant-temperature incubation experiment by adding different proportions of LHG vine biochar. We analyzed the changes in SOC mineralization, different carbon fractions, and soil nutrient content in LHG farmland. The main results showed that, compared with the control group, the cumulative mineralization (CumulMine) of SOC increased by 3% to 51%, and organic carbon content increased by 52.43% to 193.87%. As the LHG vine biochar application rate increased, the metabolic entropy (qCO₂) rose, whereas the microbial entropy (qMBC) showed an opposite trend. Similarly, compared with the control group, the addition of 1.0%, 2.0%, and 4.0% LC increased water-soluble organic carbon by 45.87 mg·kg⁻¹, 67.00 mg·kg⁻¹, and 81.73 mg·kg⁻¹, respectively, and soil nutrients also increased, but microbial biomass carbon (MBC) and readily oxidizable organic carbon (ROC) contents decreased. The main conclusions indicate that adding LHG vine biochar increases SOC content, which is associated with reduced microbial activity. Biochar-derived DOC may serve as a substrate for microbial respiration, thereby contributing to increased CO₂ release and accelerated nutrient release. The application of LHG vine biochar enhanced the carbon sequestration capacity of LHG farmland soil while improving soil nutrient content, with the 4% application rate treatment performing the best.