Soil Nitrogen Mineralization and Phosphorus Availability Differ Among Long-Term Land-Use and Restoration Plots in a Karst Ecosystem

Coupled nitrogen (N) and phosphorus (P) cycling in calcareous karst soils, and how it responds to long-term land use and restoration, remains poorly quantified. We compared seven plots on a karst slope platform: three development types (degraded disturbed land, pasture, and fruit orchard) and four restoration types (planted evergreen forest, planted deciduous forest, evergreen–deciduous mixed forest, and naturally restored forest). Gross N transformation rates were measured by 15N isotope dilution, alongside soil properties, available P, microbial biomass N, and enzyme activities. Conservation plots generally had higher soil organic carbon, total N, microbial biomass N, and enzyme activity than development plots; soil organic carbon peaked under naturally restored forest (57.5 g·kg−1) and was lowest in disturbed land (47.9 g·kg−1). Naturally restored forest also showed the highest gross nitrification and total N mineralization, whereas disturbed land had the weakest ammonification, with negative gross rates pointing to N immobilization. Available P (up to 15.9 mg·kg−1) tracked alkaline phosphatase activity, organic carbon, and total N rather than total P. Across the alkaline range (pH 7.2–7.8), random forest models ranked ammonium and enzyme activity, not pH, as the main predictors of N mineralization. Long-term land use and restoration were thus associated with consistent differences in karst soil N and P supply.