Agricultural cultivation duration affects soil inorganic N turnover and supply capacity: Evidence in subtropical karst regions

Abstract

The conversion of indigenous woodlands to agricultural lands has significantly altered nitrogen (N) cycling, impacting both ecosystem productivity and environmental health locally and globally. The relationship between cultivation duration and soil N availability and the mechanisms that drive these changes, however, remain unclear. In this study, we aimed to investigate how the duration of agricultural reclamation influences soil N cycling in the karst landscapes of southwestern China. We selected economic crops that have been cultivated for 1, 5, 15, and 30 years and conducted a regional survey using ¹⁵N labeling and molecular biology techniques to assess the effects of cultivation duration on soil N cycling. Our results show that short-term reclamation (< 5 years) caused minimal changes in soil N dynamics, with little effect on the net production rates of NH₄⁺ and NO₃^–. However, as cultivation duration increased, we observed progressive declines in mineralization, nitrification, and microbial immobilization rates of NH₄⁺ and NO₃^–. This led to a substantial reduction in soil inorganic N availability (–39 % for NH₄⁺ and –70 % for NO₃^–) and a significant increase in the mean residence time of NH₄⁺ and NO₃^–, indicating a slower N turnover. Long-term reclamation (30 years) resulted in the most pronounced effects, reducing the soil’s capacity to supply inorganic N by impairing soil organic matter input, degrading soil structure, and lowering soil pH. Key soil variables such as soil organic carbon content, pH, total N, and soil aggregate stability explained over 80 % of the variance in N turnover rates. Overall, our findings suggest that while short-term reclamation has little impact, long-term agricultural practices significantly impair soil N cycling and availability. Sustainable agricultural practices that enhance soil organic matter content and promote soil aggregate stability could help preserve soil health and maintain productivity in karst and similar regions worldwide.