Effects of Vegetation Restoration on Soil Nitrogen Pathways in a Karst Region of Southwest China

Abstract

Nitrogen (N) is an important element for plant growth in terrestrial ecosystems. Studying soil N cycling is crucial for understanding the structures and functions of an ecosystem. However, our knowledge of soil N dynamics in karst regions is still limited. In addition, while China's karst regions have conducted a series of vegetation restoration projects, the vegetation restoration effects on soil N pathways are still largely unknown. Therefore, this study selected four typical ecosystems representing four main vegetation restoration stages (i. e., cropland, grassland, shrubland, and forest) in a karst region in Huanjiang Province, southwest China. In these ecosystems, soil N pathways, including net ammonization rate (net ammonization, fungal ammonization, and bacterial ammonization), net nitrification rate (i. e., net nitrification, heterotrophic nitrification, autotrophic nitrification, fungal nitrification, and bacterial nitrification), net N mineralization rate (net N mineralization, fungal mineralization, and bacterial mineralization), and soil properties were measured. Our results showed that nitrification rate was high in all ecosystems, but the ammonization rate was low, resulting in nitrite being the main inorganic N form in karst soil. Autotrophic and heterotrophic nitrification rates accounted for 80% and 20% of the net nitrification rate, respectively. After the addition of fungal and bacterial inhibitors, ammonization rates increased for all treatments, but the nitrification rates decreased. Following vegetation restoration, soil N mineralization and nitrification rates all increased, but the ammonization rates significantly decreased. This pattern was significantly correlated with soil organic carbon, total nitrogen, nitrate, microbial biomass, and the activity of N-acquisition enzymes in these ecosystems. Our findings provide very useful information for understanding soil N cycling in the karst regions.