[Abundance of denitrifying genes and their driving factors in soil under different land use types in the karst region of Northwest Guangxi]. / æ¡‚è¥¿åŒ—å²©æº¶åŒºä¸åŒåœŸåœ°åˆ©ç”¨æ–¹å¼ä¸‹åœŸå£¤åç¡åŒ–åŸºå› ä¸°åº¦åŠå ¶é©±åŠ¨å› ç´ .

To understand the role of denitrifying microbes during vegetation recovery in karst regions, we determined the basic physicochemical properties and abundance of denitrifying microbial functional genes (nirS, nirK, fungal nirK, p450nor, and nosZ) of 13 collected soil samples under three land use types (cropland, grassland, and plantation) in Northwest Guangxi, and investigated the changes in the abundance of denitrifying microbial functional genes and their driving factors. Results showed that soil pH, soil organic carbon, total nitrogen (TN), and exchangeable calcium (Caexe) in plantation soil were significantly higher than those in cropland and grassland. The abundance of nirS, nirK, p450nor, and nosZ in plantation soil were significantly higher than those in cropland and grassland. Soil pH, TN, and Caexe were positively correlated with the abundance of denitrifying functional genes nirS, nirK, and p450nor. Results of redundancy analysis showed that soil Caexe, pH and TN were the primary factors influencing the abundance of denitrifying functional genes, which accounted for 34.1%, 20.1%, and 16.1% of the total variation, respectively. Such a result suggested that Caexe was the main driver of changes in denitrifying functional genes under different land use types. Overall, vegetation restoration (plantation) could effectively increase soil denitrifying microbe genes abundance in the karst region of Northwest Guangxi, and consequently influence soil nitrogen cycling.

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

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.

[Effects of Topography, Tree Species and Soil Properties on Soil Enzyme Activity in Karst Regions].

Soil extracellular enzymes are crucial in biogeochemical cycle and ecosystem functioning. Yet uncertainty exists in terms of major determinants on soil extracellular enzyme activity (EEA), especially at calcareous areas. In this study, soil samples (0-15 cm) were collected from different topographic conditions (different slope positions and aspects) and different tree species (Cryptocarya concinna and Eurycorymbus cavaleriei) in a forest located at Mulun natural reserve, a typical karst area. Six hydrolytic enzymes, related to carbon, nitrogen or phosphorus cycling, and soil physiochemical properties were investigated. The effects of topography, tree species and soil properties on soil enzyme activities were analyzed by multi-response permutation procedures (MRPP), redundancy analysis (RDA) and variation partitioning. The enzyme profiles were significantly (P<0.05) separated among the four slope positions but not significantly separated between the two aspects or two tree species according to multi-response permutation procedure (MRPP) analysis. Variation partitioning indicated that topography, tree species and soil properties together explained 55.3% of EEA variation. Soil property was the key factor influencing EEA variation, which explained 44.2% of EEA variation. The result of redundancy analysis (RDA) showed that soil pH, total nitrogen and inorganic nitrogen were the most important variables among the nine soil properties for EEA variation. This study first quantified the effects of small-scale topographic, tree species, and soil properties on soil enzyme activity variation in karst areas. Our results suggest that EEA variation can be well explained by soil properties at a small scale in the karst area.