Relationships between nitrogen-fixing bacteria community structure in Vicia villosa nodules, soil properties and rocky desertification degree in karst area southwest China

Abstract

Rocky desertification, a common phenomenon in karst ecosystems, significantly impacts soil fertility and vegetation restoration. Therefore, understanding the relationship between nitrogen-fixing bacteria and soil properties under different degrees of rocky desertification is crucial. Our experiment was conducted to investigate the bacterial community structure and the main environmental factors affecting the distribution of the nitrogen-fixing bacteria in the nodules of V. villosa. Based on nifH gene sequence analysis, we found that the community composition of nitrogen-fixing bacteria in the nodules was significantly correlated with the degree of rocky desertification. The soil physicochemical properties affecting community composition were analyzed. The results revealed that: (1) The soil water content, alkali-hydrolyzable nitrogen content, and total nitrogen content in the slight rocky desertification area GJ4 were significantly higher than those in the moderate rocky desertification areas KY and MZ3. (2) There were significant differences in the community composition of nitrogen-fixing bacteria across the four rocky desertification areas (R2 = 0.448, P = 0.001). Within the same area, the Shannon index in slight rocky desertification was significantly higher than that of moderate rocky desertification. Rhizobium was the dominant genus. (3) In Gejiu, Yunnan Province, it has been observed that there is a clear negative correlation was observed among rocky desertification grade, soil water content, and nitrogen-fixing bacterial diversity in the typical karst ecosystem. Specifically, intensifying rocky desertification significantly reduces soil moisture and bacterial diversity. The degree of soil rocky desertification, total nitrogen content, total phosphorus content, and pH of soil are the main factors that play a key role in the community composition of the nitrogen-fixing bacteria in the nodules of V. villosa. This study provides a theoretical basis for the control of rocky desertification.

Fig 1. The species distribution stacking diagrams of the nitrogen-fixing bacteria within the nodules of V. villosa.

(A) Phylum Level: Shows the distribution of nitrogen-fixing bacterial communities at the phylum level among different sites (GJ1, GJ4, MZ3, KY2). (B) Order Level: Illustrates the composition of nitrogen-fixing bacterial communities at the order level. (C) Family Level: Provides a breakdown of the bacterial communities at the family level. (D) Genus Level: Details the distribution at the genus level of nitrogen-fixing bacteria.

Fig 2. Non-metric Multidimensional Scaling (NMDS) analysis of nitrogen-fixing bacterial communities.

Different colors represent samples from various sites: blue for GJ1, red for GJ4, green for MZ3, and cyan for KY2. The statistical results reveal a significant separation among the groups, with a stress value of 0.076, R² = 0.4475, and P = 0.001000. The ellipses depict the 95% confidence ellipses for each group, indicating the spread and clustering of the samples within each group.

Fig 3. Heat map of soil physicochemical properties and the nitrogen-fixing bacteria in the root nodules of V. villosa.

Color Gradient: Correlation coefficient ranging from −1 (strong negative) to 0.5 (strong positive).Significance:* indicates a significant correlation at the P < 0.05 level; ** indicates an extremely significant correlation at the P < 0.01 level.

Fig 4. Canonical Correspondence Analysis (CCA) of soil physicochemical properties and the nitrogen-fixing bacteria in the root nodules of V. villosa.

Sample Points: Different colors represent samples from various sites (GJ1, GJ4, MZ3, KY2). Environmental Factors: Arrows indicate the direction and strength of the relationship between soil properties and bacterial communities. CCA1 (40.26%): Explains 40.26% of the total variance in the community structure; CCA2 (8.65%): Explains 8.65% of the total variance. Ellipses: Represent the 95% confidence ellipses for each group, indicating the spread and clustering of the samples within each group.