Abstract
Plants associated with symbiotic nitrogen-fixers and soil free-living nitrogen-fixing bacteria are good indicators for detecting the source of nitrogen in natural ecosystems. However, the community composition and diversity of plants associated with symbiotic nitrogen-fixers and soil free-living nitrogen-fixing bacteria in karst shrub ecosystems remain poorly known. The community composition and diversity of soil free-living nitrogen-fixing bacteria and plants, as well as the soil physical–chemical properties were investigated in 21 shrub plots (including different topographies and plant types). The frequency of plants associated with symbiotic nitrogen-fixers was found to be low in the 21 shrub plots. The soil free-living nitrogen-fixing bacterial community structure varied among the 21 shrub soils. Based on a variance partitioning analysis, topography, plant type, and soil pH explained 48.5% of the observed variation in bacterial community structure. Plant type had a predominant effect on community structure, and topography (aspect and ascent) and soil pH had minor effects. A negative correlation between the abundance of the soil free-living nitrogen-fixing bacterial community and the richness index for plants associated with symbiotic nitrogen-fixers was observed. The result of the low frequency of plants associated with symbiotic nitrogen-fixers highlights the importance of sources of fixed nitrogen by soil free-living nitrogen-fixing bacteria in the nitrogen limitation shrub ecosystem of the karst regions.
Highlights
Nitrogen is an important limiting factor for plant growth [1,2]
Significant negative relationships were observed between the abundance of nitrogen-fixing bacterial community and soil Olsen-P (F = −0.561, p < 0.05), pH (F = −0.719, p < 0.01), and the richness index for plants associated with symbiotic nitrogen-fixers (F = −0.443, p < 0.05; Table 3)
It is interesting that soil free-living nitrogen-fixing bacterial abundance is negatively corrected with the richness index for plants associated with symbiotic nitrogen-fixers
Summary
Nitrogen is an important limiting factor for plant growth [1,2] It is limited during the early stages of the natural recovery of degraded soils. In the karst shrub ecosystem, nitrogen is considered a key limiting factor for shrub growth [3,4]. Nitrogen-fixing bacteria are highly diverse and are divided into two groups: free-living nitrogen-fixing bacteria (e.g., Azotobacter, Azospirillum, and Pseudomonas) and symbiotic nitrogen-fixing bacteria (e.g., Rhizobia and Frankia; [6]). The fixation of atmospheric nitrogen by free-living nitrogen-fixing bacteria is significant in a few types of soil [7,8] and is the most important nitrogen source for natural ecosystems [9,10]. In nitrogen-limited natural ecosystems lacking legumes, soil free-living nitrogen-fixing bacteria may be important nitrogen sources for plant functioning
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