Low Frequency of Plants Associated with Symbiotic Nitrogen-Fixers Exhibits High Frequency of Free-Living Nitrogen Fixing Bacteria: A Study in Karst Shrub Ecosystems of Southwest China

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.