The persistence of bacterial diversity and ecosystem multifunctionality along a disturbance intensity gradient in karst soil

Extensive, progressive rock emergence causes localized variations in soil biogeochemical and microbial properties that may influence the capacity for the regeneration of degraded karst ecosystems. It is likely that karst ecosystem recovery relies on the persistence of soil functions at the microbial scale, and we aimed to explored the role of interactions between soil bacterial taxa and identify keystone species that deliver key biogeochemical functions, i.e. carbon (C) and nutrient (nitrogen, N and phosphorus, P) cycling. We applied high-throughput sequencing and phylogenetic molecular ecological network approaches to topsoils sampled at rock-soil interfaces and adjacent bulk soil along an established gradient of land-use intensity in the Chinese Karst Critical Zone Observatory. Bacterial α-diversity was greater under increased perturbation and at the rock-soil interface compared to bulk soils under intensive cultivation. However, bacterial ecological networks were less intricate and connected fewer keystone taxa as human disturbance increased and at the rock-soil interface. Co-occurrence within the bacterial community in natural primary forest soils was 13% larger than cultivated soils. The relative abundances of keystone taxa Acidobacteria, Bacteroidetes and Chloroflexi increased with land-use intensity, while Proteobacteria, Actinobacteria and Verrucomicrobia decreased by up to 6%. In general, Bacteroidetes, Verrucomicrobia and Chlorobi were related to C-cycling, Proteobacteria, Actinobacteria and Chloroflexi were related to N-cycling, and Actinobacteria and Nitrospirae were related to both N- and P-cycling. Proteobacteria and Chlorobi affected C-cycling and multiple functionality indexes in the abandoned land. We conclude that increasing land-use intensity changed the soil bacterial community structure and decreased bacterial interactions. However, increases in α-diversity at the rock-soil interface in cultivated soils indicated that major soil functions related to biogeochemical cycling were maintained within keystone taxa in this microenvironment. Our study provides foundations to test the success of different regeneration practices in restoring soil microbial diversity and the multifunctionality of karst ecosystems.