Nitrogen-cycling microbial diversity and function in tiankengs at different evolutionary stages

Abstract

Tiankengs in karst regions, characterized by distinct geomorphology and diverse microbial communities, serve as critical nitrogen (N) cycling zones. While microbial roles in karst ecosystems are recognized, the dynamics of N-cycling microorganisms across tiankengs at different evolutionary stages remain understudied. This study examined the composition, diversity, and function of N-cycling microorganisms inside and outside tiankengs at immature (Baidong), mature (Shenmu), and degraded (Dacao) stages. Significant environmental differences were observed, with mature tiankengs exhibiting the highest soil organic carbon, N, and available phosphorus levels, shaping microbial communities. Notably, denitrifying bacteria (DNB) (2.16) and ammonia-oxidizing archaea (AOA) (0.97) had the lowest Shannon diversity indices in mature tiankengs. Ralstonia pickettii and unclassified c_Betaproteobacteria emerged as potential indicators of tiankeng maturity and degradation, respectively. Microbial networks were relatively fragile, yet microbial activity, assessed via fluorescein diacetate (FDA) and peroxidase (POD) activity, was highest in mature tiankengs. AOA played a more significant role than bacteria in N cycling. As tiankengs degraded, increased connectivity with the external environment shifted N-cycling dominance from ammonia oxidation and nitrification to denitrification. This study identifies karst tiankengs as nitrogen sequestration hotspots, highlighting their importance for ecological restoration and sustainable management of vulnerable karst ecosystems.