Nitrogen response and transformation processes in karst water system using bacterial indicators

Abstract

Concentrated recharge processes significantly influence the biogeochemical behavior of nitrogen in karst water systems. To further understand the nitrogen response and transformation processes under these conditions, this study employs hydrological monitoring, hydrogeochemistry, and bacterial indicators to investigate the water environment changes between the inlet and outlet of Qinglongkou spring, South China. The results show that nitrifying bacteria drive nitrification, dominating biogeochemical processes of nitrogen. Heavy rainfall events alter the bacterial community structure associated with nitrogen cycling. The abundance of nitrification contributors (Ellin6067, MND1, and Nitrospira) exhibit a positive correlation with the flow rate of the QLK spring. The changes in nitrification functional genes abundance are associated with alterations in the dominant bacterial community structure. Nitrogen (NH₄⁺-N and NO₃^–-N) and functional genes (AOB-amoA, nirK, narG) accumulate to relatively high levels after rainfall event at both sinkholes and spring outlet, with AOB-amoA abundance positively correlated with flow rate, peaking at 7.46 × 10⁵ copies/mL. The AOB-amoA flux in Qinglongkou spring is similar to that in the sinkholes, reflecting the resilience and biogeochemical behavior of nitrifying bacteria under heavy precipitation conditions, but 16S rDNA, nirK, and narG fluxes decrease by 52.7 % to 68.3 %. This research sheds light on the impact of concentrated recharge on nitrogen response and transformation in typical karst areas, providing valuable bacterial information for mitigating nitrate pollution during rainstorm.