Comparing the effects of algae and macrophyte residues' degradation on biological nitrogen fixation in freshwater lake sediments

Abstract

The degradation and mineralization of organic residues are important factors that drive biochemical processes in lake ecosystems. However, the effect of organic matter's degradation on biological nitrogen fixation (BNF) in freshwater lake sediments remains poorly understood. This study investigated the response of sediment nitrogen fixation to the degradations of algae and macrophyte residues through continuous flow mesocosms combined with nifH sequencing analysis and isotope tracing. The results suggested that the active nitrogen fixation of sediments only occurred in the first two weeks of the rapid degradation of organic residues. Degradation of algae and macrophytes residues quickly increased the nifH abundance and the nitrogenase activity (NA) in sediments; however, the maximum NA triggered by algae's degradation (658.2 ± 16.8 ng g−1 day−1) was six times higher than that induced by the degradation of macrophytes residues. There was no significant difference in NA of sediments with the degradation of Potamogeton and Phragmites. Redundancy analysis (RDA) showed that the variation of diazotrophic community in sediment was significantly (p < 0.01) correlated with the concentrations of SO42− and NH4+ in overlying water and the Fe(II) content and Eh in sediment. Overall, the BNF of sediments can quickly respond to the degradation of organic residues, and the degradation of algae has a stronger promoting effect on the nitrogen fixation in sediments than that of macrophyte residues.