The shift from macrophytic to algal particulate organic matter favours dissimilatory nitrate reduction to ammonium over denitrification in a eutrophic lake

Abstract

Abstract In eutrophic lakes, the shift from a macrophyte‐dominated state to an algae‐dominated state changes the particulate organic matter (POM) sources, which in turn alters organic matter quality that is released and that sinks to the benthos. However, the influences of this shift on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are unclear in eutrophic lakes. Here, we elucidated how various POM sources influenced these nitrate reduction pathways in a eutrophic lake. The lake sediments from August were incubated under algae, macrophyte, and soil treatments in aquarium tanks. Potential denitrification and DNRA rates and related functional gene abundances were measured periodically. Meanwhile, the n‐alkanes composition of POM and fluorescence excitation‐emission matrices of dissolved organic matter were measured before and during incubations, respectively, to aid in the analysis of the potential mechanisms by which organic matter quality affect nitrate reduction. The results indicated that algal detritus was more labile and decomposed faster than other POM, coupled with higher DNRA rates and nrfA gene abundances. Macrophyte treatments resulted in nitrate accumulation and the increase of gene abundances related to denitrification, but no increase in denitrification rates was observed. In soil treatments, nitrate reduction processes were not significantly influenced by the addition of POM. Overall, compared with other sources, algae‐derived POM can produce high‐quality organic carbon, low dissolved oxygen and nitrate concentrations, which is more conducive to DNRA than denitrification. A loss of macrophytes and increase in algal biomass will induce changes in autochthonous POM quality and potentially drive nitrogen recycling in eutrophic lakes.