Nitrate reduction mechanisms and rates in an unconfined eogenetic karst aquifer in two sites with different redox potential

Abstract

AbstractThis study integrates push‐pull tracer tests (PPTT) with microbial characterization of extracted water via quantitative polymerase chain reaction (qPCR) and reverse transcriptase qPCR (RT‐qPCR) of selected functional N transformation genes to quantify nitrate reduction mechanisms and rates in sites with different redox potential in a karst aquifer. PPTT treatments with nitrate (AN) and nitrate‐fumarate (ANC) were executed in two wells representing anoxic and oxic geochemical end‐members. Oxic aquifer zero‐order nitrate loss rates (mmol L−1 h−1) were similar for AN and ANC treatment, ranging from 0.03 ± 0.01 to 0.05 ± 0.01. Anoxic aquifer zero‐order nitrate loss rates ranged from 0.03 ± 0.02 (AN) to 0.13 ± 0.02 (ANC). Microbial characterization indicates mechanisms influencing these rates were dissimilatory nitrate reduction to ammonium (DNRA) at the anoxic site with AN treatment, assimilatory reduction of nitrate to ammonium (ANRA) with ANC treatment in the water column at both sites, and additional documented nitrate reduction that occurred in unsampled biofilms. With carbon treatment, total numbers of microbes (16S rRNA genes) significantly increased (fourteenfold to thirtyfold), supporting stimulated growth with resulting ANRA. Decreased DNRA gene concentrations (nrfA DNA) and increased DNRA activity ratio (nrfA‐cDNA/DNA) supported the assertion that DNRA occurred in the anoxic zone with AN and ANC treatment. Furthermore, decreased DNRA gene copy numbers at the anoxic site with ANC treatment suggests that DNRA microbes in the anoxic site are chemolithoautotrophic. Increased RT‐qPCR denitrification gene expression (nirK and nirS) was not observed in water samples, supporting that any observed NO3‐N loss due to denitrification may be occurring in unsampled microbial biofilms.