Print PageLand which has been subject to intense agricultural activity can play a key role in determining the health of aquatic ecosystems. Fertilizer runoff can lead to extremely high concentrations of nitrogen in streams, thus altering these aquatic resources. However, the microbial process of denitrification can remove nitrogen from aquatic ecosystems and help prevent downstream effects of nitrogen enrichment, such as eutrophication. Agricultural activity, including fertilizer application, varies throughout the year, with corresponding variation in nitrogen inputs to streams. How this environmental variation is reflected in community structure and nitrogen retention is not well understood. We examined temporal variation in denitrifying community structure (by targeting the nitrous oxide reductase gene, nosZ) and associated changes in benthic denitrification rates in an agricultural watershed in Indiana. In 2010, sediment samples were collected seven times, along with physical and chemical data from three stream sites. Sampling dates were selected based on hydrology and seasonality. Microbial community composition was determined using 16S rRNA and nosZ terminal restriction fragment length polymorphism (TRFLP) analysis. Quantitative polymerase chain reaction (qPCR) was used to determine the number of copies of the nosZ gene.
Results/Conclusions
To meet the objectives of the study, sediment samples were collected in triplicate from two sites along the main stem of Sugar Creek (Hancock County, Indiana, USA) and from a tributary of Sugar Creek (Leary Weber Ditch). For all sites, turbidity was relatively low in the winter months of January and February, ranging from 2.05 to 4.50 NTU, and increased steadily from April through August, ranging from 15.73 to 52.07 NTU. Nitrate concentration varied seasonally at each site and among sites. Leary Weber Ditch had the highest concentrations of nitrate on each date (ranging from 2.6 to 11.6 mg N/L). Samples collected in May showed the highest nitrate concentrations at all three sites, with values between 7.1 and 11.6 mg N/L. Microbial community structure was examined via DNA extraction, PCR amplification of the 16S rRNA and nosZ genes, and TRFLP analysis. TRFLP data suggests the microbial community fingerprint varied temporally, indicating a change in the community composition during times of increased nitrogen input. These results suggest that agricultural activity, through effects on nitrogen inputs, played a role in temporally altering microbial community composition and benthic denitrification rates.
