Denitrification and the Relative Abundance of nirK and nirS Genes in Tributary Sediments of the Santa Fe River, Florida.

Intensive agriculture and fertilizer use have increased nitrogen loads to adjacent aquatic ecosystems. However, buffer systems such as riparian, tributaries and floodplains around agriculture ecosystems can decrease nitrogen concentrations through denitrification. We investigated the role denitrification in tributary sediments of the western Santa Fe River watershed, Florida, in processing nitrogen loads from adjacent land use activities (Nursery and Ranching). In sediments with a range of organic matter contents, N fertilization increased NO₃^- concentrations in the tributary sediment with low organic matter (less than 1% soil organic carbon determined by loss on ignition). However, the system impacted by nursery activities exhibited lower denitrifying enzyme activity (DEA) than the system impacted by ranch activity with relatively high organic matter (more than 1% soil organic carbon determined by loss on ignition). In low organic matter tributary sediments, DEA was low in both land activities. In addition, denitrification activity exhibited a positive relationship with liable organic carbon, indicating that available carbon may be a main factor controlling denitrification. The high organic matter system in both tributaries contained higher concentrations of NH₄⁺, TOC, and MBC, and exhibited higher DEA than the relatively low organic matter sediments from either system. All systems yielded nirS (encoding nitrite reductase) PCR products, with higher intensity PCR bands from high organic matter sediments than from low organic matter sediments. However, relatively few nirK (encoding alternate nitrite reductase) PCR amplification products were visible, and only in those impacted by ranching activities.