Benthic nitrate reduction in estuaries : process rates and key functional genes for denitrification ( DN ) , nitrate

Benthic nitrate reduction in estuaries: process rates and key functional genes for denitrification (DN), nitrate ammonification (DNRA) and anammox (AN). David B. Nedwell University of Essex The rates of DN, DNRA and AN have been investigated at three sites along the nutrified Colne estuary, U.K. using N isotope pairing techniques, together with seasonal changes. DNRA was favoured at low ambient nitrate concentrations, and denitrification at higher nitrate. AN accounted for at most 30% of N2 formation in the upper estuary site, was occasionally detected in the middle estuary but not detected at the estuary mouth. Key functional genes (nar, nap, nir, nrf) for nitrate reduction, DN and DNRA were measured along the estuary and with depth in the sediment, and 16S rRNA for anammox bacteria. Similar work was carried out in three tropical estuaries in Thailand, Indonesia and Fiji, and the data from these tropical estuaries will be compared to those from the temperate Colne estuary. Denitrification in Terrestrial Ecosystems Peter M. Groffman Cary Institute of Ecosystem Studies Denitrification is a miserable process to study in terrestrial ecosystems. Methods for quantifying the process are lousy, variability in activity is absurdly high, and temporal and spatial scaling challenges are extreme. These difficulties are unfortunate, as the need for information on terrestrial denitrification is great. Global analysis suggest that the majority of denitrification,especially of anthropogenically produced nitrogen, occurs in the terrestrial environment (Seitzinger et al. 2006). At the regional scale, there is great interest in understanding and managing denitrification to reduce eutrophication in coastal waters (Van Breemen et al. 2002, Schaefer et al. 2009). And at the ecosystem/field scale, denitrification is a key controller of the nitrogen availability to plants and hydrologic exports to groundwater (Gentry et al. 2009). Available methods for measuring terrestrial denitrification are problematic for a variety of reasons; they change substrate concentrations, disturb the physical setting of the process, lack sensitivity or are prohibitively costly in time and expense (Groffman et al. 2006). Most fundamentally, it is very difficult to quantify the dominant end-product (N2) of denitrification given its high background concentration in the atmosphere. However, new techniques for direct measurement of N2 flux are becoming more common (Swerts et al. 1995, Butterbach-Bahl et al. 2002), although these still rely on the use of extracted cores. N-based in situ methods are suitable for sites with high soil nitrate concentrations, but more sensitive mass spectrometers will be needed for tracer-level studies in natural ecosystems (Stange et al. 2007). Spatial and temporal variation in denitrification is high due to control of the process by multiple factors (oxygen, nitrate, carbon) that each vary in time and space (Parkin 1993). A particular challenge is that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of N gas flux activity (Parkin 1987, McClain et al. 2003). These phenomena are challenging to account for in measurement, modeling and scaling efforts (Groffman et al. 2009). However, all is not bleak. Recent advances in measurement methods (cited above), new conceptual approaches for addressing hotspot and hot moment dynamics (Groffman et al. 2009), and clever new remote sensing and geographic information system-based scaling methods (Ollinger and Smith 2005) suggest that we are poised to make great improvements in our understanding of terrestrial denitrification over the next 5 – 10 years. These improvements will increase our basic science understanding of a complex biogeochemical process and our ability to manage widespread nitrogen pollution problems.