Impact of agricultural activities on anaerobic processes in stream sediments

Streams draining agriculture watersheds are subject to significant anthropogenic impacts, including sedimentation from soil erosion and high nitrate input from heavy fertilizer application. Sedimentation degrades habitat and can reduce hydrologic exchange between surface and subsurface waters. Disconnecting surface and subsurface flow reduces oxygen input to hyporheic water, increasing the extent of anoxic zones in stream sediments and creating hotspots for anaerobic processes like denitrification and methanogenesis that can be important sources of nitrous oxide and methane, both powerful greenhouse gases. Increased nitrate input may influence greenhouse gas fluxes from stream sediments by stimulating rates of denitrification and potentially reducing rates of methanogenesis, either through direct inhibition or by increasing competition for organic substrates from denitrifying bacteria. We hypothesized that accumulation of fine sediments in stream channels would result in high rates of methanogenesis in stream sediments, and that increased nitrate input from agricultural runoff would stimulate denitrification and reduce rates of methane production. Our work focused on streams in northern and central Minnesota, in particular on Rice Creek, a small stream draining an agricultural watershed. We used a variety of approaches to test our hypotheses, including surveys of methane concentrations in surface waters of streams ranging in sediment type and nitrate concentration, bottle incubations of sediment from several sites in Rice Creek, and the use of functional gene probes and RNA analyses to determine if genes for these processes are present and being expressed in stream sediments. We found higher methane concentrations in surface water from streams with large deposits of fine sediments, but significantly less methane in these streams when nitrate concentrations were high. We also found high potential for both methanogenesis and denitrification in sediment incubations. Results from nitrate amendments of these incubations were mixed, with sediments from some sites showing stimulation of denitrification and inhibition of methanogenesis as hypothesized, and others showing little change in denitrification and a small increase in methanogenesis. We speculate that these results can be explained by differences in resource availability and a shift from nitrogen to carbon limitation of denitrification. Analyses of microbial DNA samples showed that the genes for both processes were ubiquitous in Rice Creek sediments, while RNA analyses showed a strong connection between the expression of these genes and process rates measured in bottle experiments. These results suggest that a complex relationship between sedimentation, carbon availability, and nitrate input in stream sediments determines the dynamics of anaerobic processes. This work highlights the need to understand how anthropogenic impacts interact if we are to successfully forecast ecosystem responses, and how these responses may feedback on environmental changes at a range of scales.