Intermittent flooding of organic‐rich soil promotes the formation of denitrification hot moments and hot spots

Anthropogenic activities have altered the nitrogen cycle, necessitating management on the landscape level. Isolated time periods and areas, termed hot moments and hot spots, respectively, frequently account for a large percentage of nitrate removal in aquatic ecosystems. A series of experiments were conducted to determine the effect of hydrologic connectivity on denitrification rates, gene abundances, and nitrous oxide fluxes. Experimental areas were divided into flooded (always inundated), floodzone (intermittently inundated), and nonfloodzone (not inundated) locations in low‐organic and organic‐rich soil. Our results demonstrate that intermittent flood events enhance denitrification rates from days to weeks after flooding, depending on the inundation period. Microbial analysis demonstrated that short‐term flood events did not lead to increases in denitrifying gene abundances. Enhanced denitrification rates did not have a corresponding increase in the ratio of incomplete to complete denitrification. Incomplete to complete denitrification ratios were high in always‐inundated low‐organic sandy soil, peaking at 40%. Our results suggest that management strategies that promote hydrologic connectivity and intermittent flooding of organic‐rich floodplain soils promote the formation of denitrification hot moments and hot spots, with relatively low incomplete denitrification (<3% of the total denitrification rates). Anthropogenic activities have greatly altered the nitrogen cycle in the past century. While the Haber‐Bosch process has allowed for the modernization of agriculture and supports billions of people worldwide (Smil 2002), the rate of anthropogenic nitrogen fixation is now nearly double the natural rate of terrestrial, bacterial‐derived, fixation (Canfield et al. 2010). Nitrogen recovery by agricultural crops is typically <50% worldwide (Fageria and Baligar 2005), and approximately 25% of the nitrogen added to the biosphere is exported from rivers to oceans or inland basins (Mulholland et al. 2008). This inefficiency in crop nitrogen uptake and excess fertilizer applications in the intensively managed agricultural region of the Midwestern United States has caused negative impacts to human health, such as methemoglobinemia (Fan and Steinberg 1996, Powlson et al. 2008, Ward et al. 2010), and ecological consequences, such as alteration of the food web, a reduction in net photosynthesis, and the formation of oceanic hypoxic zones (Camargo and Alonso 2006, Turner et al. 2006, Rabalais et al. 2007). These consequences have led to an increased need in