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

Abstract

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).