Denitrification potential of low-grade weirs and agricultural drainage ditch sediments in the Lower Mississippi Alluvial Valley

Abstract

To enhance nitrogen transformations and increase agricultural nitrogen removal, best management practices (BMPs) need to focus on identifying and improving mechanistic removal pathways. Best management practices can be implemented which favor denitrification, the single most effective removal pathway in the nitrogen cycle. Low-grade weirs in agricultural drainage ditches provide conditions conducive for denitrification to occur; however, there is very little information on denitrification potential of agricultural drainage ditches, let alone low-grade weirs in these systems. This study aimed to provide the first examination of denitrification potential of agricultural drainage ditches with and without low-grade weirs and highlight how the age of weirs may result in changes in denitrification rates. Samples were collected in three distinct seasons (summer, winter, spring) during 2011 and 2012 from ditches with and without weirs. Sediment cores were plumbed as flow-through chemostats and inflow and outflow samples were analyzed for N2 gas to determine net denitrification rates. Overall there were no significant differences between net denitrification rates between sites with weirs and sites under conventional drainage (z=0.4526, P=0.65). Average net denitrification rates for weirs and conventional drainage were 2215±440 and 2541±727μgm−2h−1, respectively. This finding suggests that the potential for denitrification exists within both weired and conventional ditches, but a precipitating environment is limiting. Denitrification rates were significantly different between seasons (F=6.1879, P=0.0004) in all sediment cores, regardless of drainage characteristics, with summer>winter>spring. Differences due to age were also significant (F=2.6483, P=0.0224), with higher rates of denitrification in younger systems (≤1 year) vs. older systems (≥2 years), suggesting denitrification potential of weirs decreases with time. These data suggest that use of weirs can provide enhanced hydrological residence time in these systems, and thus promote conditions conducive for nitrogen removal via denitrification to occur. Thus low-grade weirs should be considered as BMPs for nitrogen removal in source agricultural landscapes, as they promote conditions conducive for denitrification.