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Denitrification in agricultural drainage ditches under various hydrologic management regimes

Robert Kröger, Thad Scott

Reactive nitrogen (NR) loss from human-impacted landscapes has been implicated as a cause of water quality degradation in coastal environments, such as hypoxia in the Gulf of Mexico. However, NR in the form of plant fertilizer is essential for maintaining productive agriculture. Therefore, agriculture management techniques are needed that maximize N use efficiency and minimize NR loss to downstream environments. Agricultural drainage ditches are an obvious choice for management because they provide the necessary habitat (permanently or semi-permanently flooded soil) to transform NR into unreactive N2 through denitrification. However, there are few data on denitrification in agricultural ditches, and even fewer that were derived using modern techniques that are the most accurate.

We propose to measure denitrification in agricultural drainage ditches in the Mississippi River Delta which are experiencing various types of hydrologic management. Intact sediment cores will be collected from ditches with and without weirs, and ditches which have been managed with weirs over a time-gradient. Cores will be collected 4 times, twice in the summer/early fall of 2011, once in the winter of 2011-2012, and once more in the spring 2012. Ten intact cores will be collected on each trip and returned to the University of Arkansas, Fayetteville for incubation experiments and analysis. In addition to cores, ~25L of water will be collected for use in incubations.

In the lab, site water will be vacuum filtered through a 1.5 µm pore size filter and autoclaved. Intact cores will be plumbed with inflow and outflow lines and filtered sterilized water (receiving continuous aeration) will be pumped over the sediment surface. Inflow and outflow samples from cores will be collected daily for 4-5 days for the determination of 28N2/Ar, 29N2/Ar, 30N2/Ar, O2/Ar, NO2-N+NO3-N, and NH4-N. The concentration difference between inflow and outflow samples, corrected for flow rates, overlying water volume, and core surface area, represent the flux rates of each chemical species. After 2 or 3 days of incubation, the inflow incubation water will be amended with 15NO3 to a final concentration of 1 mg/L in order to trace potential denitrification from 29N2/Ar, 30N2/Ar using the isotope pairing technique (see Scott et al. 2008 for details). Final data from each core will include: Net N2 flux, potential denitrification, sediment oxygen demand, net NO2-N+NO3-N flux, and net NH4-N flux. All flux rates will be reported in µg XX m-2 hour-1. Replicate cores from each management regime will be used to calculate mean flux rates and differences among management techniques will be evaluate using Analysis of Variance in SAS 9.1.

Gulf of Mexico Research

Completed Projects