Investigating the role of organic carbon amendments and microbial denitrification gene abundance in nitrogen removal from experimental agricultural drainage ditches with low‐grade weirs

Abstract

Low-grade weirs placed within agricultural drainage ditches in the Lower Mississippi Alluvial Valley can be used as a management practice to enhance nitrogen removal. The addition of organic carbon amendments within ditches that contain weirs could further increase nitrogen removal. Through repeated trials, changes in NO − 3 -N concentration between inflow and outflow were variable in the ditch without weirs, while only decreases in concentration were observed in ditches with weirs. Significant differences in NO − 3 -N concentrations were observed between treatments, with greater removal of NO − 3 -N observed in dissolved organic carbon treatments compared to control and particulate organic carbon treatments. At medium-and high-flow rates, respectively, dissolved organic carbon treatments resulted in greater NO − 3 -N concentration decreases of 31.6% and 27.1% compared to 19% and 11.6% in particulate organic carbon treatments and 18.6% and 17.2% in control treatments. Significant effects of weirs and sampling date on nirS, nirK, nosZ, and 16S rRNA gene abundances were observed. Observed increases in NO − 3 -N removal with organic carbon amendments, provides support for continued investigation on improving the efficacy of organic carbon amendments as a best management practice for NO − 3 -N removal in agricultural drainage ditches. • Practitioner points• Dissolved organic carbon amendments increased nitrate-nitrogen removal. • Only decreases in nitrate-nitrogen concentration were observed in ditches with weirs. • Increasing flow rate did not affect nitrate-nitrogen removal. • Abundance of denitrification-performing microbes likely did not affect N removal. • Lack of anaerobic soil conditions and short residence time reduced nitrate-N removal.

Bioremediation of Nitrate in Agricultural Drainage Ditches: Impacts of Low-Grade Weirs on Microbiomes and Nitrogen Cycle Gene Abundance

“…This is in contrast to the previous studies, in which no difference was seen in the microbial communities between treatment and control ditches in Mississippi [49]. The impact of low-grade weirs on the abundance of denitri cation genes was also inconsistent [47,49]. The signi cance found in our study could be attributed to the long-term effects of low-grade weirs on sedimentation and organic carbon accumulation [46].…”

“…The signi cance found in our study could be attributed to the long-term effects of low-grade weirs on sedimentation and organic carbon accumulation [46]. The low-grade weir used in this study was placed in the ditch channel in 2006, 14 years before the sampling campaign began, while the samples being analyzed in previous studies were collected two years after the installation of the weirs [47,49]. Stable water ow through the experimental ditches, and the constant in ow of nutrient-rich tile-drained water could facilitate the development of a specialized microbiome for denitri cation, particularly in the ditch with the low-grade weir.…”

“…The installation of low-grade weirs in agricultural drainage ditches has shown some promising performance in terms of drainage volume reduction and water quality improvement [19,23,46,47]. However, limited studies have looked into the long-term effect of low-grade weirs on the microbiomes in agricultural drainage ditches.…”

“…This may be related to the concentration of substrates such as nitrogen and carbon, which are typically found at higher concentrations near the inlet of systems such as woodchip bioreactors receiving tile-drained water [52]. Faust et al [47] also found that the addition of labile carbon to the ditch systems equipped with low-grade weirs could enhance the nitrogen removal rate and the abundance of denitri cation functional genes. In order to better understand the biogeochemical processes affecting the formation of denitri cation "hotspots'' in unmanaged ditch systems, liquid chromatography-mass spectrometry approaches could be used to qualitatively and quantitatively assess the labile carbon pool across the ditch channel [53].…”

Editorial

“…Guo, Guan, Sun, Liu, and Zhao (2020) assessed the removal of both nutrients and heavy metals such as zinc from swine operations using microalgae‐based treatments. Beyond municipal and confined animal operations as point source contributions of nutrients, crop‐based agricultural contributes substantially to nutrient loading, and implementation of low‐grade weirs coupled with organic carbon amendments can stimulate nitrate removal via microbial denitrification in agricultural drainage ditches (Faust et al, 2020).…”