Contrasting Nutrient Mitigation and Denitrification Potential of Agricultural Drainage Environments with Different Emergent Aquatic Macrophytes

Abstract

Remediation of excess nitrogen (N) in agricultural runoff can be enhanced by establishing wetland vegetation, but the role of denitrification in N removal is not well understood in drainage ditches. We quantified differences in N retention during experimental runoff events followed by stagnant periods in mesocosms planted in three different vegetation treatments: unvegetated, cutgrass [ (L.) Sw.], and common cattail ( L.). We also quantified denitrification rates using membrane inlet mass spectrometry from intact cores extracted from each mesocosm treatment. All treatments retained 60% or more of NO-N loads during the 6-h experimental runoff event, but mesocosms planted with cutgrass had significantly higher (68%) retention than the cattail (60%) or unvegetated (61%) treatments. After the runoff event, mesocosms planted in cattail reduced NO-N concentrations by >95% within 24 h and cutgrass achieved similar reductions within 48 h, whereas reductions in the unvegetated mesocosms were significantly less (65%). Cores from cutgrass mesocosms had significantly higher average denitrification rates (5.93 mg m h), accounting for as much as 56% of the immobilized NO-N within 48 h, whereas denitrification rates were minimal in cores from the unvegetated (-0.19 mg m h) and cattail (0.2 mg m h) mesocosms. Our findings have implications for mitigating excess NO-N in agricultural runoff. While vegetated treatments removed excess NO-N from the water column at similar and significantly higher rates than unvegetated treatments, the high denitrification rates observed for cutgrass highlight the potential for permanent removal of excess N from agricultural runoff in vegetated ditches and wetlands.

Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass

“…Agricultural ditches are the first point of contact for cropland runoff entering freshwater networks. Sediments vegetated with cutgrass immobilize a significant fraction of NO 3 − at relatively high rates of NO 3 − loading and permanently remove up to 50% of the NO 3 − load via denitrification during the growing season (Taylor et al, 2015; the current study). Given the extent of ditch networks in LMRB agricultural landscapes, establishing cutgrass in ditches at the landscape scale may provide permanent (via denitrification) and temporary (via plant assimilation) removal mechanisms for excess NO 3 − that can contribute to reductions in N pollution to downstream ecosystems.…”

“…Cutgrass [ Leersia oryzoides (L.) Sw.] and cattail ( Typha latifolia L.) reduced NO 3 − concentrations more rapidly than bur‐reed ( Sparangium americanum Nutt. ; Tyler et al, 2012), and Taylor et al (2015) demonstrated that cutgrass, a common native emergent plant that establishes quickly within ditch channels in the LMRB, enhances denitrification significantly more (56% of NO 3 − uptake) than cattail (2% of NO 3 − uptake) in ditch sediments. This is likely due to certain functional traits associated with cutgrass, including higher biomass turnover rates (Farnsworth and Meyerson, 2003), which may result in an increased supply of organic C to denitrifying bacteria (Weisner et al, 1994), and lower convective flows, which inhibit transfer of O 2 to root systems and enhance favorable redox conditions at the sediment–water interface (Pierce et al, 2009; Konnerup et al, 2011).…”

Urea Release by Intermittently Saturated Sediments from a Coastal Agricultural Landscape

“…3c) and reflected the release of sediment NO 3 − ‐N initially present plus any new nitrification (Qiu and McComb, 1996). Rapid denitrification was the mostly likely explanation for the universal decrease in NO 3 − ‐N after 8 h (Qiu and McComb, 1996; Taylor et al, 2015). Denitrification can be inhibited by ditch cleaning (Smith and Pappas, 2007); however, the lack of significant differences in NO 3 − ‐N between the cleaned and uncleaned ditches shows that the denitrification potential of cleaned ditch sediments had recovered in the year since it was dredged.…”

“…Sedimentation of eroded soil and organic matter requires periodic dredging of ditches to maintain hydrologic function (Smith and Pappas, 2007). Ditch cleaning results in removal of organic matter, plants, and microbial communities that facilitate biogeochemical cycling (Smith and Pappas, 2007; Strock et al, 2007; Taylor et al, 2015). Agricultural drainage ditches in this area are cleaned as needed to remove vegetation and sediment and reestablish the original depth.…”

Nitrate availability affects denitrification in Phragmites australis sediments

“…Rooted macrophytes act as "ecosystem engineers" for N removal (Jones, Lawton, & Shachak, 1994;Vila-Costa et al, 2016) by influencing denitrification in several ways: for example, by (i) increasing organic matter accumulation (i.e. root exudates, decaying plant biomass, trapped suspended material), thus providing labile organic carbon whose mineralization promotes anoxic conditions (Hang et al, 2016;Schoelynck et al, 2017;Taylor, Moore, & Scott, 2015); (ii) releasing oxygen in the rhizosphere and establishing oxic−anoxic interfaces where the coupling of aerobic and anaerobic processes (such as organic N mineralization, nitrification, and denitrification) occurs (Rehman, Pervez, Khattak, & Ahmad, 2017;Soana et al, 2015); and (iii) promoting microbial population growth and diversity through the provision of submerged surfaces (e.g., stems and leaves) available for colonization (Soana, Gavioli, et al, 2018;Toet, Huibers, Van Logtestijn, & Verhoeven, 2003). Denitrification results in permanent removal of bioavailable N. Thus, assessing this process is a pivotal scientific and management task for decreasing eutrophication of aquatic ecosystems in human-impacted watersheds.…”