Vegetated Ditch Habitats Provide Net Nitrogen Sink and Phosphorus Storage Capacity in Agricultural Drainage Networks Despite Senescent Plant Leaching

Abstract

The utility of vegetated ditch environments as nutrient sinks in agricultural watersheds is dependent in part on biogeochemical transformations that control plant uptake and release during decomposition. We investigated nitrogen (N) and phosphorus (P) uptake and release across four P enrichment treatments in ditch mesocosms planted with rice cutgrass (Leersia oryzoides) during the summer growing and winter decomposition seasons. Measured N retention and modeled denitrification rates did not vary, but P retention significantly increased with P enrichment. At the end of the growing season, root biomass stored significantly more N and P than aboveground stem and leaf biomass. Decomposition rates were low (<10% organic matter loss) and not affected by P enrichment. Nitrogen and P export during winter did not vary across the P enrichment gradient. Export accounted for <10% of observed summer N uptake (1363 mg m−2), with denitrification potentially accounting for at least 40% of retained N. In contrast, net P retention was dependent on enrichment; in unenriched mesocosms, P uptake and release were balanced (only 25% net retention), whereas net retention increased from 77% to 88% with increasing P enrichment. Our results indicate that vegetated ditch environments have significant potential to serve as denitrification sinks, while also storing excess P in agricultural watersheds.

Vegetation and Residence Time Interact to Influence Metabolism and Net Nutrient Uptake in Experimental Agricultural Drainage Systems

“…Additionally, a previous comparison of one and two-station models in larger experimental ditches found minimal differences in estimated denitrification rates, suggesting that biological activity within short reaches can have a larger effect on N metabolism than potential upstream influences, providing some support for our estimates [49]. Further, our previous smaller scale mesocosm studies have demonstrated that a significant proportion of N retention can be attributed to high denitrification efficiencies in L. oryzoides beds [36,37,39].…”

“…Under in situ field conditions, up to 57% reduction in dissolved inorganic nitrogen (DIN) has been observed over the course of a growing season [15], and denitrification from vegetated ditches has been reported to reduce N loads 28-56% [36,37]. In contrast, ditch sediments can act as sources and sinks of P depending on prior land application rates [38,39]. Ditches may mediate downstream P transport via hyporheic exchange [40], and Everglades ditches were reported to retain up to 55% of soluble reactive phosphorus over a 7d period [41].…”

Dynamic Interception Effect of Internal and External Nitrogen and Phosphorus Migration of Ecological Ditches

“…In addition, we find that the Shannon index of the reed root sediment in the ecological ditch was lower than that of the soil ditch sediment. The reason for this result may be that the reed root sediments increased a relatively large number of new species over a certain period compared with other root sediments, resulting in a dilution effect [30]. Instead, its Shannon community diversity index has declined.…”

Natural sources and controlling factors of urea–nitrogen concentrations in agricultural drainage ditches

“…(1) higher rates of nitrification and restricted denitrification from the increased supply of oxygen under a low water table, (2) reduced biological inputs in the ditch, (3) uptake by ditch vegetation, and/or (4) reduced sediment suspension because of the root establishment of the dense ditch vegetation (Borin & Tocchetto, 2007;Moore et al, 2010;Taylor et al, 2020).…”