Vegetation and Residence Time Interact to Influence Metabolism and Net Nutrient Uptake in Experimental Agricultural Drainage Systems  
[PERSON]  
1 USDA-ARS Southeast Watershed Research Laboratory, Tifton, GA 31794, USA11 NIGong, R.L. ; [PERSON]. 1 USDA-ARS Southeast Watershed Research Laboratory, Tifton, GA 31794, USA11 NIGong, R.  
ditches remain common features within agroecosystems of the MAP and other alluvial plain regions of the world. Constructed wetlands are a common management practice for remediation of nutrients in agricultural runoff [8; 9; 10; 11]. Plant uptake, microbial nitrification and denitrification, and substrate adsorption facilitate nutrient removal in constructed wetlands [12; 13; 14]. However, constructed wetlands can be perceived as taking valuable farmland out of production, thus limiting their adoption as a management practice in agricultural watersheds. Alternatively ditches within existing agricultural drainage networks can act as linear constructed wetlands as they have hydric soils, a diverse community of hydrophytes, and a wide range of soil saturation [15]. Furthermore, ditches may provide other ecosystem benefits including provisioning of food resources and habitat within heavily modified landscapes. Within the MAP and other regions of the world, land managers are beginning to recognize the ecological value of ditches and actively manage them for nutrient processing and other ecosystem benefits [13; 14; 15; 16; 17; 18]. Two factors can be managed to increase the nutrient processing properties of agricultural ditches: hydraulic residence time (HRT) and wetland vegetation. Increased HRT, the time a parcel of water spends in a given reach, has been shown to be an important factor in the efficacy of nutrient mitigation [19] through either biological or physical processes [20]. A variety of management approaches including alteration of channel morphology, weirs, the construction of two-stage ditches, and vegetation have been used to reduce water velocities and increase HRT in ditches [21; 22; 23]. Residence time has been shown to increase when vegetation is present in wetlands [24], streams [20], and ditches [25], with dominant species morphology influencing both water flow and resulting sediment dynamics [26; 27; 28]. Furthermore, increasing vegetation density can reduce turbulence intensity while reducing mean flow rates [27]. Establishment of wetland vegetation can also reduce nutrients and pesticides in ditches by enhancing biological processes common in wetlands [29; 30; 31; 32]. Primary N removal mechanisms include plant uptake, sediment sorption, and microbial processing such as nitrification and denitrification [33]. Metabolism can be a determinant of nitrogen processing rates [34]; in turn, vegetation can amplify gross primary production (GPP) and ecosystem respiration (ER) [35]. Moreover, vegetation has been shown to be particularly effective in reducing nitrate prior to transport downstream. 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]. However, there is a lack of experimental evidence regarding how interactions between increasing HRT and vegetation affect nutrient retention and removal from agricultural runoff in aquatic agroecosystems. Most studies have relied on empirical observations [18; 42]. This is in large part due to the substantial difficulty in replicating agricultural ditches under field conditions as well as challenges associated with controlling HRTs across ditches.