Nitrogen Cycling in Piedmont Vegetated Filter Zones: II. Subsurface Nitrate Removal

Abstract

Subsurface flow often constitutes the major pathway for movement of dissolved nutrients such as NO3‐N from agricultural fields. The objectives of this study were (i) to determine the changes in shallow groundwater chemistry along a piezometric gradient from agricultural fields, across grass‐vegetated field edges and through adjacent forest on two Piedmont watersheds and (ii) determine the relative importance of dilution, denitrification, and plant uptake in subsurface NO3 attenuation. We monitored changes in groundwater chemistry at three depths along a piezometric gradient from an agricultural field through a grass field edge and through a forested filter zone (FFZ). We measured marked decrease in nitrate concentrations from 8 to 10 mg L−1 at the field edge to almost 0 at the forest edge; Cl concentrations remained within the range of 8 to 10 mg L−1, suggesting that dilution was not an important factor in NO3 concentration reductions. At a third site, we introduced NO3‐N and a conservative tracer, bromide, into the soil profile at both the grass‐vegetated field border and the forested area, to determine mechanisms responsible for the observed decrease in NO3‐N concentrations. Using ion concentration ratios we determined that nitrate attenuation in the grass‐vegetated field edge was low compared to the forest. Nitrate loss in the forest was almost exclusively through denitrification; plant uptake was insignificant in these experiments. Although grass‐vegetated field borders were less effective than riparian forests at NO3‐N removal, considerable reductions were observed in these areas on the experimental watersheds. Similar reductions would be expected over shorter distances in riparian forests.

A diagnosis-prescription system for nitrogen management in environment

“…Experimental results have indicated that grassed and forested riparian buffers and filter strips are effective at removing NO 3 -N from subsurface flow, mostly through denitrification. [3,13,15] Also, some studies reveal efficient surface flow removal of nutrients, sediments and pesticides. [16] However, the filtration capabilities of the riparian buffers, as determined in field studies and laboratory experiments, may not always be fully realized at the watershed scale.…”

Effect of margin strips on soil mineral nitrogen and plant biodiversity

“…Margin strips may reduce nitrate leaching to surface waters (Hefting, 2003) or function as a filter preventing runoff of sediments and agrochemicals from reaching nearby habitats with susceptible or vulnerable organisms such as ditches or nature reserves (Mander et al, 1997;Verchot et al, 1997). Apart from the combined effect of direct N uptake and N incorporation (immobilization) in litter, buffer strip vegetation has a significant indirect role in N removal by stimulating denitrification activity through the supply of organic matter by litter and root exudates (Hefting, 2003).…”

“…However, information is not abundant concerning mineral N rates and losses in soil horizons under field margin strips during winter months, or concerning the optimal width of field margin strips for reducing mineral N content in soil. Most studies mainly deal with forested buffer strips (Haycock and Pinay, 1993;Schultz et al, 1995;Mander et al, 1997;Verchot et al, 1997). Grass strips were found to reduce soil nitrate nitrogen concentrations by approximately 50% (Verchot et al, 1997).…”

“…Most studies mainly deal with forested buffer strips (Haycock and Pinay, 1993;Schultz et al, 1995;Mander et al, 1997;Verchot et al, 1997). Grass strips were found to reduce soil nitrate nitrogen concentrations by approximately 50% (Verchot et al, 1997). Nor are there many data about the optimal dimensions of margin strips to function as a buffer against drift of agrochemicals.…”

Using time‐domain reflectometry to characterize shallow solute transport in an oak woodland hillslope in northern California, USA

“…As a result, there is a continuing need to develop methods to assess and manage NPS pollution. In order to model the production of NPS pollution, relationships between pollutant resident time and degradation of labile contaminants during transport must be understood (Fetter, 1977;Verchot et al, 1997;). Estimates of solute residence time are necessary as dilution and degradation of pollutants can increase with time, depending on interactions with the soil matrix, resulting in lower pollutant concentrations reaching streams.…”