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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #308660
Title: Nitrate sinks in perennial vegetation filter strips in the toeslopes of agricultural watersheds

Author
item MITCHELL, DAVID - Iowa State University
item XIABO, ZHOU - Iowa State University
item Parkin, Timothy
item HELMERS, MATTHEW - Iowa State University
item CASTELLANO, MICHAEL - Iowa State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2014
Publication Date: N/A
Citation: N/A

Interpretive Summary: Vegetation growing between agricultural fields and streams help prevent nutrient runoff from croplands to surface waters, and are often referred to as filter strips. Nitrate (NO3) is one nutrient that has several potential fates as it passes through a filter strip. Some of the nitrate can be removed by plants, fixed into soil organic matter, or converted to gas by a specialized group of soil bacteria. It is important to understand the rates of these potential reactions so that filter strips can be designed to maximize nitrate removal. This study was performed in central Iowa where the fate of nitrate added to a vegetative filter strip was determined. We found that plant uptake of nitrate and fixation of nitrate in the soil organic pool, together accounted for less than 30% of the nitrate added. The bacterial process that converts nitrate to nitrogen gas (called denitrification) was responsible for the nitrate removal. These results indicate that denitrification, which is the only NO3 sink that does not saturate in the long term, was the most important NO3 sink in these perennial vegetation strips. This information will be useful to scientists and land managers who design and evaluate vegetative filter strips to protect surface waters from agricultural pollution.

Technical Abstract: Integration of perennial filter strips (PFS) into the toeslope of agricultural watersheds may decrease downstream NO3 losses, especially if subsurface flow interacts with the rooting zone of the perennial vegetation. However, the long-term effectiveness of NO3 removal depends on the relative importance of several NO3 sinks in the PFS. Plant biomass and labile soil organic matter (SOM) are temporary NO3 sinks, while stable SOM is a long-term, but potentially finite, NO3 sink, and denitrification is a permanent NO3 sink. We investigated the relative importance of these NO3 sinks in perennial filter strips at the toeslope of row crop watersheds in Iowa, USA. Using 25 x 30 cm in situ mesocosms, we added 15NO3 to PFS soils and quantified 15NO3-N recovery in plant biomass and SOM after one growing season. Further, we compared 15NO3-N recovery in particulate organic matter (relatively labile) and mineral-associated organic matter (relatively stable) pools in soils with and without growing perennial vegetation. To determine the potential strength of the denitrification sink in these buffers, we compared denitrification enzyme activity in soils from the paired watersheds with and without PFS. Transfer of 15NO3-N into labile and stable SOM pools was rapid and initially independent of growing vegetation; however, the presence of growing vegetation increased 15NO3-N retention in SOM by the end of the growing season. Stable SOM was a more important sink for 15NO3-N than labile SOM. Nevertheless, recovery in SOM and plant biomass accounted for <30% of 15NO3-N inputs. Denitrification enzyme activity data indicated that dissolved organic carbon from perennial vegetation increased potential denitrifier activity in PFS soils. Together, these results constrain potential SOM and biomass NO3 sinks and indicate that denitrification is likely to be the most important NO3 sink in perennial filter strips.