Date of Award

1997

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Natural Resources

First Advisor

Art Gold

Abstract

Although riparian zones can remove nitrate (NO3) from groundwater, the interaction between vegetation type, subsurface patches of elevated organic matter and NO3 removal rates is still uncertain. The goal of my study was to examine the effects of vegetation type on groundwater NO3 removal rates in poorly drained riparian subsoils with four treatments, two "forested" and two "suburban" treatments. I also examined the variability in NO3removal rates between riparian sites and the relationship between patches of elevated organic matter in the subsoil and groundwater NO3 removal rates. My study was based on a mesocosm approach to stimulate NO3 dynamics in the shallow groundwater. These mesocosms were obtained from paired forested and suburban (mowed vegetation dominated by non-woody species) vegetated areas at two different sites. Intact horizontal mesocosms (15 cm diam., 40 cm long PVC cores) were extracted from fine to medium textured sands. The soils were derived from stratified glacial drift and were classified as Typic humaquepts or Umbric endoaquods. At each site, I obtained mesocosms from at least 35 cm below the dormant season water table depth. These mesocosms were placed in a controlled environment chamber and continuously dosed with bromide and NO3 amended groundwater for 71 days. Mass balance calculations were completed to determine the rate of groundwater NO3-N removal from each saturated mesocosm.

There was no significant difference in groundwater NO3 removal rates between forested and suburban vegetated areas within sites. However there were significantly differences in groundwater NO3 removal rates between sites. Groundwater NO3 removal rates were correlated with the total mass of carbon contained within small, amorphous pr root-like, dark stained patches of elevated organic matter. By determining the mean mass of patch per mesocosm per treatment, these patches were found to constitute 0.5 to 16.8% of the mesocosm mass. There was a significant correlation between dry mass of roots and the mass of patch material C contained within mesocosms.

The inflow groundwater contained negligible concentrations of ammonium-N (NH4-N). However,, NH4-N was found in the outflow of groundwater even before NO3 amendments were commenced. This suggests that the NH4 was generated within the mesocosms and was produced by mineralization of pools of organic N within the mesocosms rather than by dissimilatory reduction of NO3. Temporal and treatment related trends in NH4 generation were not the same as NO3-N removal trends indicating that different processes are responsible for the fate of each form of N.

The existing type of vegetation cover did not influence groundwater NO3 removal, rather the quantity and/or quality of subsoil patches of elevated organic matter appear to be a critical factor in NO3 removal from groundwater in vegetated riparian zones. These results suggest that the NO3 abatement function of vegetated riparian zones is not limited to native forest ecosystems. Further investigations on the linkage between vegetation type, subsurface patches of elevated organic matter and groundwater N removal are warranted. However, suburban vegetated riparian zones may prove useful for limiting the export of NO3 from watersheds.

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