The role of woody vegetation in preventing ground water pollution by nitrogen from septic tank leachate

Abstract

The ability of woody vegetation to remove nitrogen from septic tank leachate was studied in pine upland, oak upland and hardwood wetland habitats of the New Jersey Pinelands. The study was stimulated by the incorporation of a term for plant uptake in nutrient dilution models used for Pinelands land-use management decisions. Plant response was studied at sites involving septic tank leach fields and matched control sites for each habitat type. At each site, total biomass, net production, and tissue N concentrations for the dominant species was determined. The hardwood wetland habitat had a total biomass of 15.9 MT ha −1, a net primary production of 5.4 MT ha −1yr −1 and a net N uptake 75–80 kg N ha −1yr −1. Tissue N values for wetland trees and shrubs did not show significant differences between control and experimental sites. The pine upland communities had a biomass of 55 MT ha −1 and a net production of 5.7 MT ha −1yr −1; net N uptake ranged from 45 kg N ha −1yr −1 (control sites) to 56 kg N ha −1yr −1, (experimental sites). The oak upland communities had a biomass of 59 MT ha −1 and a net primary production of 5.0 MT ha −1yr −1; net uptake ranged from 55 kg N ha −1yr −1 in the control sites to 69.3 kg N ha −1yr −1. Tissue N concentrations showed significant increases for tree but not shrub species in both upland habitats. The capacity of the upland woody plants to increase N uptake and storage appears to be related to rooting depth and to the proximity of the plants to the drain tile: only plants with deep taproots, growing close to (within 1 m) the trench showed significant increases in uptake. It is suggested that only by augmenting appropriate natural vegetation with supplementary tree plantings can upland vegetation be utilized to reduce nitrogen movement to groundwater from septic systems in sandy soil; vegetation at naturally-occurring densities will not have a significant effect on water quality.