Impact of long-term application of biosolids to surface water nitrogen in a working forest plantation in King County, Washington

Abstract

Forest fertilization with municipal biosolids has been shown to increase tree growth and enhance forest soils. However, there are concerns that nitrogen from the biosolids could impact surface waters through movement from subsurface flow. Here we analyzed data on soil and surface water nitrogen from a working tree plantation that has used biosolids for over three decades to see if there was evidence of N movement through the soil to surface waters. GIS (Geographic Information System) was used to map application units over time and LiDAR (Light Detection and Ranging) was used to delineate watersheds. The program is located in King County Washington with biosolids provided by the King County Wastewater Treatment program. We assembled records to determine if there is any evidence of movement of NO3− through soils or any enrichment in surface waters. While soils show evidence of NO3− enrichment following biosolids application with cumulative loading rates up to 26 Mg ha−1, this is generally limited to the ’A’ soil horizon and does not increase linearly with increased biosolids loading rates. There was no indication of increased surface water NO3− concentration relative to biosolids application rates, with a small trend of decreasing water NO3− over time. Surface water NO3− concentration was not correlated with the fraction of the watershed area that had been amended with biosolids, and there was no observable increase in surface water NO3− with increased frequency of biosolids applications to the tree plantations. These results suggest that the current biosolids program is sufficiently protective of ground and surface waters. These observations suggest that biosolids application can be conducted on a large scale with multiple benefits and no discernible impact to surface waters.