Nitrate in Forest Soils Following Biosolids Applications in a Working Plantation

Abstract

Biosolids applications to commercial tree plantations is standard practice in the Pacific Northwest. It has shown to increase stand growth, improve the quality of forest soils, soil and above ground carbon sequestration capacity, and plantation economic returns. However, biosolids applications are currently limited to minimize the potential for nitrate leaching to groundwater. Plantation managers reduce the risk of nitrate leaching by determining site-specific application rates that aim to match the nitrogen demand of the forest with that of available mineral nitrogen after additions and losses (e.g. mineralization, volatilization, and denitrification). This study examines a dataset collected through a 30 year-long biosolids monitoring program in a Douglas-fir plantation located in King County, Washington, to identify risks to groundwater that successive applications and variable rates of biosolids may pose. Soil nitrate, soil total carbon and total nitrogen were examined relative to biosolids application rate, application number, as well as a range of environmental variables including soil series, precipitation and temperature. The results of this study show little evidence that biosolids applications pose a risk to groundwater quality at the rates and frequencies used during the study period. While nitrate concentration in the A horizon was found to increase as the number of applications increased, nitrate in the A horizon had no linear relationship to the application rate. Additionally, there was no evidence of downward movement of nitrate; nitrate in the B horizon did not have any linear correlation with application number or rate. Additionally, the results of this study demonstrate that the nitrate concentration in the A and B horizons showed little change in concentration incrementally (δ, i.e. between applications), or over the course of the application period studied (Δ, i.e. t0 to tn, where n is the total number of applications a given unit received during the study period for a given unit). Collectively, these results suggest that the nitrate being added to plantation soils in biosolids applications is largely being taken up by the forest system, rather than being lost to leaching.