The mobility and fate of phosphorus following municipal biosolids application to forest soils

Abstract

Municipal biosolids are typically not used on the steepest of forested slopes in the Pacific Northwest. The primary issue in using biosolids on steep slopes is movement of constituents in biosolids to surface waters during runoff events. There is a particular concern with biosolids phosphorus (P), as this nutrient limits productivity of most fresh surface water systems in western Washington. Application rates are based on the release of nitrogen from biosolids, without consideration of the cumulative effect of repeated applications on soil P levels. This dissertation examines P mobility following biosolids application to forested slopes on two scales: whole watershed and small plot. It also examines the vertical mobility and fate of P following biosolids fertilization to two different forest soils that received heavy experimental and lesser application rates using surface application to the forest floor or direct soil incorporation. A small creek draining a 21 ha watershed was monitored for P forms before and after biosolids application to 40% of the watershed. Direct runoff from biosolids into surface water did not occur. Elevated surface water discharge generally did not change the concentration of any P form; biosolids had no effect on this relationship. Soil water interflow was suspected as a conduit for direct runoff of P from biosolids to surface waters during heavy rainfall events. Sampling soil water interflow under a range of discharge conditions showed that phosphate concentrations in soil water decline with horizon depth and, when detected, phosphate concentrations decrease with increasing discharge regardless of biosolids application. A sequential P fractionation approach was used to identify operationally defined P pools following biosolids, application to acidic forest soils representing two contrasting soil types. There is phosphorus retention in the soil following surface application of biosolids; no statistically significant differences in mean P fraction concentrations between treated and control soils were noted below 15 cm. The dominant mechanism for retention of surface applied biosolids P was adsorption to Al and Fe oxides. P availability was significantly increased in all biosolids treated soils examined between 0--15 cm using application rates between 105 and 470 Mg ha--1 (2--10 Mg P ha --1).