Redistribution of Biosolids‐Derived Total Phosphorus Applied to a Mollisol

Abstract

Abstract The amount of P applied to land with biosolids is generally more than plants need when the application rate is based on the potentially available N. The subsequent accumulation of P is of environmental rather than agronomic concern, because it could lead to eutrophication of water bodies if P is carried off‐site in surface runoff, subsurface drainage water, or eroded soil. In a large‐scale field study with three biosolids application rates (approximately 0, 7.4, and 13.0 Mg dry matter ha −1 ), we investigated the redistribution of P derived from surface‐applied biosolids in soil under poplars and switchgrass. Soil samples were collected from two depths (0–5 cm and 5–25 cm), and total P was determined by colorimetry after strong acid digestion. Total P in mobile soil water samples collected by zero‐tension lysimeters was determined by inductively coupled plasma spectrometry. After 6 yr of continuous biosolids application to poplar tree ( Populus sp.) plots, total P in soil increased significantly (α = 0.05) at both the 0‐ to 5‐ and 5‐ to 25‐cm depths under poplars. Also at both depths, a linear relationship between total P in soil and the amount of P applied with biosolids (α = 0.05) was found. Under switchgrass ( Panicum virgatum L.), total P increased significantly (α = 0.05) at only the 0‐ to 5‐cm depth. A linear relationship between total P in soil and the amount of P applied with biosolids (α = 0.05) was found at the 0‐ to 5‐cm depth under switchgrass but not at the 5‐ to 25‐cm depth. The amount of P taken up annually by switchgrass was estimated to be five times more than that taken up by poplars. The amount of P leached in mobile soil solution of switchgrass plots was estimated to be twice that leached in soil solution of poplar tree plots. Phosphorus that accumulated in the upper 5 cm of the soil accounted for ≥45% or more of the total P applied with biosolids, except for the low biosolids application rate to switchgrass plots where the value was 28%.