Phosphorus Forms in Manure and Compost and Their Release during Simulated Rainfall

TLDR

Manure from animal feeding operations accumulates high levels of phosphorus, raising the risk of eutrophication‑driven water‑quality impairment through runoff. The study aimed to quantify the amounts and relative solubilities of phosphorus in various manures and composts using a modified Hedley fractionation and simulated rainfall release. Twenty‑four samples of dairy, poultry, and swine manures and their composts were collected over two years and tested in 15‑cm laboratory columns subjected to 70 mm h⁻¹ rainfall for 30 min. Total P ranged 2,600–40,000 mg kg⁻¹, mostly inorganic, with 80 % water‑extractable; leached P (34–75 mg L⁻¹) during simulated rainfall correlated strongly (r² ≥ 0.98) with water‑extractable inorganic or organic P, suggesting this fraction predicts runoff enrichment.

Abstract

Abstract The large accumulation of P in manure from animal feeding operations in localized areas has increased the potential for P export following land application. Impairment of freshwater quality by accelerated eutrophication has focused attention on manure management and the potential for P loss in runoff. Thus, we investigated the amounts and relative solubilities of P in manures and their composts using a modified Hedley fractionation and release of P during simulated rainfall (70 mm h −1 for 30 min) in laboratory columns (15‐cm diameter). Twenty‐four samples each of dairy manure, dairy manure compost, poultry manure, poultry manure compost, poultry litter, and swine manure were collected over 2 yr. Total P concentration ranged from 2600 to 40 000 mg kg −1 , mostly as inorganic P (63 to 92%). The distribution of inorganic and organic P fractions depended on manure and compost type. Most of the inorganic P, 80%, was water extractable (2030 mg kg −1 ), while 55% was hydroxide extractable in swine slurry (16 620 mg kg −1 ) and 38% acid extractable in poultry manure (9320 mg kg −1 ). The dissolved inorganic P concentration in leachate from manure and compost (10 Mg ha −1 manure application rate) during 30‐rain rainfall ranged from 34 (poultry litter) to 75 mg L −1 (poultry manure). The amount of P leached by five simulated rainfall events was significantly correlated to respective water extractable inorganic ( r 2 = 0.98) or organic P ( r 2 = 0.99) of each material. This suggests that water extractable P may be used to estimate the potential for land‐applied manures or composts to enrich leachate and surface runoff P.