Potential release of phosphorus from soil mixed with sewage sludge

Retention capacities of phosphorus (P) in three soils (sandy loam, sandy clay, and sandy clay loam) and in soil mixed with 50 g kg−1 (5% ww) digested sewage sludge (Al- or Fe-precipitated) were measured in a laboratory study. Artificial rainwater, continuously leached through different samples, released about 15% of the total-P content (0.6–1.1 g P kg−1) of the soils. Adding Fe-precipitated sludge to the soil approximately doubled the P-concentration (1.1 to 1.7 g P kg−1) in the samples, and 20% was released. Up to 43% of the total-P (1.1 g P kg−1) was released when Al-precipitated sludge was mixed with the sandy loam soil. The release process was described by a first-order decay equation, and release rates between 0.04 and 0.07 d−1 or 35 mm water passing−1 were obtained. The released amounts could be considered as potential mobile phosphate under the experimental conditions used and are compared with potentially crop-available P. Phosphorus fractionation revealed that P adsorbed to Fe and Al was more or less exhausted, declining from about 35% at the start of the experiment to 5% of tot-P by the end. The inert P-pool increased, indicating that P transformations favored the production of more stable compounds. The accumulation of P in urban regions and the risk for nonpoint pollution of surface- and groundwaters are discussed.