Potential Release of Phosphorus from Soil Mixed with Sewage Sludge
Rydin, Emil | Otabbong, Erasmus
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.
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