Incubation Studies of the Fate of Organic Nitrogen in Soils Amended with Activated Sludge

Abstract

AbstractMineralization of sludge organic N and the effect of transformations in nitrogen compounds on soil solution composition were studied in incubated soil‐sludge mixtures. Four soils were incubated with sludge at four mixing ratios (1–15% sludge) for 90 d at 35 °C under continuous flow of water saturated air. Soil‐water contents were maintained at 60% of the 0.033 MPa tension. The extent of ammonification was 41 to 61% of the amount of sludge organic N added. Both soil properties and sludge content affected the extent of ammonification. The soils' effect was relatively small. Only the clayey and sandy soils differed significantly (54.5 vs. 47.9%). The effect of sludge loading was more pronounced. At 1 and 5% sludge contents the ammonification was significantly greater (53 and 56% ammonified) than at 10% (48%) or 15% (43%) sludge contents. The reduced ammonification at the higher sludge and clay content might be related to the more intense nitrification‐induced salinization observed in these treatments. Sludge content and soil properties affected the initial rates of both ammonification and ammonia volatilization. Ammonification was linearly related to sludge and clay content, whereas the volatilization rate was similarly related to sludge content but inversely related to cation exchange capacity (CEC). Estimated half‐lifes for the added organic N is 2 to 8.5 d. Up to 87% of the ammonified sludge N, was lost as gaseous species, mostly as ammonia. Sludge application to soils and subsequent transformations could greatly influence soil solution chemistry and composition of exchangeable cations, depending mainly on its content.