Nitrogen dynamics in forest soils after municipal sludge additions

Nitrogen dynamics were assessed for a forested site in southern New Hampshi re during the first and second growing seasons after surface application of an aerobically-digested, limed, liquid municipal sludge. Sludge was applie d in June 1989, at 3.3, 6.9, and 14.5 M ha(-1) (or 199, 396, and 740 kg TKN ha(-1)). Elevated net N mineralization (in situ buried bags) occurred in t he organic and upper mineral soil horizons during the first two months afte r sludge application, but was similar to control levels thereafter. Net N m ineralization was negligible at 30 and 60 cm soil depths. Foliar N concentr ation increased with sludge loading rates. Concentrations of NH4+ and NO3- in soil leachate were low, except in early fall when microbial activity was still high and plant demand was low. In trenched subplots where vegetative uptake was eliminated, NO3- concentrations in soil leachate (60 cm) rose t o between 15 and 35 mg N L-1 in the first year and remained high in the sec ond year. Other studies reported higher magnitudes of NO3- leaching from tr eated plots. These studies and the findings reported here shown the charact eristics of the sludge being applied to land are at least as important as t he physical and chemical characteristics of the site to which they are to b e applied.