Nitrogen Transformations in Sewage Sludge—Amended Soils as Affected by Soil Environmental Factors

The objectives of this study were to determine the effects of soil pH, soil moisture tension, sludge application rate, incubation temperature, and nitrification inhibitor on N transformations in soils amended with an 15 N‐labeled sewage sludge. Less than 0.1% of added sludge‐N was lost from any of the samples due to NH 3 volatilization. Nitrification rates were faster in sludge‐amended soils as compared to nonamended acidic soils treated with NH 4 + ‐N. Adding calcareous sewage sludge to acid soils produced a more favorable pH for nitrification. Nitrification was faster in sludge‐amended soil having an initial pH of 7.5 than in samples at pH 6.0 or 5.3. Nitrite‐N accumulated to levels of 100 µg/g in sludge‐treated soils with high pH values (∼ 7.5). Nitrification proceeded at a faster rate in sludge‐amended soils adjusted to −0.25 and −0.5 bar moisture tension than in soils at −1 bar. Nitrification rates were not greatly affected by sludge application rate, although more time was required for complete nitrification of greater quantities of NH 4 + ‐N. Nitrapyrin inhibited nitrification for 112 days, after which nitrifying ability recovered. Temperature was the only factor evaluated which markedly affected mineralization of sludge‐organic N and immobilization of added inorganic‐N. The rates of both mineralization and immobilization increased as temperatures increased. At the end of 168 days of incubation at 21°C, 40% of added sludge‐organic N was mineralized in silt loam soils, but up to 26% of added NH 4 + ‐N was immobilized. There were only small net changes in inorganic‐N concentrations in sludge‐amended soils after 168 days of incubation.