Decomposition of Sewage Sludge Compost in Soil: IV. Effect of Indigenous Salinity

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Abstract
To determine the effects of indigenous salinity on rates of compost decomposition and nitrogen transformations, a sewage sludge‐woodchip compost was leached with water. Leached and unleached composts were mixed with a loamy sand at rates of 0, 112, and 224 t ha−1 (dry wt), equivalent to 5 and 10% of the dry soil weight, and incubated at 25°C in a constant atmosphere of CO2‐ and NH3‐free air for 160 d. Rates of compost C decomposition and N mineralization were determined by monitoring an air stream for CO2 and NH3 evolution, and by measuring changes in the extractable organic and inorganic fractions of C and N in soil with time.The initial rate of CO2 evolution from the soil amended with leached compost lagged behind that of the soil amended with unleached compost. However, after 33 d of incubation, the leached compost treatments mineralized C at a faster rate, and after 160 d had evolved 14 and 32% more CO2 for the 112 and 224 t ha−1 rates, respectively. The initial lag may have resulted from partial removal of the more readily decomposable water‐soluble C by leaching. The total cumulative compost C evolved as CO2 from the unleached and leached compost mixtures after 160 d of incubation were about the same, i.e, 14% and 16%, respectively. However, the amounts of net N mineralized in the unleached and leached systems were greatly different. Approximately 14% of the N in the unleached compost had mineralized after 160 d, whereas the soil amended with leached compost exhibited a net immobilization of mineral N. These results indicate that salts and other compounds that are present in sewage sludge compost (water soluble) can inhibit compost decomposition when added to soil, and enhanced decomposition results in more immobilization of mineral N when these inhibitory compounds are removed by leaching.