Organic Nitrogen Mineralization Rate in Sewage Sludge‐Amended Mine Soil

Abstract

This study was carried out on lignite mine tailings at Meirama (north‐western Spain). A laboratory experiment was conducted to investigate the effects of amending lignite mine spoil with municipal sewage sludge at rates of 5, 25, and 50 Mg ha−1 triplicate samples of soil, and each treatment was incubated from 0 to 90 days at constant moisture and temperature (85% air relative humidity and 25°C). Each sewage sludge dose was packed into PVC columns and inserted vertically into the recipient containing the upper layer of mine soil. A set of the incubated columns was removed at 15, 45, and 90 days, and the soil underlying each column was sampled for analysis. Nitrogen (N) mineralization rate and the evolution of soil pH, C/N ratio, organic matter, total N, organic‐N, NO3 −‐N, and NH4 +‐N content were measured. Sewage sludge application produced an immediate increase of the inorganic N at all dosage levels, mainly in the ammonium form. The nitrogen mineralization in the amended soils was higher than in the control soil, mainly in the case of those treated with 25 Mg ha−1. The net mineralized organic N was always positive, except at the end of the period of incubation in the 5 Mg ha−1sewage sludge treated soil. The nitrogen mineralization rate, calculated as a decrease in the organic nitrogen content, overestimates the amount of inorganic nitrogen that is available. The measurement of the cumulative amount of NH4 +‐N and NO3 −‐N provided a much lower estimation of N mineralization under incubation conditions.