Mineralization of soil and legume nitrogen in soils treated with metal-contaminated sewage sludge

Abstract

Eighty percent of urban sewage sludge in southeastern Australia is destined to be reused on agricultural land to improve soil fertility. However, this sludge is usually contaminated with industrial pollutants, in particular with heavy metals. As heavy metals are known to be toxic to microorganisms, concern has been raised that treating soils with these sludges may adversely affect the mineralization of the organic N in the soil, sludge or plant material incorporated into the amended soils. In the absence of historically contaminated soils, dewatered sewage sludges with total heavy metal contents to 4658 mg kg −1 were ground and mixed with different soils at rates up to 240 t ha −1. The soil–sludge mixtures were then ‘aged’ through seven cycles of wetting and drying, in the presence of plants, over a period of twelve months. Total heavy metal concentrations to 1026 mg kg −1 soil, with individual metal concentrations (mg kg −1 soil) to Zn (481), Cu (249), Cr (187), Ni (86), Pb (80) and Cd (2.5), were achieved with the treatments. Samples of the processed soils were incubated at the field capacity water contents, with or without incorporated lucerne, before determining the concentration of available N (nitrate+ammonium) in the soil. More available N occurred in soils treated with sludge than in unamended soils, and available N increased with the amount of sewage sludge added. The N in lucerne was mineralized in all treatments, and there were few cases in which the increase in available N due to lucerne was reduced in sludge-amended soils. These cases involved nitrate loss and could not be ascribed to an effect of high concentrations of heavy metals in soil. Soil amendment with sludge increased the concentrations of total N, C and exchangeable cations, as well as pH, in the soil. Of these factors, only total N and exchangeable cations were positively correlated with available N. Higher concentrations of C (or heavy metals) in soil and higher pH were associated with less available N, but these effects were quantitatively inferior to the positive effects of total N and exchangeable cations. Based on the results of these studies the current limits on the allowable concentrations of heavy metals in soils, as defined by the New South Wales Environmental Protection Agency, can be substantially increased without affecting the benefits in N obtained from the incorporation of legume N with soil.