Date of this Version
Soil quality changes resulting from repeated or single injection of liquid municipal sewage sludge were evaluated in terraced cropland in eastern Nebraska, USA. Differences in soil properties among sampling locations were explained primarily by two factors, landscape position and sludge injection. Selected chemical properties (pH, EC, NO 3-N) did not generally differ between landscape positions, but soil organic matter (organic C and N) and microbial activity indices (soil respiration, biomass N, available N, mineralization and nitrification rates) were more sensitive indicators of change. Values of these indicators generally increased down-slope from the upper terrace to the grassed waterway on a west facing slope. A probable similar pattern on the east slope was obscured by repeated application of sludge. Single or repeated (long-term) sludge injection increased the readily decomposable organic matter, ammonium- and available-N in soil (0-30.5 cm depth). These changes stimulated soil microbial activity as evidenced by increased basal respiration, net mineralization and nitrification rates. Consequently, nitrification of ammonium-N was rejected in soil chemical properties as increased soil nitrate-N (to levels that were more than two times higher than sufficiency levels for corn) and EC and by decreased pH. In-field measurements detected changes in physical properties such as a decrease of infiltration rate caused by sludge injection and soil compaction as a result of traffic operations. Differences between the sites of single and repeated sludge injection were found in soil pH, ammonium-N, organic matter and microbial activity. Recent sludge injection resulted in higher ammonium-N concentration and higher microbial activity in soil, and repeated sludge injection resulted in lower pH and in greater organic matter content. Regardless of these differences in soil properties between the sites of single and repeated sludge application, the overall changes that were caused by sludge injection had both positive and negative effects on soil quality and the sustainability of this management practice. Increase of organic matter content and biological activity improved soil fertility, but excessive amounts of ammonium salts contained in liquid sludge resulted in soil nitrification, excessive nitrate formation and acidification. These processes reduce soil productivity, increase the risk of ground- and surface-water contamination and pose a threat to plant and animal health.