Impact of sewage sludge application on soil microbial biomass, microbial processes and plant growth – A review

Sewage sludge is a rich source of plant nutrients (C, N, P and K). Beside these sludge often contains heavy metals like Zn, Cu, Fe, Cd, Pd, Ni, Hg and Cr. Some of these metals at low concentration are essential micronutrients. The repeated addition of sludge contaminated with heavy metals may lead to build up of heavy metals in soil and thereby exert adverse effects on soil health. The concentrations of heavy metals close to or less than current permissible limits have no effect on microbial biomass carbon and other activities. Soil microbial biomass carbon and activities increased with the application of uncontaminated sewage sludge. But the application of metal contaminated sewage sludge leads to the decline in microbial biomass C and N. At higher level metal contaminated sewage sludge resulted in the accumulation of organic matter due to less degradation or carbon dioxide evolution. The urease and dehydrogenase activities are adversely affected while other enzyme activities stimulated by metal contaminated sewage sludge. Soil microbial community structure and plant growth are also adversely affected by metal contaminated sewage sludge.