Benefits and risks of using a combination of sewage sludge and chemical fertilizer on rice in acid sulfate soil

Field experiments were conducted in acid sulfate soils in the CentralPlain of Thailand to study a combination of sewage sludge and urea as nitrogensources for rice. Rice yield, nitrogen mineralization, heavy metal availableanduptake were quantified. The experiment design was a split plot in 2×4 factorialdesign. Two rates of nitrogen (75 and 150 kg Nha^–1) were assigned to main plots and four combinationsof urea and sewage sludge as source of N to subplots. The four combinationswerein % of N from urea (U) and from sewage sludge (S), as follows: 33% N from U + 67%N from S, 67% N from U + 33% N from S, 100% N from U + 0% N from S, and 0% N fromU + 100% N from S. Nitrogen mineralization in these treatments was tested in bothopen and closed systems to study N balance at different growth stages of rice.The results showed that nitrogen at 75 kg N ha^–1 gavea better grain yield than at 150 kg N ha^–1 and control(0 kg N ha^–1). At the higher nitrogen applicationrate, an increase in the biomass with reduced grain yield was observed. Amongthe sources of N, the combination of urea 33% N and sewage sludge 67% Nresultedin highest grain yield. Nitrogen mineralization of sewage sludge in acidsulfatesoil was low. Nitrogen mineralization from urea and urea combination withsludgetreatment were greater than sewage sludge application alone. Due to lowextractable heavy metals in sewage sludge applied soil, the heavy metal contentin milled grain and straw were below allowable limits set by US standards.Results indicated that a combination of urea and quality sewage sludge could beused as a fertilizer nitrogen source for rice, without risks associated withtoxic heavy metals.