Effects of sewage sludge compost and chemical fertilizer application on microbial biomass and N mineralization rates

To determine the effect of sewage sludge compost and chemical fertilizer application on the soil chemical properties, net N mineralization rates, CO2 evolution rates, microbial biomass C and N, a non-leaching experiment was conducted under controlled conditions of temperature (25degC) and moisture content (80% WHC) using 2 types of soils that had been previously treated with sewage sludge composts or chemical fertilizer during short or long periods of time. The incubation lasted for 90 d. The treatments included: soils samples taken from 3 plots, each receiving sewage sludge compost with rice husk (RH), sewage sludge compost with sawdust (SD), and chemical fertilizer (CF), each applied at the rate of 200 kg N ha**-1 biannually for 22 years (Tama farm) and 2 years (Chiba farm). The application of SD and RH led to a significant increase in the total N and C and soluble organic C reservoirs of the two soil types regardless of the application history. Soil pH was always higher in the sludge-treated soils than in the CF-treated soils. The SD- and RH-treated soils at the Tama farm showed significantly higher emission rates of CO2, cumulative CO2, and microbial biomass C and N than the CF-treated soils and SD- and RH-treated soils at the Chiba farm. The metabolic quotients of the soils treated with sewage sludge compost were significantly higher than those of the CF-treated soils. However the soils treated with sewage sludge compost of both Tama and Chiba farms exhibited a mixed response during the incubation period. Net N mineralization rates were significantly higher in the soils treated with SD followed by RH at the Tama farm; while the SD, RH, and CF-treated soils of the Chiba farm and CF soils of the Tama farm exhibited a negative N mineralization rate (immobilization). These results showed that the soils amended with sewage sludge composts displayed larger and more active microbial biomass than those treated with CF, which has implications for current interests in shifting from chemical fertilizer application to organic wastes for promoting efficient nutrient cycling in agro-ecosystems.