Initial and long-term effects of sewage-sludge composts on soil chemical and biological properties

Sewage-sludge compost contains nutrients and organic matter as well as heavy metals which can affect soil microbial activities. The objective of this study was to investigate the initial and long-term effects of two kinds of sewage-sludge compost applications, either with saw dust (SD) or rice husk (RH), on soil heavy metal and microbial biomass (SMB) contents in mesocosms compared to chemical fertilizer (CF) alone, during the first 3 years application period, and 21 years post application period. Zinc (Zn) contents tended to increase in the first 3 years and kept higher (+5–10%) in RH and SD than CF plots, also in upper (1–16 cm) soil layer than lower (16–31 cm) soil layer in RH, while copper (Cu) contents tended to be higher (+15–27%) in RH than SD and CF. Nitrous oxide (N2O) emission showed sharp increases soon after compost and fertilizer application in the initial third year, being higher in RH and SD than CF plots. SMB C and N also showed increase after compost and fertilizer application in the initial period, tending to be higher in SD and RH than CF plots. Twenty-one years after application period, Zn contents in upper soils of SD and RH plots were 5–10% higher than that of CF plot, while Cu contents were 15–27% higher in SD and RH than CF not only in upper layers but also in lower layers. ATP (adenosine 5’ triphosphate) contents in the upper soil were 45–52% lower in SD and RH than CF plot, while no significant change was observed in the lower soil. Soil surface in Petri dishes had visible algal colony formation in the upper soil only of CF, while lower soil of all plots after growth-chamber incubation. Contents of chlorophyll-type compounds in the upper soil amended with RH were 71% lower than in CF and tended to be so in SD (23% lower) after incubation. These results suggest that initial effects of compost on SMB and N2O emission increases attribute organic matter in the sewage sludge composts even with heavy metal increases in soil, while residual suppressive effects on SMB and its activities remained with residual heavy metals in the soil 21 years after sewage sludge compost application stopped, which can be monitored by soil microbial properties as useful sensitive indicators for soil health. Heavy metals in sewage sludge and its residual effects in soil should be monitored carefully in the long term.