Effect of organic and mineral amendments upon soil respiration and microbial biomass in a saline-sodic soil

An understanding of soil carbon stocks and fluxes in saline-sodic soils is becoming critical in environmental management because salinity and sodicity are predicted to increase worldwide. The effects of amendment with sewage sludge (SW), mined gypsum (MG) and synthetic gypsum (SG) on the soil respiration rate and soil (S) microbial biomass (SMB) of a saline-sodic soil were assessed in vitro over 60 days under controlled conditions. The treatments were: T1 = S + MG (7 t ha(-1)); T2 = S + MG (7 t ha(-1)) + SW (90 t ha(-1)); T3 = S + SG (7 t ha(-1)); T4 = S + SG (7 t ha(-1)) + SW (90 t ha(-1)); T5 = S + SW (90 t ha(-1)); T6 = S + SW (270 t ha(-1)); TO = Control (unamended soil). Soil respiration rates were determined by a respirometry method in a closed incubation chamber. SMB was measured by the chloroform fumigation-extraction procedure. The results showed that the highest respiration rate occurred when soil was amended with T6, followed by T5. Microbial respiration was the lowest with T1 and T3. The gypsum amendments did not significantly increase soil respiration, due to low organic matter (OM) intake. All treatments showed a gradual decrease of SMB over the 60-day incubation period. The saline-sodic soil was able to respond positively to large amounts of added sewage sludge, up to 270 t ha(-1).