Dynamics of 14C-labelled glucose in alkaline saline soil

The application of glucose C or sewage sludge C to undrained alkaline saline soil of the former lake of Texcoco and soils drained for 1, 5 and 8 years significantly increased CO2 production only for the 8-year site. The added organic material could have been immobilised in the microbial biomass or sequestered. To elucidate possible mechanisms involved, we investigated the fate of 14C-labelled glucose when added to soil, under laboratory conditions. Soil was sampled from an undrained plot and from plots drained for 1 , 5 and 8 years, amended with 1000 mg 14C-labelled glucose C kg−1 dry soil (DS) and 200 mg NH4+–N kg−1 DS, and incubated aerobically for 97 days at 22±1°C. 14CO2 and CO2 production, inorganic-N dynamics (NH4+, NO2−, NO3−), microbial biomass 14C, C and ninhydrin N, glucose and NH3 volatilization were monitored. CO2 production was fastest in the soil drained for 8 years with electrolytic conductivity of 2.6 dS m−1 and lowest in the soil drained for 1 year with conductivity of 79.9 dS m−1. There was a lag between uptake of glucose and production of 14CO2, which increased with increased conductivity. A priming effect of 250, 674 and 221 mg CO2–C kg−1 DS was observed in the undrained soil and soils drained for 1 and 5 years, but it was absent in the soil drained for 8 years. The priming effect was explained by a replacement of the microbial biomass by newly formed biomass in the undrained soil and soil drained for 5 years but partly by increased decomposition of soil organic matter in the soil drained for 1 year. Nitrification, as indicated by increases in NO3− concentrations, occurred even in the soil drained for 1 year, but oxidisers of NO2− were inhibited as indicated by temporal increases in concentrations of NO2−. Calculated efficiencies for 14C were affected by soil characteristics. nd that sequestration of added organic C as found in previous experiments was not confirmed but soil characteristics changed by drainage affected decomposition of added organic material. Care should be taken when salts are applied to soil to study possible effects of conductivity on C and N dynamics as micro-organisms adapt to these specific conditions.