Incorporation of glucose-14C and NH4+ in microbial biomass of alkaline saline soil

The turnover of organic material determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. Approximately 10% of 1,000 mg of C-14-labelled glucose added to this soil remained in the microbial biomass C and half of 200 mg NH4+-N added could not be accounted for within a day of application. We investigated whether the added C-14 and NH4+ could be liberated by adding a further 1,000 mg C-14-labelled or unlabelled glucose C kg(-1) dry soil and 200 mg NH4+-N kg(-1) dry soil to soils with electrolytic conductivity (EC) 1.2, 24.6 and 32.7 dS m(-1) previously amended with 1,000 mg unlabelled glucose or C-14-labelled glucose. Application of unlabelled glucose at day 70 increased production of (CO2)-C-14 in soil with EC 32.7 dS m(-1), but not in the other soils. Approximately 100 mg NH4+-N kg(-1) could not be accounted for within a day after application of 200 mg NH4+-N kg(-1) and that remained so for the entire 70 days. However, after the second application, NH4+ was released and oxidized to NO3- in soil with EC less than or equal to 24.6 dS m(-1), but only to NO3- in soil with EC 32.7 dS m(-1). It was found that the second application of glucose only affected dynamics of C-14 in soil with EC 32.7 dS m(-1). The soil of the former Lake Texcoco was depleted in N, and an EC of 32.7 dS m(-1) inhibited oxidation of NO2_.