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

Abstract.

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 ¹⁴C-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 ¹⁴C and NH₄ ⁺ could be liberated by adding a further 1,000 mg ¹⁴C-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 ¹⁴C-labelled glucose. Application of unlabelled glucose at day 70 increased production of ¹⁴CO₂ 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 ≤24.6 dS m^–1, but only to NO₂ ^– in soil with EC 32.7 dS m^–1. It was found that the second application of glucose only affected dynamics of ¹⁴C 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 ^–.