Influence of sorption on the biological utilization of two simple carbon substrates

The utilization of glucose and citrate either with or without being mixed with one of four commonly-occurring soil mineral phases was assessed using both bacterial cultures and soil incubations. Citrate and glucose, both ¹⁴C-labelled, were added separately to bacterial cultures. In each case they were rapidly: degraded with > 60% recovered as CO₂. In the presence of soil the respired proportion declined especially for glucose. The presence of mineral phases (illite-mica, kaolinite, mixed soil clay or ferric hydroxide) had little observable affect on the utilization of glucose in either bacterial cultures or soil. However, ferric hydroxide did induce an alteration in the way ¹⁴C was partitioned within the microbial cells. In contrast, citrate degradation was greatly reduced over short periods (6-22 h) in the presence of the three clay materials and almost completely inhibited by the presence of ferric hydroxide. The importance of substrate sorption in the context of carbon utilization in the rhizosphere is discussed.