Question

What do you understand by exploitation and assimilation efficiency?

Answer 1

Microbial carbon assimilation efficiency is commonly defined as microbial biomass produced as a proportion of total carbon utilized. It is also called the yield factor. Accumulated evidence suggests that a big proportion of root exudates is utilized and released as CO2 in a very short period of time; only a small portion becomes microbial biomass (Dyer et al. 1991; Harris and Paul 1991). The microbial assimilation efficiency of these exudates (6.5–15%; Helal and Sauerbeck 1989; Liljeroth et al. 1990; Martin and Merckx 1992), is considerably lower than the theoretical maximum of 60 percent (Payne 1970) and of other sources of carbon in the soil. The microbial assimilation efficiency is 61 percent for glucose added to the soil after about 40 hours of incubation (Elliott et al. 1983), 27 percent after 61 weeks of incubation (Johansson 1992), and 47 percent for rye shoots added to the soil after 7 weeks of incubation (Cheng and Coleman 1990). Why is the microbial assimilation efficiencyof root exudates so low? What mechanisms are there behind this lower efficiency?

The occurrence of biological N2-fixation in the rhizosphere may contribute to the low microbial assimilation efficiency of root exudates. Biological dinitrogen fixation requires high amounts of energy, especially those of an associative nature. At least 16 ATP molecules are consumed to convert one N2 to two NH3 molecules, in addition to other processes required for associative N2-fixation. If a large proportion of root exudates is used by diazotrophs in the rhizosphere, the assimilation efficiency of root exudates will be much lower than if it is being used by non-nitrogen-fixing microbes. Several studies (Liljeroth et al.