aSignificantly different (P < 0.1%) from rhizobial treatment. bSignificantly different (P < 0.01%) from rhizobial treatment. From Kucey and Paul, 1982.

grasses (the same species studied by McNaughton, 1976, and McNaughton et al., 1998), when compared with noninoculated control plants. Some elegant field studies in Scottish grassland soils near Edinburgh have demonstrated significant flows of carbon from roots to mycorrhiza. Within 21 hours of pulse-labeling a grassland sward in the field with 13CO2, between 3.9 and 6.2% of the 13CO2 passed through the external mycelium of the AM fungal symbionts to the atmosphere (Johnson et al, 2002). This is the first in-the-field verification of similar results measured using pot experiments. Additional recent pot experiments have exposed mycorrhizal plants to fossil (14C-depleted) carbon dioxide and collected samples of extraradical mycelium (ERM) hyphae over the following 29 days. Analyses of their 14C content by accelerator mass spectrometry (AMS) revealed that most ERM hyphae of AM fungi live, on average, 5 to 6 days (Staddon et al, 2003). This high turnover rate indicates the existence of a large and rapid mycorrhizal pathway of carbon in the soil carbon cycle.

Recent field research has demonstrated the significant effects of an additional flow of carbon from plant roots to mycorrhiza and into the soil. The glycoprotein glomalin, which is produced by arbuscular mycorrhizal fungi (AMF), has a marked effect on soil aggregate water stability (Wright, et al, 1999). We discuss this further in the section on microbial interactions in soil (in Chapter 3).

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