CO2 Production in Soils

Who is Respiring?

There are two major sources contributing to the efflux of CO2 from soils: root (autotrophic) respiration and heterotrophic respiration. The C sources used are sugars, derived more or less directly from plant photosynthesis, and a range of more or less complex plant macromolecules. In the soil food-web, of course, secondary products such as dead bacteria, fungi, soil animals, etc. are also ultimately decomposed by the heterotrophic community. However, although the above divisions may seem simple and straightforward, the belowground world is in reality more complex (see Gleixner, Chapter 3).

Plant roots are commonly mycorrhizal, i.e., forming symbiotic associations with fungi, which by definition are heterotrophs. Yet these fungi receive, like the roots of the autotrophic plants, sugars more or less directly from the plant foliage (Smith and Read, 1997). In boreal forests, the dominant coniferous trees engage in ectomycorrhizal symbiosis, in which up to several tens of micrometer-thick fungal sheaths commonly cover close to 100% of the fine root tips (Taylor et al, 2000; Fig. 4.1). From these sheaths an extramatrical mycelium, with a biomass of roughly the same size (Wallander et al, 2001), ramifies further out into the soil. Although the growth and activity of this fungal tissue is supported by photosynthates from

Figure 4.1 The organic mor-layer of a boreal forest soil. Tree roots, with the fine roots colonized by a white ectomycorrhizal fungus, are clearly visible, while bacteria, and many other heterotrophic microorganisms decomposing the organic matter constituting the dark background matrix cannot be seen by the naked eye. Note the rhizomorphs and fungal strands extending outwards from clusters of ectomycorrhizal fine roots; these structures are damaged if the system is disturbed by soil or root sampling. Photo courtesy of Kjell Olofsson.

Figure 4.1 The organic mor-layer of a boreal forest soil. Tree roots, with the fine roots colonized by a white ectomycorrhizal fungus, are clearly visible, while bacteria, and many other heterotrophic microorganisms decomposing the organic matter constituting the dark background matrix cannot be seen by the naked eye. Note the rhizomorphs and fungal strands extending outwards from clusters of ectomycorrhizal fine roots; these structures are damaged if the system is disturbed by soil or root sampling. Photo courtesy of Kjell Olofsson.

the trees, there also seems to be a significant leakage of C compounds from the fungal cells to the wider community of soil organisms (Garbaye, 1994; Timonen et al, 1998; Hogberg and Hogberg, 2002). Heterotrophic activity, on the other hand, encompasses anything from decomposition of simple sugars leaked from plant roots to the degradation of secondary products like the large complex macromolecules collectively termed humus. Hence, the division between autotrophic and heterotrophic is not clear-cut.

One of the simpler and technically feasible ways (see later) of classifying components of soil respiration is to group organisms into those that receive photosynthates more or less directly from the plant canopy and those that receive their C mainly through decomposition of dead or dying organic matter. This means that the former group would include the classic autotrophic component plant roots, but also heterotrophic organisms like mycorrhizal fungi and a wide range of other organisms in the rhizosphere and mycorrhizosphere, which largely depend on the flux of recent photo-synthates. We will call this the autotrophic component or root respiration, although in a classic sense it also involves heterotrophic organisms. Thus, the heterotrophs will, in our view, be those organisms that degrade more complex molecules, and do not depend on recent plant photosynthates.

With the above division in major components, the C mass balance and the C isotope mass balance of the CO2 efflux from the soil become:

CO2total X <513CtotaI = (CO2hetero X <513Chetero) + (C02auto X <513Cauto)

in which CO2 is the respiration rate, <5,3C is the 13C abundance of the CO2, and subscripts total, hetero, and auto stand for total, heterotrophic, and autotrophic respiration, respectively. We will examine the different factors controlling the variables in this equation. It follows from the above that:

which means that it is sufficient to determine total respiration and one of its major components in order to calculate the other component.

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