Root respiration is highly variable among plant species, and in space and time. It is, furthermore, technically difficult to separate from heterotrophic respiration because mycorrhizal roots and true heterotrophs occur in a complex and delicate mix in the soil (Fig. 4.1). As a result, estimates of the contribution by root respiration to total soil respiration vary from 10 to 90% (Hanson et al, 2000). Part of this variability clearly relates to methodological problems.
In an attempt to obtain a robust division of the major components of soil respiration, we recently girdled (ring-barked) a boreal Scots pine forest (Hogberg et al, 2001). This involved girdling of all trees on 30 m x 30 m plots, with an average of 120 trees per plot. The treatment terminates the flux of recent photosynthates to roots and their associated microorganisms, but does not, in a shorter time perspective, affect the uptake of water by the trees or the soil temperature. Nor does the treatment involve any physical disturbance of the soil biota. During the first year we found that up to 56%
of the soil respiratory activity was lost from the girdled plots when compared to the untreated control plots. Apparently, this was a conservative estimate of the contribution by root respiration, as we observed that starch reserves were more rapidly depleted in roots of the girdled trees. It was thus no surprise that the respiration on girdled plots during the second vegetation period after girdling, when the stores of starch were depleted in roots of girdled trees, was up to 65% lower than in control plots (Bhupinderpal-Singh et al, 2003).
The decline in soil respiration after girdling could be remarkably rapid (Hogberg et al, 2001). When girdling was carried out in August, the time of the year when the C allocation to roots is supposed to be at maximum in northern temperate conifers (Hansen et al, 1997), 40% of the soil respiratory activity was lost in 5 days and 56% in 14 days. These data clearly show that root respiration can account for 50%, or more, of total soil respiration in boreal forests. Ongoing girdling experiments in two other boreal forests support this estimate (Hogberg et al, unpublished; Nordgren et al, unpublished). Planned experiments of the same kind, but outside the boreal zone, will determine if the large contributions from root respiration found in boreal forests are exceptional or not.
The amount of plant biomass produced, which ultimately in the longer term feeds the heterotrophic community, is constrained by the supply of nutrients. Likewise, the fraction of plant C allocated to roots and myco-rrhizal fungi is strongly affected by the nutrient supply, but decreases as the nutrient supply increases. This means that the nutrient supply should have opposing effects on the C allocation to roots and heterotrophs, and that the ratio root/heterotrophic respiration should decrease with increasing nutrient supply (Hogberg et al, 2003). The sum of the two components should increase in response to greater nutrient supply when measured per unit area, but there are as yet not many data from boreal forest ecosystems to test this assumption. An exception is the detailed study of Scots pine by Linder and Axelsson (cited in Cannell, 1989) demonstrating that the fraction of photosynthate C allocated belowground was 32% in fertilized trees while it was 59% in N-limited control trees, at the same time as fertilization doubled photosynthetic C fixation. The partitioning between root growth and respiration was not measured, but it seems likely that root respiration accounts for 75% of the C allocated belowground in a strongly nutrient-limited boreal forest (Hogberg et al, 2002).
Disturbances may create large variations in the ratio root/heterotrophic respiration. For example, clear-felling should cause a decrease in soil respiration, and hence a shift in the ratio autotrophic/heterotrophic respiration. Another example is that of a forest with large stores of soil C in SOM, and which acted as a source of CO2 (Valentini et al, 2000), which was attributed to increased heterotrophic respiration caused by forest drainage several decades earlier of the previously anaerobic soil system.
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