Implications of Soil Biodiversity for Sustainable Organic Matter Management - Sustainable Management of Soil Organic Matter - page 381

Fig. 5.5.2. Soil microbial biomass 14 C at increasing distance from the root mat. Bars indicate SE

(n= 4).

from labelled SOM was suppressed in the presence of living roots (e.g.
Jenkinson, 1977; Sparling et al ., 1982; Martin, 1987), while others (Helal
and Sauerbeck, 1986; Sallih and Bottner, 1988; Cheng and Coleman,
1990) found a stimulatory effect. In this experiment, the increase in SMB
14 C in the rhizosphere of the fertile soil clearly showed that carbon was
moved from a pool that was not extractable after fumigation, to one that
was. Two possible explanations exist: (i) 14 C-labelled SOM was taken up
by the SMB, and hence decomposed (a priming effect); or (ii) 14 C-labelled
microorganisms in a dormant state were revitalized by the exudates, and
became prone to fumigation. The total changes in soil 14 C were not
significant in this short experiment and it is uncertain whether the roots in
the long term would have exerted a priming effect on the SOM. Different
changes in soil biological and physical factors induced by roots, depending
on experimental set up, may explain the conflicting results in the literature
(Cheng and Coleman, 1990; Dormaar, 1990). The complexity of the
situation is accentuated by the fact that no significant priming effect was
found in the nutrient-depleted soil. We propose three explanations: (i) the
SMB was nutrient limited which hampered decomposition of the labelled
SOM; (ii) the SMB in the nutrient-depleted soil was not capable of
decomposing the 14 C bound in microbial residues; and (iii) unlike in the fer-
tile soil, dormant SMB in the nutrient-depleted soil was not brought back into
an active state. The first explanation appears unlikely since microbial resi-
dues would contain the necessary nutrients for microbial growth.