FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 382

1.5.2 THE ROLE OF PRIMING EFFECTS: AN HYPOTHESIS

The priming effect was first described by Bingeman et al. (1953‚ in Jenkinson et al.‚ 1985)
as the stimulation of soil organic matter decomposition by the addition of fresh organic
material. In more general terms‚ the priming effect was defined by Jenkinson (1966) as
a positive or negative change in the decomposition rate of soil organic matter caused by
the addition of fresh organic matter. A simple experiment illustrated this effect: a 125 mg
sample of Rye Grass ( Lolium perenne ) foliage uniformly labelled with was mixed
with 100 g of soil and subsequent carbon dioxide evolution was monitored during an
incubation period of 78 days. Production of unlabelled carbon dioxide resulting from
mineralisation of the soil organic carbon was greater in the mixture than in the control
soil. The difference observed was attributed to a positive priming action.
Most examples of priming effects described in the literature are positive: the addition
of fresh organic matter to soils as green manure‚ or the addition of mineral nitrogen as
fertiliser‚ usually stimulates the mineralisation of soil organic matter (reviewed in
Jenkinson et al.‚ 1985). Nonetheless‚ negative effects have been observed in some
circumstances. For example‚ during the first few days following the addition of glucose
to an organic soil (Bingeman et al.‚ 1953‚ in Jenkinson et al.‚ 1985). The observed priming
effects may be apparent or real. Apart from experimental artifacts ( e.g.‚ the exchange of
labelled for unlabelled carbon in calcareous soils‚ or errors due to an heterogeneous
labelling of the introduced material)‚ some observed priming effects may be due to
environmental changes associated with the introduction of the organic material. In this
last circumstance‚ changes in soil organic matter mineralisation may result from an
altered pH or oxygen supply‚ irrespective of the effect of the fresh organic materials (see
e.g.‚ Barrow‚ 1960 and Parr and Reuszer‚ 1959 in Jenkinson‚ 1966). Jenkinson (1966)
attributes real priming effects to three different causes:
(i) An increased rate of spore germination leading to greater overall microbial activity.
(ii) Interactions between compounds derived from the added and 'native' organic mat-
ter‚ which render the latter more labile (Mandl and Neuberg‚ 1956‚ in Jenkinson‚ 1966).
(iii) An increased production of extra-cellular enzymes by micro-organisms‚ leading to
the accelerated organic matter decomposition.
In soils‚ several classes of water-soluble organic materials (referred to as 'soluble
resources' in Chapter 1.3.2.5) are normally available to prime the decomposition of resis-
tant organic matter. The former materials have been named 'ecological mediators' for
their unique roles in mediating soil function (Lavelle et al.‚ 1994b). They include:
(i) High-energy‚ water-soluble organic leachates percolating from the above-ground
vegetation and leaf litter.
(ii) Root exudates are water-soluble organic substances‚ readily assimilated by micro-
organisms‚ that become intimately mixed with the soil and penetrate the very fine pores
within micro-aggregates. Real priming effects on evolution due to maize root

mucilages have been measured by Mary et al. (1992).
(iii) Earthworm cutaneous and intestinal mucus. Intestinal mucus produced by earth-
worms is considered to be responsible for much of the priming of soil micro-organism
activity observed during transit through the gut (Lavelle and Gilot‚ 1994). The remaining
part being an apparent priming due to the addition of water and an intense mixing.