Agriculture Reference
In-Depth Information
The priming effect started after an incubation period of one day and after seven days,
40.4 % of the C in the mucilage had been mineralised together with C from soil organic
matter equivalent to 5.8 % of the C added as mucilage. In a comparative experiment
with glucose, 44.7 % of C from glucose was mineralised, plus an extra mineralisation
due to priming on the soil carbon, equivalent to 9.2 % of the C in the added glucose.
Dead root material decomposed at a much slower rate (30.8 %) and priming was limited
to 3.5 % of C introduced as root material.
Beyond these general results, an interesting feature is the form of the microbial
response to the introduction of mucilage: mineralisation is very slow during the first two
days and suddenly accelerates to attain maximum values between the third and the sixth
days. This contrasts with the addition of glucose and roots that triggers a large and
immediate response (Figure IV.39). Such a response may be interpreted as an adaptation
to the specific conditions in the rhizosphere: during the few days following deposition
of mucilage in the root cap region, the root will have grown and maximum microbial
activity will occur in the elongation zone behind the root tip. This microbial activity
is therefore likely to lead to nutrient release in close proximity to the root hair zone
where absorption will occur.
This priming effect probably occurs throughout the rhizosphere soil, associated with
the mucilage. Although the sites of the priming effect are as yet unclear, bacteria
accumulated in the rhizosphere mucilage may release enzymes which diffuse to the zone
of contact with the rhizosphere soil. They digest the soil organic matter at this location
thereby releasing mineral nutrients that will accumulate initially in the microbial biomass.
Priming has been clearly demonstrated by Sallih
et al.
(1987) in a pot experiment
lasting 700 days. They observed a high microbial biomass in the presence of living roots
while there was a 25 % increase in mobilised nitrogen (
i.e.,
mineral + root nitrogen +
microbial-biomass nitrogen) in treatments where living roots were continuously present,
compared with a control treatment without plants. A large microbial biomass was
maintained by the living plant providing clear evidence that the mineralisation of soil
organic matter and the subsequent release of mineral nitrogen was enhanced through
the priming effect of root exudates (Figure IV.40).
