Agriculture Reference
In-Depth Information
Evidence for the functional importance of micro-foodwebs
The functional importance of micro-foodwebs has been demonstrated several times in
small-scale laboratory chambers called 'microcosms'. Their effects on carbon flows and
ecosystem-level nutrient cycles have been observed and quantified.
C flows: Although microfaunal respiration only represents a small percentage of
overall soil respiration (0.6 to 2 % for nematodes according to Freckman‚ 1988)‚ their
effects are disproportionate to their sizes and respiration rates. The microfauna signifi-
cantly stimulates the growth and turnover of microbial populations thereby promoting
more rapid rates of decomposition‚ mineralisation and thus nutrient turnover
(Wasilewska‚ 1979; Hendrix
et al.‚
1986; Hunt
et al.‚
1987; Sohlenius
et al.‚
1987‚ 1988;
Moore
et al.‚
1993).
The N cycle is particularly affected by micro-foodweb interactions since bacteria
have average C:N ratios of 6:1‚ close to that of the protists (5:1 according to Reich‚ 1948‚
in Stout and Heal‚ 1967) and slightly lower than that of nematodes (10:1). Consequently‚
predation on micro-organisms results in the release of mineral-N that may be further
used by plants. In the presence of nematodes‚ ammonification by bacteria is greatly
increased (Clarholm‚ 1981; Ingham
et al.‚
1986b). N-fixation may be greatly enhanced
through the maintenance of predominantly young populations‚ provision of stimulatory
compounds or decreased oxygen concentrations (Darbyshire and Greaves‚ 1973).
The P cycle is also affected although contrasting results have been found. Coleman
et al.
(1984b) observed that‚ in the presence of amoebae‚ a significant part of the P accu-
mulated within the microbial biomass is transformed into bicarbonate-extractable P.
However‚ the flux of P affected by this transformation can be small and this may explain
why few effects on the P cycle have been noted‚ even in microcosms (Bååth
et al.‚
1980).
Positive effects of micro-foodweb activities on plant growth have been measured
in microcosms. The productivity of wheat plants grown in sterilised soil into which
bacteria and protists had been introduced‚ increased by 80 % and N mineralisation from
soil organic matter was increased by 59
%
in relation to a control soil without plants.
This emphasises the role of root-derived carbon in providing energy to the system
(Clarholm‚ 1984). In another 35-day microcosm experiment‚ bacterial biomass was
reduced eightfold and N uptake by plants was increased by 20 % through the activity
of protists. The shoot:root ratio of plants was increased and more than 65 % of
bacterial
was taken up by the plants (Kuikman and van Veen‚ 1989 ).
