Environmental Engineering Reference
In-Depth Information
activity were correlated with improved palatability of detritus to consumers (
Arsuffi and
Suberkropp 1984
). These observations led to the proposal that the microbial biomass was
the digestible portion of the detritus-microbe complex and much of the actual detrital
organic matter was essentially unusable as a source of carbon or energy for the consumer
(the “peanut butter and crackers” model;
Cummins 1974
). In this model the microbes
are the digestible “peanut butter” and the plant material itself is analogous to the lower-
quality crackers.
Once quantitative estimates of microbial biomass became common and it appeared that
microbial carbon was insufficient to meet carbon demands of consumers (
Findlay et al.
1986
), other explanations for the observed correlations were proposed. Microbes inhabiting
litter can alter the digestibility of the plant material itself by releasing extracellular
enzymes capable of “predigesting” some of the macromolecules (
Barlocher 1982
) and there
can be substantial production of extracellular polymers that can serve as a carbon source
for consumers. Additionally, even relatively low assimilation efficiencies applied to the
large quantities of ingested plant material would allow for the nonliving detritus to con-
tribute significantly to overall consumer carbon demand. There is even a wider recognition
that high microbial degradation can reduce detrital standing stocks, thus ultimately
depleting the food resources for animals (
Benstead et al. 2009
). Microbes thus can both
facilitate and compete with invertebrates for detritus.
The presence of microbial biomass somehow makes the pool of detrital organic matter
more palatable and available to invertebrates. In contrast, there are also examples where
microbial metabolism causes losses of organic carbon to CO
2
, thereby reducing the pool of
food for other consumers. As argued earlier, an appreciation of which pathways of mass
loss are important at various times, places, and under what environmental conditions can
greatly alter our conception of how allochthonous inputs contribute to secondary produc-
tion. For instance, release of DOC can support bacterial growth but is generally unavail-
able for macroinvertebrates. Fine particles of organic matter can be used as a food
resource by animals downstream or transported to other ecosystems, whereas conversion
of detritus to CO
2
means that amount of carbon is of no further use to heterotrophs.
CONTROLS ON DECOMPOSITION
For the sake of simplicity the various factors potentially controlling decay rates will be
separated into
intrinsic
versus
extrinsic
factors although the two actually interact. For
instance, an intrinsic factor such as plant nitrogen content will be affected by (and will
eventually affect) environmental nutrient content. Similarly, low nutrient availability at a
site or presence of herbivores (extrinsic factors) can alter plant biochemical composition
(an intrinsic factor). Nonetheless, it is useful and informative to separate some inherent,
species-specific controls on plant litter decay from regulation by environmental conditions.
Intrinsic controls are most often associated with either the bulk macromolecular compo-
sition of plant litter or the availability of some required nutrient that is present in low con-
centrations in the litter (together often referred to broadly as litter quality). Structural
carbohydrates (e.g., cellulose, hemicellulose), often complexed with lignin, make up a
major portion of plant biomass with obvious and well-known differences among plant
