Environmental Engineering Reference
In-Depth Information
Given the broad quantitative importance of decomposition processes and the ease of
manipulation of many participating organisms there have been several attempts to investi-
gate the effects of species diversity or richness of both plant litter and decomposer organ-
isms on mass loss or respiration. Human activities frequently alter the richness of both the
plants contributing litter to an ecosystem and the macro-organisms. For instance, planting
monocultures, hybrids, or exotic tree species in riparian zones clearly alters the pool of
POM and there are several examples of whole-ecosystem effects of such plantings (Leroy
and Marks 2006;
Swan and Palmer 2004
). Additionally, human-caused climate change will
likely lead to rapid change in species distribution. There will probably be some period of
increasing richness during the period when both extant and new species are present, but
ultimately species better suited to the new conditions will replace previously abundant
species. Therefore, direct examination of effects of species richness of either the litter or
detritivores has become a major research topic.
Recent studies have included invertebrates representing different taxa (
Jonsson and
Malmqvist 2000
) or functional types (
Heemsbergen et al. 2004
). In general, these efforts
have shown that decomposition is stimulated as species are added to the mixture; these
effects usually are attributed to differences in functional capacity among test animals.
Since microbes are responsible for the bulk of decomposition there have been several stud-
ies of bacterial taxonomic richness/diversity on decay of detritus. These studies typically
show a leveling off of the effect of species richness above some number of species, sug-
gesting there is some functional redundancy among taxa (
Bell et al. 2005
). Several studies
have asked whether mixtures of litter decompose differently from what would be expected
from their individual decay rates. The entire range of outcomes has been observed from
faster than expected, slower than expected, and no effect (
Swan and Palmer 2006; Lecerf
et al. 2007; Srivastava et al. 2009
). While individual results have been reconciled, there is
yet to be a synthesis that accounts for the diversity of observations on consequences of
mixing together different species of litter.
INTERACTIONS WITH OTHER
ELEMENT CYCLES
One of the key chemical characteristics of most (but certainly not all) plant litters is the
relative shortage of some elements, most notably nitrogen and phosphorus. Plants under-
going normal senescence withdraw valuable nutrients from tissues destined for abscission
or death at the end of the growing season. Therefore, microbes and other organisms using
these detrital materials as a food resource are faced with a shortage of nutrients. Microbes
can draw nutrients from the surrounding water or soil solution and so they often use the
particulate organic matter as a carbon source while assimilating inorganic nutrients from
their environment. This capability provides the mechanism whereby external nutrient sup-
ply can alter decomposition rates, but more importantly it allows ecosystems with large
pools of detritus to have a substantial potential to remove and sequester nutrients. Tidal
wetlands are well known for their capacity to act as a sink for inorganic nutrients, and
while there are multiple mechanisms
(plant uptake, denitrification, burial, etc.),
