Agriculture Reference
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they need for cellular maintenance and growth and release this excess N into the envi-
ronment. This can be understood through simple stoichiometric relationships between
organismal C and N. Microbes have a C:N ratio of about 9:1 (Cleveland and Liptzin, 2007),
which is lower than the C:N ratio of many microarthropods and protozoans. To maintain
their C:N ratios, grazers need to release some of the excess N that results from ingest-
ing microbes (Osler and Sommerkorn, 2007). Wickings and Grandy (2011) carried out an
experiment showing the strong effects of microarthropods on decomposition and nutrient
cycling. They compared litter decomposition dynamics in the lab with and without the
oribatid mite Scheloribates moestus . Litter decomposing with this microarthropod had 19%
greater respiration rates, almost fivefold greater NH 4 + -N concentrations, and substantially
more NO 3 - -N (7 vs. 1 mg g -1 dry litter). Further, it was found that mites altered the chem-
istry of litter during decomposition by changing the relative abundance of litter polysac-
charides and by producing frass with unique structural chemistry relative to the original
plant litter. Although the broader significance of these changes in chemistry could not be
derived from a short-term study, feedbacks from changes in litter chemistry may influence
decomposer communities and N mineralization.
The microbial loop is an example of N availability exceeding decomposer demand.
Often, however, microorganisms need additional N to decompose organic matter with
a C:N ratio that is greater than 20-25:1. When the N content of organic matter does not
meet the requirements dictated by cellular stoichiometry, organisms need to acquire N
from the surrounding environment. This process is termed N immobilization and results
in declines in soil inorganic N as decomposer communities utilize it to break down
organic matter. Given that N mineralization-immobilization is key to soil N availability,
we need to understand better the factors tipping the balance toward one or the other pro-
cess. Again, we can look to the microbial requirements for C and N for a general answer.
Microbial growth efficiency (the amount of substrate converted to biomass relative to the
total amount of the resource consumed) varies depending on substrate quality, microbial
community structure, and other factors, but 45% is a typical approximation (Six et al., 2006;
Thiet et al., 2006). If we assume that the average microbial C:N ratio is about 9:1 (Cleveland
and Liptzin, 2007; Kallenbach and Grandy, 2011), then a substrate C:N ratio of about 20:1
represents the threshold for N mineralization and immobilization. Above this threshold,
microbes will utilize soil inorganic N to decompose organic matter, resulting in immobi-
lization. Below this threshold, there is adequate N in the litter to support decomposition,
and any excess N will be released to the soil environment (i.e., mineralization). Indeed,
C:N ratios provide reasonable broad-scale predictions of N mineralization-immobilization
dynamics, but the specific chemical structure of organic matter (i.e., the concentrations of
lignin, lipids, and polysaccharides) can be just as important as C:N ratios (Melillo et al.,
1982; Berg, 2000; Grandy and Neff, 2008).
Nitrogen mineralization is thus strongly dependent on substrate C:N ratios and
organic matter chemistry, but like other biological processes, it is also sensitive to soil
moisture and temperature (Cook et al., 2010). Further, microbial community structure can
account for as much as 15-20% of the variation in decomposition rates, and changes in
trophic-level diversity can strongly influence decomposition and N mineralization (Ayres
et al., 2006; Strickland et al., 2009). Given the range of controls over N mineralization and
the different temporal and spatial scales at which they operate, it should come as no sur-
prise that predicting N release from organic sources is difficult. It is even harder to try to
synchronize N mineralization with plant N needs over space and time.
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