Environmental Engineering Reference
In-Depth Information
Substrate
Microbe
Products
New cells
Residue
C = 50, N = 1
C:N = 50
C = 5
N = 1
C = 40
C:N = 5
5 CO
2
Small microbial biomass
(limited by N), incomplete
organic matter decomposition
and no N release
C = 50, N = 5
C:N = 10
C = 25
N = 5
C = 0
25 CO
2
Higher microbial biomass, complete
organic matter decomposition
and no N release
C = 50, N = 25
C:N = 2
C = 25
N = 5
N = 20
25 CO
2
High microbial biomass, complete
organic matter decomposition
and N is released
FIGURE 4.8
Effects of litter stoichiometry on potential nutrient immobilization. Assumptions are that the micro-
bial decomposers have a biomass C: N of 5 and their growth efficiency is 50%. As microbes grow on a substrate
they require 10 units of carbon (5 are lost to respiration) and 1 unit of nitrogen to make 1 new unit (5 carbon and
1 nitrogen) of microbial biomass. In the top case they become nitrogen-limited and cannot metabolize the residual
carbon. The example assumes no external nitrogen supply but in the top case the residual carbon could be metabo-
lized if a nitrogen source were available. In the second case the C: N ratio of the material is such that complete
decomposition occurs, generating 25 units of new biomass and consuming all the detrital organic matter. In the last
case there is excess nitrogen after microbial metabolism has consumed all carbon. Thus, in the first case detritus
decomposition yields a “demand” for inorganic nitrogen, and in the last the system can release nitrogen to the
environment.
defense against herbivores. Most attention has been given to a wide array of phenolic com-
pounds since they should be inherently difficult to decompose and have the capability to
bind proteins (
Hattenschwiler and Vitousek 2000
). Protein binding may inactivate
enzymes important to the decay process and decrease the availability of this source of
organic nitrogen. These secondary compounds vary among plant species and may be
induced by herbivore attack on the living plants. Tannins, which affect protein metabolism
by binding and possibly inactivating enzymes, are especially common secondary com-
pounds. Effects of tannins on decomposition are demonstrable but not always strong.
For instance, reindeer browsing on beech shifted tannin content but not uniformly across
habitats or years (
Stark et al. 2007
). In a study of litter decay in a set of tropical streams
Ardon et al. (2006)
found little effect of variation in abundance of various classes of pheno-
lic compounds. The effect of secondary compounds may be large in particular cases but is
perhaps less general than consequences of variation in major macromolecules or macronu-
trient content.
