Principles of terrestrial ecosystem ecology.pdf

206 9. Terrestrial Nutrient Cycling
high litter quality (see Fig. 7.14) also promote
nitrogen mineralization.
Environmental conditions that promote
microbial activity enhance both gross and net
nitrogen mineralization. Net nitrogen mineralization
rates are therefore generally higher in
tropical than in temperate forest soils, whereas
arctic soils show net nitrogen immobilization
(a net decrease in the concentrations of inorganic
nitrogen) during the growing season
(Nadelhoffer et al. 1992). Even within a biome,
factors that improve the soil temperature and
moisture environment for microbial activity
enhance net nitrogen mineralization. Across
the Great Plains of the United States, for
example, rates of mineralization are positively
related to precipitation and temperature.
Recently deforested areas also typically have
higher rates of net nitrogen mineralization
than do undisturbed forests, at least in part due
to warmer, moister soils (Matson and Vitousek
1981). Soil moisture that is high enough to
restrict microbial activity also restricts net
nitrogen mineralization.
Why do favorable litter quality, moisture, and
temperature lead to net nitrogen mineralization
rather than immobilization of nitrogen in
a growing microbial biomass? Several factors
contribute to this pattern. First, the limitation
of microbial activity by carbon availability,
particularly in environments where nitrogen is
readily available and litter nitrogen is relatively
The critical C:N ratio that marks the dividing
line between net nitrogen mineralization
and net nitrogen uptake by microbes can be
calculated from the growth efficiency of
microbial populations and the C:N ratios of
the microbial biomass and their substrate.
Assume, for example, that the microbial
biomass has a growth efficiency of 40% and a
C:N ratio of 10:1. If the microbes break
down 100 units of carbon, they will incorporate
40 units of carbon into microbial
high, causes microbes to use some DON to
meet their carbon requirements for growth
and maintenance, secreting the ammonium as
a waste product. Second, warm temperatures
increase maintenance respiration and therefore
the carbon demands for microbial activity.
Finally, increases in microbial productivity
promote predation by soil animals, causing
greater microbial turnover and release of nitrogen
to the soil.
Substrate quality influences nitrogen mineralization
rate, not only through its effects on
carbon quality, which governs decomposition
rate (see Chapter 7) but also through its effects
on the balance between carbon and nitrogen
limitation of microbial growth. The carbon to
nitrogen (C:N) ratio in microbial biomass is
about 10:1. As microbes break down organic
matter, they incorporate about 40% of the
carbon from their substrates into microbial
biomass and return the remaining 60% of the
carbon to the atmosphere as CO2 through
respiration. With this 40% growth efficiency,
microbes require substrates with a C:N ratio of
about 25:1 to meet their nitrogen requirement
(Box 9.1). At higher C:N ratios, microbes
import nitrogen to meet their growth requirements,
and at lower C:N ratios nitrogen
exceeds microbial growth requirements and is
secreted into the litter and soil. In practice,
microbes vary in their C:N ratio (5 to 10 in bacteria
and 8 to 15 in fungi) and in their growth
Box 9.1. Estimation of Critical C:N Ratio for Net Nitrogen Mineralization
biomass and respire 60 units of carbon as
CO 2.The 40 units of microbial carbon require
4 units of nitrogen to produce a microbial
C:N ratio of 10:1 (= 40:4). If the 100 units of
original substrate is to supply all of this nitrogen,
its initial C:N ratio must have been
25:1 (= 100:4). At higher C:N ratios,
microbes must absorb additional inorganic
nitrogen from the soil to meet their growth
demands. At lower C:N ratios, microbes
excrete excess nitrogen into the soil.