Environmental Engineering Reference
In-Depth Information
and temperature. The stimulation of nitrification and lack of plant uptake creates larger
pools and hydrologic losses of NO
3
2
. As vegetation begins to grow back following distur-
bance, hydrologic losses decrease, often to levels lower than before the disturbance, as the
young, actively growing vegetation creates a strong demand for water and nutrient
uptake. Once biomass stops increasing, nutrient losses then increase and should come into
rough balance with inputs (
Figure 7.6
). While the patterns of N cycling with ecosystem
development described earlier are a useful way to think about N cycling in terrestrial eco-
systems, there is much variation in response to disturbance from site to site due to differ-
ences in inherent site fertility (e.g., nutrient-rich sites lose more nutrients following
disturbance), hydrologic conditions (e.g., high precipitation, permeable soils facilitate
nutrient losses), and the details of the disturbance (e.g.,
just how are trees cut and
removed from the system).
There are two interesting unresolved nitrogen mysteries that have arisen over the past
couple of decades. The first mystery is the persistence of N limitation of above-ground net
primary production (ANPP) over long (millions of years) time frames. Even though pools
of N in soils and vegetation become very large over time, additions of N in fertilizer still
lead to increases in ANPP. Persistent N limitation is likely due to uncontrolled gaseous
and hydrologic N losses. A second mystery, which somewhat contradicts the first mystery,
is low hydrologic losses of N even in mature (nonaggrading) ecosystems in regions with
relatively high atmospheric N deposition. This high N retention is likely due to storage of
excess N in dead wood, soil pools that increase over time, or poorly quantified and under-
estimated gaseous fluxes.
FIGURE 7.6
The watershed-
scale mass balance of nitrogen
at Hubbard Brook Experimental
Forest Watershed 6 (New
Hampshire) is shown for a 37-year
period. This northern hardwood
forest was aggrading (building up)
biomass until about 1982. During
the aggrading period, there were
“missing inputs” to support the
large accumulation in biomass, and
in the nonaggrading period, there
were “missing outputs” as stream
output, which is measured very
accurately at Hubbard Brook, and
is much lower than atmospheric
input. These missing outputs must
be either to gases or accumulation
in soil organic matter. (From Yanai,
R.D., S.P. Hamburg, M.A. Arthur,
M.A. Vadeboncoeur, C.B. Fuss, and
T.G. Siccama, unpublished.)
25
20
15
Missing term
10
Total atmospheric
input
5
Total stream
output
0
-5
Δ
total biomass
(live + dead)
-10
Δ
forest floor +
woody debris
-15
-20
-25
1965-77
1977-92 1992-2007
