16. Ecosystem Nutrient Balance and Dynamics
257
A.
Growing season
Donnant season
Lysimeter at
15 em depth
Lysimeter at
50 em depth
D NH4
N03
Ground water
II DON
0
15 30 45 60 75 90 105 120
0
15
30 45 60
75 90 105 120
B.
Lysimeter at
15 em depth
Lysimeter at
50 em depth
Ground water
0
15
30 45 60
75 90 105 120 0
15
30 45 60
75 90 105 120
(~moles liter -1 )
(~oles liter -1 )
FIGURE 16.3. Mean concentrations of N species captured in zero-tension lysimeters and measured in groundwater
during the growing season (April 15 to October 15) and dormant season (October 16 to April 14) over a 2-year
period in a forested watershed developed on sandy substrates, Waquoit Bay, Cape Cod, MA. Values for two sites
with different soil textures are shown: A, fine sand and B, loamy sand. (Redrawn from Seely et al. [1998].)
The Hubbard Brook "sandbox" experiment (Bormann et a1. 1987; Bormann et a1. 1993), was created
in 1982 specifically to calculate complete plot-level
element budgets. Large monolith lysimeters were
dug to a depth of 1.5 m, lined with reinforced membranes on the sides and outfitted with bottom drains
to measure output solution chemistry, filled with
screened glacial outwash sand and 5 cm of topsoil,
and planted with seeds or seedlings of known N
content. Here an experimental ecosystem was created under field conditions where, in theory, all inputs and outputs could be measured. Even in these
carefully controlled ecosystems, though, a balanced
budget could not be made; pine sandboxes had an
unexplained input of 54 kg N ha -I yr- I (Bormann
et a1. 1993). The authors argued against dry deposition inputs at this order of magnitude, and suggested that associative rhizosphere N2 fixation
could account for this imbalance. Although they
measured rates of acetylene reduction in low-02
chambers in pitch and red pine rhizospheres, they
declined to convert these direct measurements into
annual or per-unit-area values of N2 fixation, citing
the uncertain ratio of C2H4 fixed to N2 fixed and
unknown O2 concentrations in the actual rhizospheres. Thus rapid associative fixation could not
be independently measured, and other studies of
associative fixation have uniformly measured rates
257
A.
Growing season
Donnant season
Lysimeter at
15 em depth
Lysimeter at
50 em depth
D NH4
N03
Ground water
II DON
0
15 30 45 60 75 90 105 120
0
15
30 45 60
75 90 105 120
B.
Lysimeter at
15 em depth
Lysimeter at
50 em depth
Ground water
0
15
30 45 60
75 90 105 120 0
15
30 45 60
75 90 105 120
(~moles liter -1 )
(~oles liter -1 )
FIGURE 16.3. Mean concentrations of N species captured in zero-tension lysimeters and measured in groundwater
during the growing season (April 15 to October 15) and dormant season (October 16 to April 14) over a 2-year
period in a forested watershed developed on sandy substrates, Waquoit Bay, Cape Cod, MA. Values for two sites
with different soil textures are shown: A, fine sand and B, loamy sand. (Redrawn from Seely et al. [1998].)
The Hubbard Brook "sandbox" experiment (Bormann et a1. 1987; Bormann et a1. 1993), was created
in 1982 specifically to calculate complete plot-level
element budgets. Large monolith lysimeters were
dug to a depth of 1.5 m, lined with reinforced membranes on the sides and outfitted with bottom drains
to measure output solution chemistry, filled with
screened glacial outwash sand and 5 cm of topsoil,
and planted with seeds or seedlings of known N
content. Here an experimental ecosystem was created under field conditions where, in theory, all inputs and outputs could be measured. Even in these
carefully controlled ecosystems, though, a balanced
budget could not be made; pine sandboxes had an
unexplained input of 54 kg N ha -I yr- I (Bormann
et a1. 1993). The authors argued against dry deposition inputs at this order of magnitude, and suggested that associative rhizosphere N2 fixation
could account for this imbalance. Although they
measured rates of acetylene reduction in low-02
chambers in pitch and red pine rhizospheres, they
declined to convert these direct measurements into
annual or per-unit-area values of N2 fixation, citing
the uncertain ratio of C2H4 fixed to N2 fixed and
unknown O2 concentrations in the actual rhizospheres. Thus rapid associative fixation could not
be independently measured, and other studies of
associative fixation have uniformly measured rates