16. Ecosystem Nutrient Balance and Dynamics
teristics of a forest riparian zone, groundwater dynamics, and/or in-stream processing (Billen et al.
1991). Tietema and Verstraten (1991), for example,
found that while streamwater N losses were only
21 % of extremely high atmospheric inputs of N to
an acid forest ecosystem in the Netherlands, 83%
of the amount "retained" by the ecosystem was actually denitrified. A literature compilation by Hill
(1996) found that denitrification of subsurface water inputs from agricultural and unsewered residential land uses through riparian zones ranged from
55 to 94% and were dependent on hydrologic flow
paths and residence times. In an analysis of tallgrass
prairie watersheds, Dodds et al. (1996) found that
denitrification and ammonia volatilization were a
large percentage of N inputs, and noted that large
watersheds had a greater amount of time to immobilize or to denitrify N inputs than smaller watersheds, causing stream water concentrations of N
to be lower. Holmes et al. (1996) also found stream
water outputs to be a poor measure of N exports
from the terrestrial ecosystem, and that denitrification was related to stream order and to organic
inputs.
Although the simplest answer to this problem
might be to directly measure denitrification losses
as part of the internal cycling of N, the measurement of actual denitrification rates (rather than denitrification potentials) is fraught with difficulties.
Denitrification is extremely variable in the terrestrial landscape, and is dependent on landscape position (Farrell et al. 1996; Groffman and Tiedje
1989) and the presence of rare "hot spots" (Christensen et al. 1990; Parkin 1987). Perhaps more importantly, the most common method for measuring
denitrification, the acetylene blockage technique,
has been called into question in recent years in both
oxic and anoxic soils and sediments (Seitzinger et
al. 1993; Bollmann and Conrad 1997) as acetylene
appears to scavenge intennediate nitric oxide, and
also blocks autotrophic nitrification and thus does
not measuring coupled nitrification-denitrification.
Although more complex methodologies are being
developed for both terrestrial and aquatic soils and
sediments, including N2 flux and 15N isotope pairing methods (cf. Nielsen et al. 1997; Joye et al.
1996; LaMontagne and Valiela 1995; Scholefieldet
al. 1997; Seitzinger et al. 1993; Van Luijn et al.
1996), no one technique has been accepted. Also,
most denitrification at the watershed scale may take
253
place primarily in riparian forest, stream, or lake
sediments, and thus capturing the spatial scale of
denitrification is extremely difficult.
Bums (1998) pointed out that denitrification was
only one of many aquatic and in-stream processes
that could reduce nutrient concentrations in
streams. In his study of the Neversink River (Catskill Mountains, NY), diurnal variations in stream
NO; levels of up to 30% were consistent with uptake by photoautotrophs during daylight hours, and
that both biological uptake and denitrification were
important processes reducing stream NO; over a
reach by up to 29%. Clearly, stream NO; concentrations integrate both terrestrial and aquatic retention processes. However, Bums (1998) points out
the failure of many models of watershed response
to atmospheric N deposition to acknowledge
aquatic and in-stream processes, because many attribute all retention to terrestrial processes.
Other Budget Approaches
Johnson and Van Hook (1989) discussed some of
the advantages and disadvantages of the watershedlevel versus the stand-level approach to nutrient
balance studies. Advantages of the watershed approach include little site disturbance and the fact
that the system is viewed as a unit. Disadvantages
include the fact that fluxes in the soil are rarely
accounted for, and the requirement of impenneable
bedrock is rarely met. The stand-level approach allows investigators to examine aboveground nutrient cycling, and has the advantages of reduced heterogeneity, estimates of within-soil processes, and
a decoupling from aquatic processes (if that is a
goal). Outputs can be measured using lysimeters,
which have some decided disadvantages, including
problems with interpreting the pool of soil water
sampled and obtaining an accurate estimate of water flux.
Stand-Level Budgets Using Lysimetry
Ecosystem budgets have been conducted at scales
smaller than the watershed in a variety of ways.
Often when stream sampling is impossible or impractical, or unnecessary for the hypothesis being
tested, a stand-level approach can be used, and ecosystem outputs can be estimated using lysimeters,
teristics of a forest riparian zone, groundwater dynamics, and/or in-stream processing (Billen et al.
1991). Tietema and Verstraten (1991), for example,
found that while streamwater N losses were only
21 % of extremely high atmospheric inputs of N to
an acid forest ecosystem in the Netherlands, 83%
of the amount "retained" by the ecosystem was actually denitrified. A literature compilation by Hill
(1996) found that denitrification of subsurface water inputs from agricultural and unsewered residential land uses through riparian zones ranged from
55 to 94% and were dependent on hydrologic flow
paths and residence times. In an analysis of tallgrass
prairie watersheds, Dodds et al. (1996) found that
denitrification and ammonia volatilization were a
large percentage of N inputs, and noted that large
watersheds had a greater amount of time to immobilize or to denitrify N inputs than smaller watersheds, causing stream water concentrations of N
to be lower. Holmes et al. (1996) also found stream
water outputs to be a poor measure of N exports
from the terrestrial ecosystem, and that denitrification was related to stream order and to organic
inputs.
Although the simplest answer to this problem
might be to directly measure denitrification losses
as part of the internal cycling of N, the measurement of actual denitrification rates (rather than denitrification potentials) is fraught with difficulties.
Denitrification is extremely variable in the terrestrial landscape, and is dependent on landscape position (Farrell et al. 1996; Groffman and Tiedje
1989) and the presence of rare "hot spots" (Christensen et al. 1990; Parkin 1987). Perhaps more importantly, the most common method for measuring
denitrification, the acetylene blockage technique,
has been called into question in recent years in both
oxic and anoxic soils and sediments (Seitzinger et
al. 1993; Bollmann and Conrad 1997) as acetylene
appears to scavenge intennediate nitric oxide, and
also blocks autotrophic nitrification and thus does
not measuring coupled nitrification-denitrification.
Although more complex methodologies are being
developed for both terrestrial and aquatic soils and
sediments, including N2 flux and 15N isotope pairing methods (cf. Nielsen et al. 1997; Joye et al.
1996; LaMontagne and Valiela 1995; Scholefieldet
al. 1997; Seitzinger et al. 1993; Van Luijn et al.
1996), no one technique has been accepted. Also,
most denitrification at the watershed scale may take
253
place primarily in riparian forest, stream, or lake
sediments, and thus capturing the spatial scale of
denitrification is extremely difficult.
Bums (1998) pointed out that denitrification was
only one of many aquatic and in-stream processes
that could reduce nutrient concentrations in
streams. In his study of the Neversink River (Catskill Mountains, NY), diurnal variations in stream
NO; levels of up to 30% were consistent with uptake by photoautotrophs during daylight hours, and
that both biological uptake and denitrification were
important processes reducing stream NO; over a
reach by up to 29%. Clearly, stream NO; concentrations integrate both terrestrial and aquatic retention processes. However, Bums (1998) points out
the failure of many models of watershed response
to atmospheric N deposition to acknowledge
aquatic and in-stream processes, because many attribute all retention to terrestrial processes.
Other Budget Approaches
Johnson and Van Hook (1989) discussed some of
the advantages and disadvantages of the watershedlevel versus the stand-level approach to nutrient
balance studies. Advantages of the watershed approach include little site disturbance and the fact
that the system is viewed as a unit. Disadvantages
include the fact that fluxes in the soil are rarely
accounted for, and the requirement of impenneable
bedrock is rarely met. The stand-level approach allows investigators to examine aboveground nutrient cycling, and has the advantages of reduced heterogeneity, estimates of within-soil processes, and
a decoupling from aquatic processes (if that is a
goal). Outputs can be measured using lysimeters,
which have some decided disadvantages, including
problems with interpreting the pool of soil water
sampled and obtaining an accurate estimate of water flux.
Stand-Level Budgets Using Lysimetry
Ecosystem budgets have been conducted at scales
smaller than the watershed in a variety of ways.
Often when stream sampling is impossible or impractical, or unnecessary for the hypothesis being
tested, a stand-level approach can be used, and ecosystem outputs can be estimated using lysimeters,