16
Ecosystem Nutrient Balance
and Dynamics
Kate Lajtha
Introduction
Many attempts at balancing ecosystem-level nutrient budgets have been made in the past few decades
using a variety of approaches, and for a variety of
different purposes. Relatively simple mass balance
equations have been used at the level of the watershed that might comprise single forested ecosystems (e.g., Binkley et al. 1982; Stohlgren et al.
1991; Likens and Bormann 1995; Hedin et al. 1995;
Stottlemyer and Troendle 1992), or at the level of
a larger-scale region (e.g., Gold 1990; Frink 1991;
Jaworski et al. 1992; Howarth et al. 1996; Valiela
et al. 1997) that might include different land uses
and nutrient inputs. These budgets have been developed with varying efforts at measuring, or modeling, internal processes of nutrient retention and
release.
The whole-ecosystem-Ievel budget has a variety
of purposes, including:
1. directly testing mechanistic models and understanding processes that control nutrient retention, turnover, and leaching in a field setting;
2. estimating processes by difference that are difficult to measure directly (such as weathering),
or to test measurement methods for complex
processes when the measurement techniques are
called into question (such as dry deposition inputs, nitrogen fixation, or denitrification);
3. understanding controls on regional water
quality;
4. in an experimental setting, understanding responses to watershed perturbations such as harvest, fire, or anthropogenic pollutant deposition
(e.g., Likens et al. 1978; Pardo et al. 1995);
5. assessing the potential for long-term depletions
in critical nutrient pools, such as cation stocks
in forests affected by acidic deposition (e.g.,
Federer et al. 1989; Likens et al. 1996).
Each specific purpose may require a unique level
of analysis and detail of measurement.
Often the lack of balance in an ecosystem budget
has proven far more interesting than a budget that
has balanced. For example, Bormann et al. (1977)
found a higher rate of nitrogen (N) storage (as accumulation in biomass) plus loss (as streamwater)
in the Hubbard Brook, NH, watershed than they
found in inputs of N measured as bulk deposition.
At the time, they suggested that the imbalance was
probably due to unmeasured N2 fixation. Thus the
imbalance led to a testable hypothesis that directed
future research, and later measurements showed
that fixation inputs were unlikely to correct the imbalance. Only a portion of the imbalance could be
corrected by improved methods of measuring atmospheric inputs such as dry deposition. However,
when the imbalance was compared with the mass
of N in the soil pool (7200 kg ha - 1), it became
clear that a fluctuation of 0.5% in this pool would
dwarf the budget imbalance, and thus much of the
N uptake by vegetation probably came from a small
net change in the mineral soil. This study showed
the difficulty in comparing relatively wellquantified external fluxes with poorly quantified
and considerably larger internal fluxes, and showed
the importance of examining the common assumption of ecosystem steady state.
In this chapter I will review some of the basic
methods and problems of creating whole249
Ecosystem Nutrient Balance
and Dynamics
Kate Lajtha
Introduction
Many attempts at balancing ecosystem-level nutrient budgets have been made in the past few decades
using a variety of approaches, and for a variety of
different purposes. Relatively simple mass balance
equations have been used at the level of the watershed that might comprise single forested ecosystems (e.g., Binkley et al. 1982; Stohlgren et al.
1991; Likens and Bormann 1995; Hedin et al. 1995;
Stottlemyer and Troendle 1992), or at the level of
a larger-scale region (e.g., Gold 1990; Frink 1991;
Jaworski et al. 1992; Howarth et al. 1996; Valiela
et al. 1997) that might include different land uses
and nutrient inputs. These budgets have been developed with varying efforts at measuring, or modeling, internal processes of nutrient retention and
release.
The whole-ecosystem-Ievel budget has a variety
of purposes, including:
1. directly testing mechanistic models and understanding processes that control nutrient retention, turnover, and leaching in a field setting;
2. estimating processes by difference that are difficult to measure directly (such as weathering),
or to test measurement methods for complex
processes when the measurement techniques are
called into question (such as dry deposition inputs, nitrogen fixation, or denitrification);
3. understanding controls on regional water
quality;
4. in an experimental setting, understanding responses to watershed perturbations such as harvest, fire, or anthropogenic pollutant deposition
(e.g., Likens et al. 1978; Pardo et al. 1995);
5. assessing the potential for long-term depletions
in critical nutrient pools, such as cation stocks
in forests affected by acidic deposition (e.g.,
Federer et al. 1989; Likens et al. 1996).
Each specific purpose may require a unique level
of analysis and detail of measurement.
Often the lack of balance in an ecosystem budget
has proven far more interesting than a budget that
has balanced. For example, Bormann et al. (1977)
found a higher rate of nitrogen (N) storage (as accumulation in biomass) plus loss (as streamwater)
in the Hubbard Brook, NH, watershed than they
found in inputs of N measured as bulk deposition.
At the time, they suggested that the imbalance was
probably due to unmeasured N2 fixation. Thus the
imbalance led to a testable hypothesis that directed
future research, and later measurements showed
that fixation inputs were unlikely to correct the imbalance. Only a portion of the imbalance could be
corrected by improved methods of measuring atmospheric inputs such as dry deposition. However,
when the imbalance was compared with the mass
of N in the soil pool (7200 kg ha - 1), it became
clear that a fluctuation of 0.5% in this pool would
dwarf the budget imbalance, and thus much of the
N uptake by vegetation probably came from a small
net change in the mineral soil. This study showed
the difficulty in comparing relatively wellquantified external fluxes with poorly quantified
and considerably larger internal fluxes, and showed
the importance of examining the common assumption of ecosystem steady state.
In this chapter I will review some of the basic
methods and problems of creating whole249