14
Nutrient Transformations
John M. Stark
Introduction
A question often asked in ecosystem studies is:
What controls rates of release of plant-available
nutrients? Nutrient elements are converted from
unavailable to available forms through a wide variety of transformation processes (Stevenson 1986;
Schlesinger 1991); however, there are some general
cycling patterns that are common to all nutrients
(Fig. 14.1). For example, most plant macronutrients
exist in both organic and mineral forms. Therefore,
the processes of mineralization (conversion of organic to mineral forms) and assimilation (conversion of mineral to organic forms) are common to
these nutrients. Other nutrients also form insoluble
compounds and, thus, dissolution and precipitation
processes represent important pathways for release
and immobilization of plant-available forms. While
procedures used to measure concentrations of different nutrients vary considerably, the approaches
used to measure these transformation rates are generally quite similar.
In this section, I discuss a variety of approaches
that can be used to measure nutrient transformation
rates. Rather than describing the details of specific
procedures, I emphasize the theory, assumptions,
advantages, and disadvantages of the various approaches. For descriptions of specific procedures,
the reader should consult one of several excellent
methodology texts (e.g., Weaver et al. 1994;
Knowles and Blackburn 1993; Sparks et al. 1996).
In the following discussion, the methods are divided into two groups: non-isotope methods and
isotope methods. Methods for measuring nitrogen
(N) cycling processes will often be used as exampIes for two reasons: N is the nutrient that most
frequently limits net primary production in ecosystems, and many of the methods were originally developed to measure rates of N transformations.
With appropriate modifications, however, these
methods can be used to measure transformation
rates of a variety of other nutrients.
Before discussing specific approaches, it is important to distinguish among some terms often used
to describe rate estimates. The term "gross rate"
refers to the unidirectional flow of a nutrient from
one pool to another. It is the rate that is represented
by individual arrows in Fig. 14.1. In contrast, a "net
rate" is the difference between the flows into a pool
(production processes) and the flows out of a pool
(consumption processes). Net rates are determined
simply by measuring the change in a pool size (nutrient concentration) over a period of time:
net production rate
PI - Po
t
~ gross production rates
- ~ gross consumption rates (14.1)
where Po is the nutrient concentration at the beginning of the incubation (time 0) and PI is the concentration at the end of the incubation (time t). For
consistency and ease of use in mass balance calculations, rates should be expressed in terms of the
amount of element transferred, not the amount of
compound. For example, rates of assimilation of
phosphate (PO~ -) into plant biomass would be
expressed in units of g P ha - I day - I rather than
g PO~- ha- I day-I.
215
Nutrient Transformations
John M. Stark
Introduction
A question often asked in ecosystem studies is:
What controls rates of release of plant-available
nutrients? Nutrient elements are converted from
unavailable to available forms through a wide variety of transformation processes (Stevenson 1986;
Schlesinger 1991); however, there are some general
cycling patterns that are common to all nutrients
(Fig. 14.1). For example, most plant macronutrients
exist in both organic and mineral forms. Therefore,
the processes of mineralization (conversion of organic to mineral forms) and assimilation (conversion of mineral to organic forms) are common to
these nutrients. Other nutrients also form insoluble
compounds and, thus, dissolution and precipitation
processes represent important pathways for release
and immobilization of plant-available forms. While
procedures used to measure concentrations of different nutrients vary considerably, the approaches
used to measure these transformation rates are generally quite similar.
In this section, I discuss a variety of approaches
that can be used to measure nutrient transformation
rates. Rather than describing the details of specific
procedures, I emphasize the theory, assumptions,
advantages, and disadvantages of the various approaches. For descriptions of specific procedures,
the reader should consult one of several excellent
methodology texts (e.g., Weaver et al. 1994;
Knowles and Blackburn 1993; Sparks et al. 1996).
In the following discussion, the methods are divided into two groups: non-isotope methods and
isotope methods. Methods for measuring nitrogen
(N) cycling processes will often be used as exampIes for two reasons: N is the nutrient that most
frequently limits net primary production in ecosystems, and many of the methods were originally developed to measure rates of N transformations.
With appropriate modifications, however, these
methods can be used to measure transformation
rates of a variety of other nutrients.
Before discussing specific approaches, it is important to distinguish among some terms often used
to describe rate estimates. The term "gross rate"
refers to the unidirectional flow of a nutrient from
one pool to another. It is the rate that is represented
by individual arrows in Fig. 14.1. In contrast, a "net
rate" is the difference between the flows into a pool
(production processes) and the flows out of a pool
(consumption processes). Net rates are determined
simply by measuring the change in a pool size (nutrient concentration) over a period of time:
net production rate
PI - Po
t
~ gross production rates
- ~ gross consumption rates (14.1)
where Po is the nutrient concentration at the beginning of the incubation (time 0) and PI is the concentration at the end of the incubation (time t). For
consistency and ease of use in mass balance calculations, rates should be expressed in terms of the
amount of element transferred, not the amount of
compound. For example, rates of assimilation of
phosphate (PO~ -) into plant biomass would be
expressed in units of g P ha - I day - I rather than
g PO~- ha- I day-I.
215