Environmental Engineering Reference
In-Depth Information
exudation of carbon from roots. To calculate NEP, he would need to combine his estimates
of biomass carbon accumulation with measurements of soil carbon accumulation (which
are very difficult to make in most forests), imports and exports of organic carbon (e.g.,
export by organic carbon leaching from the soil), and any losses of carbon due to fire dur-
ing the measurement period.
The geologist is searching for the remains of past primary production. The coal that
accumulated from an ancient forest is a component of the NEP of that forest. The geologist
could calculate the NEP if she knew the time period over which the organic matter accu-
mulated in the soil, and if she assumed (1) that the forest biomass was in steady state;
(2) that there were no carbon imports, exports, or fires in the forest; and (3) that there
were no losses of organic carbon from the stored coal during the millions of years that the
organic matter was stored in the soil and transformed to coal in the rocks.
While all of these measurements pertain to primary production, each has a particular
scale associated with it, and they are not comparable to one another without a significant
effort at spatial and temporal scaling and a clear understanding of exactly which compo-
nents of productivity are being measured. Nevertheless, each of these investigations pro-
vides insights of relevance to the others as our four researchers discuss later while having
a beer together. The leaf measurements of the physiologist help explain the mechanisms
that lead to the timing and rates of gas exchange by the forest. Knowing the types and
amounts of plant matter accumulation provided by the plant ecologist allows the geologist
to evaluate what forests and what conditions produced long-term preservation of organic
matter. The studies of different scales and processes provide a richer view of primary
production.
SUMMARY
To summarize the main points of this chapter, primary production provides the organic
carbon that ultimately supports the metabolism of autotrophs and heterotrophs in ecosys-
tems. Understanding the pathways that organic carbon follows through an ecosystem
leads to distinctions among GPP, NPP, and NEP. Measurement methods for primary pro-
duction reflect the questions of interest, the types and life histories of primary producers
considered, and scales over which rates are considered. Primary production is regulated
by factors such as light, temperature, herbivory, and the supply of water and nutrients.
Models of primary production in ecosystems cannot rely on a single limiting factor but
instead must consider multiple potentially limiting processes. Net primary productivity
varies widely among ecosystems as a function of temperature, light, nutrients, and precipi-
tation (for terrestrial ecosystems) and covers a range from a few to more than a thousand
grams of organic carbon produced per square meter each year. Global primary production
is about half on land and half at sea and represents an important flux in the global carbon
cycle. The most immediate fate of net primary production is either consumption by herbi-
vores or conversion to detritus with the relative rates of these two processes varying
widely among ecosystems. Ultimately the fate of most primary production is conversion
back to CO
2
via respiration; however, some residual carbon accumulates in biomass and
