Environmental Engineering Reference
In-Depth Information
Plant respiration is responsible for a major loss of fixed
carbon, but (unlike wasteful photorespiration) this loss
is a function of fundamental gains: the released CO
2
is
an inevitable by-product of biochemical reactions that
support plant growth (including the transport of photo-
synthates and biosynthesis of complex compounds)
and maintenance. Respiration is thus a metabolic bridge
from photosynthesis to plant structure and function
(Amthor and Baldocchi 2001). Respiration rates (usually
expressed as the quotient R
A
/GPP) are generally lowest
in crops (often as low as 0.3; agriculture could be defined
as a quest for maximized GPP and minimized respira-
tion). They range between 0.3 and 0.65 in grasslands
and between 0.45 and 0.70 in young forest, and can
even be over 0.9 in mature forests. The rate of 0.5 is a
good first-order approximation.
NPP is a fundamental concept, but its value cannot
be measured directly. A variety of methods (including
recurrent harvesting with corrections for litter fall
and heterotrophic losses, gas-exchange techniques, and
mathematical treatments relating plant growth to various
environmental variables) have been used to determine
NPP. The task is obviously most difficult with forests.
Until recently highly accurate gas-exchange studies were
practical only on a very small scale (10
0
-10
1
m
2
), and
reliable complete destructive harvesting is limited by
logistics and cost to 10
3
m
2
. Good approximations of
above-ground NPP can be based on short-term measure-
ments of fixation rates. Typical daily means sustainable
for several weeks of the most rapid growth are about 20
g/m
2
for C
4
plants and 5-15 mg (average 13) for C
3
species. All the latest NPP estimates rely on increasingly
complex models that take into account many relevant en-
vironmental variables (Cramer et al. 1999).
Phytomass is subject to heterotrophic consumption
that ranges from rapid microbial decomposition to sea-
sonal grazing by large ungulates. Subtracting heterotro-
phic respiration (R
H
) by microbes, invertebrates, and
vertebrates from the NPP of a particular ecosystem yields
the net ecosystem production (NEP, the annual rate of
storage), whose ultimate aggregate is the net primary
biospheric production. NEP is the phytomass that is po-
tentially available for human harvests, but because of
difficulties in quantifying R
H
, the NPP has become the
preferred measure of primary production. It has received
a great deal of research attention, particularly with re-
spect to the anthropogenic releases of CO
2
affecting the
carbon cycle.
Estimates of the Earth's primary productivity have a
long history. In 1862 one value was based on the seem-
ingly unrealistic assumption of all land entirely covered
with a green meadow and yielding annually 5 t/ha (Lie-
big 1862). Surprisingly, the result, about 63 Gt C, falls
within the range of estimates published since the 1970s:
the Earth's mean primary productivity does indeed re-
semble that of temperate grassland (fig. 3.5). Most of
the values offered during the intervening generations
were either serious under- or overestimates of the most
likely total. A clearer consensus emerged only during the
1970s, when appraisals using empirically derived relations
between climatic variables and NPP yielded totals be-
tween 40 Gt C and 60 Gt C (Smil 2002).
The first estimate of global phytoplankton pro-
duction—25.4 Gt C/a for the open ocean and 3.2
Gt C/a for coastal seas—was based on just a few values
for relatively unproductive areas (Noddack 1937). Riley
(1944) used productivity values for seven western Atlan-
tic sites to produce a range of 44-208 (mean 126) Gt C.
