Environmental Engineering Reference
In-Depth Information
reduced by
heterotrophic respiration
(NEP = NPP - R
H
), but it should also include
all nonrespiratory carbon losses.
Chapin et al. (2006) argued that the term NEP should be restricted to just the
difference between GPP and ecosystem respiration and proposed a new term, net
ecosystem carbon balance (NECB), for the net rate of carbon accumulation (or loss)
in ecosystems. R
H
is inl uenced by the ubiquitous microbial and insect depredations
to the recurrent seasonal grazing by large ungulates and also rel ects such irregular
episodes of massive insect attacks as invasions of crop-devouring locusts or bark
beetles destroying coniferous trees. In bioenergetic terms, agriculture should thus
also be seen as an endeavor aimed at minimizing R
H
. This quest is accomplished
most often through the application of pesticides (dominated by insecticides and
fungicides) but also through the use of protective crop cover, the fencing of i elds,
or even by deploying noisemaking devices to minimize damage done by birds or
grazing herbivores. As expected, the components of NEP vary with scale, and at a
regional or global level the term
net biome productivity
(NBP) is used as an alterna-
tive to NEP.
NPP cannot be measured directly, and the standard method of frequent harvesting
of sample plots (restricted by logistics and cost to areas of 10
3
m
2
and hard to do
in mature forests) captures reasonably well only the aboveground share of the
overall productivity; infrequent sampling would, of course, miss a great deal of lit-
terfall (once the dead phytomass is on the ground it is difi cult to assess its exact
age). But the sequential harvesting of aboveground phytomass does not capture
either the belowground increment or the carbon losses that do not involve respira-
tory l ows. The component of belowground productivity most difi cult to measure
is the often voluminous but always short-lived i ne root turnover (Fahey and Knapp
2007). As expected, overall accuracy is better at smaller scales, particularly where
many i eld observations are available. Luyssaert et al. (2009) found that their model-
based account of European (EU-25) forest NPP (annual mean of 520
75 g C/m
2
)
was within 25% of the results obtained using forest inventories or scaled-up ter-
restrial observations.
Accounts of agricultural production may be the most reliable source for quantify-
ing actual NPP, but a recent continent-wide analysis of the carbon balance of Euro-
pean (EU-25) croplands showed that even the world's best output and land-use
statistics, combined with process-oriented and remote-sensing models, still yielded
a surprisingly large range of NPP rates. Even when an identical cropland area was
assayed, different production factors resulted in estimates of mean annual NPP of
cropland in 25 EU nations as low as 646 and as high as 846 g C/m
2
(Ciais et al.
±
