Environmental Engineering Reference
In-Depth Information
2010), a 30% difference. Expressed in common yield terms (although, of course,
NPP is much larger than actual harvestable yield), this corresponds to an average
annual productivity of 13-17 t/ha.
More complete appraisals of CO 2 l uxes can be now derived using gas exchange
techniques (eddy covariance and mass balances within the convective boundary
layer) that are fairly easily accomplished with small grass or crop plots but are much
more difi cult with forest growth (they require the erection of tall towers, the use
of tethered balloons, or regular sampling with aircraft). But even these techniques
are of no help in discriminating the autotrophic (derived from roots) and hetero-
trophic (derived from bacteria) components of soil respiration, or in quantifying
non-CO 2 losses. Total CO 2 l ux methods should yield productivity estimates that
are perhaps 20%-50% higher than the standard values (Geider et al. 2001). For
example, the NPP of a Brazilian rain forest near Manaus is as high as 15.6 t C/ha,
while the total that neglected i ne root turnover was nearly 40% lower. And
Scurlock, Johnson, and Olson (2002) believe that the harvest-based estimates of
grassland NPP may be no more than 50% and perhaps as little as 20% of the real
rate. At the same time, NEP values are signii cantly lower once all nonrespiratory
l uxes are taken into account.
The most obvious of these carbon losses is from ubiquitous litterfall: a large
part of NPP is continuously discarded in plant litter, and while most of these tissues
(buds, l owers, fruits, leaves, twigs, branches) are consumed (some rapidly, others
very slowly) by resident heterotrophs, some litter carbon is carried away (dissolved
and particulate organic carbon is moved by leaching beyond root level and by
leaching and soil erosion into streams and carried away by wind) and does not
get a chance to become part of the producing ecosystem's heterotrophic respiration.
Not surprisingly, specii c information about the partitioning of litter l ows among
bacterial and fungal decomposers, invertebrate and vertebrate foraging, and losses
beyond an ecosystem boundary is not easily available. Meentemeyer et al. (1982)
estimated the worldwide annual leaf fall at 35.1 Gt and the total litter production
at 54.8 Gt, or about half the global NPP. Studies in various ecosystems have shown
ranges of 5-15 t/ha in tropical forests (11 t/ha may be a good mean) and mostly
between 4 and 8 t/ha in temperate and boreal biomes, where 4.5-5 t/ha may be
a typical loss.
Other carbon losses include those in the form of volatile organic compounds
(mainly monoterpenes) emitted in particularly copious volumes by some trees
(Fuentes et al. 2000; Tunved et al. 2006; Schurgers et al. 2009), other exudates
(sap, resins, waxes), methane produced by methanogenic bacteria, CO from
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