Environmental Engineering Reference
In-Depth Information
m 2 ) put sugarcane well ahead of other crops, even after
adjustments for the length of growing period (365 days
for cane, 90-150 days for temperate crops).
Blue light penetrates farthest in the open ocean, and
even in clean waters little red light will be available
for pigments below 10 m. Macronutrient concentrations
nearly always show a sharp decline near the surface and a
remarkable stability below 1000 m, a result of dominant
stratification of ocean waters. Even in relatively nutrient-
rich coastal water, photic zone has much lower N and P
concentrations. Areas of the highest NPP coincide with
the zones of nutrient enrichment (continental runoff in
near-shore waters or coastal upwelling). The northern
Atlantic, the Pacific shelves of Asia, and the upwelling
zones off Africa, the Americas, and in the northern In-
dian Ocean have the highest annual production (fig.
3.8). And because of a strong negative abundance-mass
scaling, energy used by all phytoplankton cells in a given
class size (regardless of species) equals that of all the cells
in other size classes (Li 2002).
The most efficient ecosystems are wetlands, which
benefit from a high influx of waterborne nutrients. Both
tropical and temperate marshes convert commonly 1.5%
of insolation into new phytomass, and the best sites do
at least twice as well. Typical grassland conversions
are much lower. US/IBP Grassland Biome sites show
a range from 0.13% for a desert grassland to 1.2% for a
mountain formation, with a mean of 0.46% for ungrazed
sites and 0.57% for grazed sites (Coupland 1979). Alpine
grasslands have efficiencies of 0.05%-0.13%, Arctic
grasses below 0.09%. The best conversions for temperate
forests are around 1.5%, for tropical rain forests about
1%, for rich, mature coniferous and deciduous forests
0.4%-0.9%, and in stressed locations 0.3%.
tion, and effective weed and pest control, traditional vari-
eties would produce no less phytomass than the modern
cultivars, but the partitioning of their photosynthates is
economically much less favorable.
In the year 2000 the mean global NPP for all crops
(calculated by enlarging the total DM harvest of @7.5
Gt by 15% in order to account for root productivity and
by another 15% to factor in the preharvest losses to het-
erotrophs) was about 10 Gt or about 7 t DM/ha (0.7
kg/m 2 , or 12 MJ/m 2 ,or < 0.4 W/m 2 ). Two studies of
the NPP of U.S. agriculture (a satellite observation-based
model and an estimate based on harvest data) found the
annual range of 0.54-0.62 Gt C (Lobell et al. 2002).
These totals prorate to 9.5-11 t DM/ha, 35-55% above
the global mean but still no better than the NPP of a
good lawn. This is not surprising because lawns have
high LAI and longer growing periods (6-9 months even
in winter climates, compared to 90-150 days for cereals).
Extreme crop NPP ranges from low-yielding cereals
and legumes in arid regions ( < 2 t DM/ha) to tropical
sugarcane ( > 50 t DM/ha). Among the leading commer-
cial crops, Iowa grain corn and Dutch wheat (8-9 t
grain/ha), would have whole-plant NPP of 19-22 t/ha,
(assuming that pesticide applications reduce R H to just
5% of NPP), identical to that of a dense lawn. Those C 4
crops that fix CO 2 year-round do as well as the best nat-
ural grasses, and when irrigated and fertilized, better than
any other plants. Even the worldwide average of sugar
cane NPP (with 15% of NPP from roots and R H equal
to at least 5% of NPP) is about 30 t/ha; the best national
average (about 150 t/ha of fresh cane in Peru) translates
into productivities of about 80 t/ha; and the highest
recorded fixation in Java amounted to 94 dry t/ha.
These record productivities (8-9.4 kg, or 135-160 MJ/
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