Environmental Engineering Reference
In-Depth Information
often creates isolated (and often poorly accessible) patches of farmland among new
buildings and roads. Some of those patches continue to be cultivated for a while,
but most are soon abandoned to weeds. Other land near industrial plants or mines
has to be abandoned because of high levels of soil contamination. A new industry
devoted to the restoration of this land (often relying on bioremediation using plants
whose metabolism concentrates soil pollutants in roots or leaves or enhances benei -
cial microbial activity) has been successful in returning this land to plant cover.
Effects on Biomass Productivity
Deforestation, grassland burning, and permanent cropping have had a number of
obvious environmental consequences for the affected ecosystems. In most cases, the
primary productivity of new plant cover has been lower than that of the displaced
natural ecosystems. Where high-yielding crops displaced short grasses the productiv-
ity may be higher, but the nature of standing phytomass has changed profoundly:
unlike crops, natural grasslands have most of their phytomass underground to boost
water storage and minimize soil erosion. The consequences have included changes
in the micro- and mesoclimate resulting from alterations in the albedo (the share
of incoming solar radiation rel ected by surfaces), evapotranspiration, and soil
moisture retention; these alterations in turn have affected primary productivity,
either by changing its overall rate or by altering the specii c composition of plant
communities.
Where the conversions have resulted in higher soil erosion rates—burning down
a closed-canopy forest could increase the typical erosion rate, particularly on a
sloping land and in a rainy climate, by several orders of magnitude—the effects
have extended further ai eld. Eroded soil, carrying particulate organic matter and
dissolved nitrogen, is deposited downstream in river channels and can eventually
change the stream course or bring more extensive l ooding, while periodic l ooding
in lowlands can accelerate the deposition of rich alluvial soil suitable for intensive
farming. A common assumption was that these changes affected productivity only
on a local or regional scale and that they could not have a larger, even global,
impact.
That assumption was challenged by Ruddiman (2005), who argued that the
anthropogenic inl uences on global climate (and hence on primary productivity)
began with the adoption of shifting agriculture by Neolithic cultivators and intensi-
i ed with subsequent permanent cropping and animal husbandry. These effects
