Page 13 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002C-60 May 17, 2001 20:23

Biomass Utilization, Limits of 169

Serious soil erosion occurs on most of the world’s agri- in a lack of water for the remaining vegetation and limits
culture,includingtheUnitedStateswhereerosiononcrop- continual forest biomass production.
land averages 13 t/ha/yr. In developing countries, soil ero-
sion is approximately 30 t/ha/yr. The rates of erosion are
C. Nutrient Losses and Water Pollution
intensifying in developing countries because of inefficient
farming practices and because large quantities of biomass Rapid water runoff and nutrient losses occur when crop
are removed from the land for cooking and heating. Rural biomass residues are harvested for fuel and rainfall easily
people who are short of affordable fuels are now being erodes soils. Water quickly runs off unprotected soil be-
forced to remove crop residues and utilize dung for cook- cause raindrops free small soil particles that, in turn, clog
ing, leaving their soils unprotected and susceptible to wind holes in the soil and reduce water infiltration. This water
and water erosion. runoff transports soil organic matter, nutrients, sediments,
Indeed soil erosion caused by wind and water is re- and pesticides to rivers and lakes where it harms natural
sponsible for the loss of about 30% of the world cropland aquatic species. For example, conventional corn produc-
during the past 40 years. For example, the rate of soil loss tion lost an average of about 20 t/ha/yr of soil compared
in Africa has increased 20-fold during the past 30 years. with only about 5 t/ha/yr with ridge- and no-till.
Wind erosion is now so serious in China that Chinese soil As mentioned, the water-holding capacity and nutrient
can be detected in the Hawaiian atmosphere during the levels of soils are lessened when erosion occurs. With
Chinese spring planting period. Similarly, soil eroded by conventional corn production, erosion reduced the volume
wind is carried from Africa to Florida and Brazil. of moisture in the soil by about 50% compared with no-till
Erosion diminishes crop productivity by reducing the corn culture. In contrast, soil moisture volume increased
water-holding capacity of the soil and reduces water avail- when corn was grown in combination with living mulches.
ability to the plants. In addition, soil nutrient levels and Estimates are that about $20 billion in fertilizer nutrients
organic matter are carried away with the eroding soil and are lost annually from United States agriculture because
soil depth is lessened. Estimates are that the continuing of soil erosion.
degradation of agricultural land will depress world food Large quantities of nutrients are also lost when fuel-
production from 15–30% by the year 2020. Others project wood and crop residues are also removed and then burned.
that Africa will be able to feed only 40% of its popula- On average, crop residues contain about 1% nitrogen,
tion in 2025 both because of population growth and soil 0.2% phosphorus, and 1.2% potassium. When burned, the
infertility in vital cropland areas. nitrogen is released into the atmosphere. Although some
phosphorus and potassium are retained in the ashes, an
estimated 70–80% of these nutrients is lost when the fine
B. Forest Land Erosion
particulate matter is dispersed into the air during burning
Forestlands lose significant quantities of soil, water, and process. Thus, only a small percentage of the nutrients in
soil nutrients wherever trees are cut and harvested. For in- crop residues are conserved even when returning the ash
stance, the surface water runoff from a forested watershed residues to the cropland.
after a storm averaged 2.7% of the precipitation, but after
forest cutting and/or farming water runoff rose to 4.5 per-
D. Water Use
cent. In addition, soil nitrogen leached after forest re-
moval was 6 to 9 times greater than in forests with normal All biomass vegetation requires and transpires massive
cover. amounts of water during the growing season. Agricul-
Also,theproceduresusedinharvestingtimberandpulp- ture uses more water than any other human activity on
wood biomass contribute to increased erosion because the planet. Currently, 65% of the water removed from all
they expose the soil to wind and rainfall energy. Typically, sources worldwide is used solely for irrigation. Of this
tractor roads and skid trails severely disturb 20–40% of the amount, about two-thirds is consumed by plant life (non-
soil surface in forests. In addition, the heavy equipment recoverable). For example, a corn crop that produces about
needed to harvest and clear the land compacts the soil, 8000 kg/ha of grain uses more than 5 million liters per
resulting in greater water runoff. hectare of water during its growing season. To supply this
For example, compaction by tractor skidders harvest- much water to the crop, approximately 1000 mm of rain-
ing Ponderosa pine reduced growth in pine seedlings from fall per hectare, or 10 million l of irrigation, is required
6 to 12% over a 16-year period. Following clearing, wa- during the growing season.
ter percolation in the wheel-rutted soils was reduced for The minimum amount of water required per capita
12 years and in log-skid trails for 8 years. This resulted for food production is about 400,000 l/yr. If the water