Agriculture Reference
In-Depth Information
Chighladze et al., 2012). Nitrate N in the soil profile (0.1-1.2 m depth) accounted for 20-38% of the
apparent fertilizer N recovery, thus, most likely resulting in the accentuation of N losses due to deep
leaching of
NO
3
−
from the soil (Turpin et al., 1998).
2.6 LOSSES THROUGH SOIL EROSION
Soil erosion results in losses of agricultural productivity and continues to be a major issue, espe-
cially under conventional agricultural practices (Lal, 1999; Veum et al., 2012). As a result, conserva-
tion management practices have become increasingly popular due to a wide range of environmental
benefits, including increased soil organic carbon (Lal et al., 1994), reduced erosion (Robinson et al.,
1996), reduced runoff (Veum et al., 2009), and increased aggregate stability (Angers et al., 1993).
From 1982 to 1997, agricultural soil erosion in the United States declined by approximately 1 billion
metric tons per year, and a quarter of this decrease has been attributed to conservation management
efforts (Wiebe and Gollehon, 2006).
Soil erosion is a natural phenomenon. But when the removal of the soil is faster than soil forma-
tion through bedrock weathering, soil erosion becomes a problem, often resulting in the reduced
ability of soils to perform their functions (Mchunu et al., 2011). Serious soil erosion is now occur-
ring in most of the world's major agricultural regions, and the problem is growing as more marginal
land is brought into agricultural production (FAO, 2008). Since soil erosion is a major threat to the
sustainability of soils and soil functions in the years to come, finding remediation to soil erosion is
a key issue (FAO, 2008).
N loss through soil erosion is a big loss in the soil-plant system. The erosion may be related to
wind erosion and water erosion or to both. Feng et al. (2011) reported that soil wind erosion and
fugitive dust emission contribute to land degradation, loss of soil productivity, and poor air quality
and visibility. Atmospheric dust also influences climate by altering the Earth's radiation balance
(Tegen and Lacis, 1996). The potential for wind erosion is higher in arid and semiarid regions
(Feng et al. 2011). During high wind events, soil entrainment is dominated by suspension processes
(Kjelgaard et al., 2004; Sharratt, 2011). Suspension of fine particulates from relatively small emis-
sion source areas can impact communities downwind. Soil and mineral particles with a diameter of
<60 µm are especially important to air quality because they contain significant amounts of soil nutri-
ents (Zobeck and Fryrear, 1986) and contaminants (Pye, 1987). Of particular concern are these par-
ticles with mean aerodynamic diameters ≤10 and 2.5 µm that are stringently regulated by the U.S.
Environmental Protection Agency (USEPA, 1990). Furthermore, wind and water erosion removes
the topsoil layer that contains a high amount of organic matter. When the top layer organic matter
provides a large amount of N to crop plants through mineralization. Hence, the loss of topsoil layer
means the loss of a high amount of N in the organic form.
2.7 LOSSES THROUGH AMMONIA VOLATILIZATION FROM FOLIAGE
The absorption and loss of N through the plant canopy is also an important part of N cycling in soil-
plant systems. Controlled as well as field studies showed that plants can absorb NH
3
from the air as well
as lose NH
3
to the air by volatilization (Farquhar et al., 1980; Fageria and Baligar, 2005). The emission
of NH
3
has considerably increased over recent decades. Factors influencing NH
3
losses include soil and
plant N status and plant growth stage (Sharpe and Harper, 1997). Abundant supply favors NH
3
losses,
especially if the supply is in excess of plant requirements (Fageria and Baligar, 2005). The loss of NH
3
through the plant canopy can occur during the whole growth cycle of a crop (Harper and Sharpe, 1995).
However, some scientists have reported that the highest NH
3
volatilization rates for major agricultural
crops occur during the reproductive growth stage (Francis et al., 1997). Absorption of atmospheric NH
3
has been associated with low plant N content and with high atmospheric NH
3
concentrations (Harper
and Sharpe, 1995). Raun and Johnson (1999) reported that N losses under field conditions are generally
attributed to the volatilization of NH
3
from leaves of N-rich plants.
