Agriculture Reference
In-Depth Information
strategy and its impact on water quality.
Reduction in surface loadings of nitrate does not
instantly reduce its concentration at discharge
points, and P management practices may take
several years to improve water quality. Also,
changes in sources of streamflow over time make
the interpretation of water quality data difficult.
The lack of improvement in ground and surface
water quality in the USA is frustrating, but what
is not knowable is what the changes would have
been without the adoption of conservation prac-
tices promoted in the last two decades.
Also, the original EU models and many oth-
ers fail to consider the risks from climate change,
which has a complex relationship with diffuse
pollution. For example, flooding may increase
transport of sediment-associated pollutants but,
also, drought may make aquatic ecosystems
more vulnerable to various pollutants due to
reduced dilution. Improvements in these types
of predictive models should include factors such
as future climate change and projected land use,
so that policy makers can have different model-
ling options to fit specific situations.
Strategies to manage diffuse sources of pollu-
tion including agriculture must address the soci-
etal interest in minimizing the cost of management
implementation while maximizing efficiency and
effectiveness. Making farmers aware of peripheral
benefits to reduced nutrient losses (e.g. reduced
mastitis associated with stream exclusion) helps
increase their willingness to pay a greater por-
tion of the bill. Also, 'pollution swapping' must be
avoided. Mitigation approaches for one pollutant
may have negative effects on another. For instance,
slurry application during spring rather than
winter reduces nitrate in runoff but higher tem-
peratures may increase ammonia emission.
Similarly, composting of manure is positively
perceived as it creates a stable, transportable,
marketable product. The 'stability' of N in com-
post, however, is largely because most ammonia
N is lost during the composting process.
input inadequate to replenish soil is as often the
problem in developing countries. In all countries
nutrient losses from agricultural land is a non-
point source of environmental degradation but
in developing countries nutrient losses from
croplands are more damaging still as they cause
depletion of soil nutrients, impairing crop pro-
ductivity. In both scenarios, deterioration of sur-
face water is the outcome. Negative nutrient
balances are observed in sub-Saharan Africa,
making depletion of soil nutrients a continual
threat (Saleem, 1998). Livestock manure is a
key source of nutrients to replenish soil nutri-
ents. Total N and P in animal excreta worldwide
in 1996 were 93.7 and 21.1 million t, respec-
tively, with cattle the dominant source for both
N and P (Fig. 8.1).
Vitousek
et al
. (2009) compared nutrient
imbalances in a low nutrient input maize-based
system in Kenya, high nutrient input wheat- and
maize double-cropping system in north-east
China, and maize-soybean rotation system in
upper Midwest USA (Table 8.1). In the Kenya
system, annual N and P input to most fields was
7 and 8 kg ha
−1
, which is insufficient to replen-
ish the nutrients extracted by the crop (59 and
7 kg ha
−1
annually). The opposite scenario was
found in China where annual N and P input
(588 and 92 kg ha
−1
) was well in excess of the
crop's requirement, leading to degradation of
surface water degrading water quality. Similarly,
during a period of 25 years (1970-1995),
addition of N and P to agricultural fields in
the Mississippi Basin far exceeded the amount
extracted by crops leading to losses of nutrients
to, and consequently eutrophication of, fresh-
waters and the coastal Gulf of Mexico. Nutrient
N and P balances in this region are improving
with annual N and P input averaging 155 and
14 kg ha
−1
, and annual output of 145 and
23 kg ha
−1
from 1997 to 2006, respectively.
Environmental policies must obviously be
created based on local or regional systems. In
the Kenya system studied, increased nutrient
addition to the fields is clearly warranted. But
the addition of N fertilizer can be reduced to half
without affecting the yield or quality of the crop
in China or in Mexico. In most cases, reduction
of nutrient input alone will not be sufficient to
restore water quality. Other management strate-
gies are needed such as proper placement and
timing of fertilization, dietary manipulation in
Nutrient Imbalance, Water Pollution
and Soil Nutrient Reserves
in Developing Countries
While the nutrient balance challenge in devel-
oped countries is often one of excess, nutrient
