Environmental Engineering Reference
In-Depth Information
cultivars (
Matson et al. 1998
), valley farmers produce some of the highest wheat yields in
the world (
FAO 1997
). The region also maintains the most productive fisheries in Mexico
with sardines and shrimp being among the most important species (
CONAPESCA 2002
). In
recent years the region also has developed the second largest shrimp aquaculture industries
in Mexico
(CONAPESCA 2002)
. However, in a world of globalized markets, reduced subsi-
dies and price supports, drought, hurricanes, and other forces, many farmers and fishers in
the region are concerned about maintaining production and household incomes.
In the mid-twentieth century, the Mexican government and the international development
community identified the Yaqui Valley as an appropriate center for agricultural development. In
1943, Norman Borlaug, working for the Mexican government and the Rockefeller Foundation,
launched a wheat research program that was the forerunner of the International Maize and
Wheat Improvement Center (CIMMYT), which remains in the region today (
Naylor et al. 2001
).
Later, a national agricultural experiment center (CIANO) also was established in the valley.
Because the region is agro-climatically representative of 40% of the developing world's wheat
growing areas, it was selected as an ideal place for the early wheat improvement program.
The use of fertilizer nitrogen has increased markedly in the past four decades; between
1968 and 1995 fertilizer application rates for wheat production increased from 80 to
250 kgN/ha, over a 200% increase. When we started the study, the most common agro-
nomic practice for wheat production in the valley was a preplanting broadcast application
of urea or injection of anhydrous ammonium (at the rate of 150
200 kg/ha), followed by
irrigation (the preplanting irrigation is intended to aid in weed control by causing germi-
nation of weeds that can then be plowed under prior to planting). The causes and conse-
quences of these management approaches were the first focus of our studies in the region.
Fertilizer Use and Nitrogen Cycling in the Yaqui's Ecosystems
When we first tackled it in 1994, it seemed like an easy question: “Why do Yaqui farm-
ers use so much nitrogen fertilizer, does it matter to yields or productivity of the crop, and
to the environment, and if so, what can be done that makes sense economically
and environmentally?” Irrigated wheat yields were about 5 tons per ha in the early 1990s
and had held steady at that for almost two decades. Nevertheless, nitrogen applications
had been dramatically increasing, the current rate of nearly 250 kg/ha seemed extraordi-
narily high, and the rate of increase in nitrogen use showed few signs of slowing down.
Given what we already understood about nitrogen cycling in any ecosystem, we
expected dramatic losses of nitrogen through a variety of mechanisms (
Figure 12.2
). At the
outset, the team carried out a series of experiments both on experimental station lands and
in farmer fields that showed that the typical farmer's fertilization practice led to very high
concentrations of ammonium and nitrate in soils following the fertilization and irrigation
events, and large nitrous oxide and nitric oxide losses to the atmosphere—the highest
fluxes ever measured (
Matson et al. 1998
, and others); ammonia flux was never quantified
(despite several aborted attempts to do so), but comparative indicators suggested they too
were extremely high. Levels of nitric oxide emissions were large enough to drive the seri-
ous air pollution that occurred during that time in Cuidad Obreg
´
n, a city just downwind
of the valley. Deposition of nitrogen oxides certainly occurred in shrubland and forested
