Agriculture Reference
In-Depth Information
plants such as Virginia springbeauty (
Claytonia virginica
L.) and the common dan-
delion (
Taraxacum officinale
F.H. Wigg.), and then on aphids in the winter wheat
and alfalfa crops (Colunga-Garcia 1996). Later in the season, after aphids have fed
on soybean, the Early Successional and Poplar communities support late-season
aphid infestations that are exploited by the coccinellids (Maredia et al. 1992b).
Landscape diversity can therefore be key for biocontrol services provided by
mobile predators. For coccinellids, the presence of heterogeneous habitats within
1.5 km of a soybean field is strongly correlated with soybean aphid suppres-
sion: Landscapes with greater proportions of the local area in corn and soybean
production have significantly less biocontrol (Fig. 2.2B; Gardiner et al. 2009).
Landis et al. (2008) estimated the value of hidden biocontrol in Michigan and three
adjacent states to be $239 million for 2007 on the basis of a $33 ha
−1
increase in
profitability from higher production and lower pesticide costs among the soybean
farmers who used integrated pest management to control aphids.
Providing Clean Water
The quality of water draining from agricultural watersheds is a longstanding envi-
ronmental problem. Sediment, phosphorus, and nitrate are important pollutants that
leave cropland and lead to compromised groundwater, surface freshwaters, and
marine ecosystems worldwide. In the United States, over 70% of the nitrogen and
phosphorus delivered to the Gulf of Mexico by the Mississippi River is derived
from agriculture (Alexander et al. 2008). Such deliveries create coastal hypoxic
zones worldwide (Diaz and Rosenberg 2008).
Must this necessarily be the case? Sediment and phosphorus loadings can be
reduced substantially with appropriate management practices: No-till and other
conservation tillage methods can often eliminate erosion and substantially reduce
the runoff that also carries phosphorus to surface waters, as can riparian plantings
along cropland waterways (Lowrance 1998). Nitrate mitigation is more problem-
atic. Because nitrate is so mobile in soil, percolating water carries it to groundwater
reservoirs, where it resides for days to decades before it emerges in surface waters
and is then carried downstream (Hamilton 2012), eventually to coastal marine
systems.
Some of the transported nitrate can be captured by riparian communities
(Lowrance 1998) or can be processed streamside (Hedin et al. 1998) or in transit
(Beaulieu et al. 2011) to more reduced forms of nitrogen, including nitrogen gas.
If wetlands are in the flow path, a significant fraction can be immobilized in wet-
land sediments as organic nitrogen or can be denitrified into nitrogen gas, either by
heterotrophic or chemolithoautotrophic microbes (Whitmire and Hamilton 2005,
Burgin and Hamilton 2007). Restoring wetlands and the tortuosity of more natu-
ral channels can increase both streamside and within-stream processing of nitrate
(NRC 1995).
But, by far, the best approach to mitigating nitrate loss is avoiding it to begin
with—a major challenge in cropped ecosystems so dependent on large quantities
of plant-available nitrogen. The average nitrogen fertilizer rate for corn in the U.S.
Midwest is ~160 kg N ha
−1
(ERS 2013), with only about 50% taken up by the crop,
