Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 34

Shipitalo and Owens (2006) observed that extreme events were responsible for a
large amount of the herbicide loss from fields. Extreme events will link agricultural
systems and the off-site impacts because of the potential impact of increased precipi-
tation amounts. However, extreme events do not have to occur for water quality to be
impacted; for example, Hatfield et al. (2009) showed that changes in nitrate concen-
trations in surface water in the Raccoon River watershed were related to changes in
cropping patterns more than changes in precipitation. Bertol et al. (2007) observed
that for soybean production in Brazil, there were positive relationships between P
and potassium concentrations in runoff water as well as concentrations in sediments.
Fullen and Brandsma (1995) observed positive linkages between erosion rates and
textural changes, with a continual loss of smaller texture materials along with sig-
nificant losses of both macronutrients and micronutrients from the topsoil. Their
conclusion was that typical erosion rates for Europe are likely to have negative con-
sequences on long-term soil fertility. The change in the textural composition of the
soil also has implications for the water holding capacity.
Water quality impacts from farming systems vary. Jaynes et al. (1999) evaluated
the surface water quality in a central Iowa watershed and found that herbicide con-
centrations of atrazine, alachlor, metolachlor, and metribuzin increased in surface
water with runoff events, while nitrate decreased. Nitrate concentrations increased
with increased subsurface drainage flow from the watershed. Jorden et al. (1997)
observed that water impacts from agricultural landscapes increased with the inten-
sity of agricultural practices. Similar results were found earlier by Mason et al. (1990)
for different watersheds. Keeney and DeLuca (1993) concluded that intensity of till-
age and subsequent N mineralization in the soil profile was the primary source of
NO 3 -N in the Raccoon River that offset the increase in commercial N fertilizer use
in the 1970s. In a companion study across the Raccoon River watershed, Lucey and
Goolsby (1993) concluded that climate variation and the subsequent amount of water
movement from the watershed and fields were responsible for the temporal variation
in nitrate concentrations in the surface water. Mason et al. (1990) observed similar
results in watersheds across Wisconsin. Water quality is affected by the water bal-
ance in the soil profile and across the landscape. We conclude from these observa-
tions that marginal soils with reduced water holding capacity will have a greater
likelihood of drainage through the soil profile or runoff from excess precipitation
than will soils with an improved water holding capacity.
Soil erosion is related to soil C loss, and Gregorich et al. (1998) evaluated
changes in soil C across a landscape and concluded that erosion affected C dynam-
ics in the soil through the redistribution of soil across a landscape. They found that
erosion was related to diminished primary productivity of the soil, which would
have a long-term impact because of the reduced return of C to the soil. Owens et al.
(2002) observed large differences in sediment and C losses from small watersheds
in Ohio due to different tillage systems; however, their primary conclusion was
that the greatest factor in reducing C movement from the watersheds was to reduce
sediment movement. A subsequent study by Larney et al. (2009) on wheat found
that over a 16-year period, grain yield declined 2.1% per centimeter of topsoil depth
removed on the dryland site and only 1.7% per centimeter on the irrigated site. They