Agriculture Reference
In-Depth Information
in terrestrial ecosystems despite the high N inputs, while P tends to be limiting in
groundwater-fed surface waters.
Estimates of NO
3
−
leaching under MCSE systems clearly show the contrast
between annual row crops and unmanaged perennial vegetation and the role of fer-
tilization and N-fixing crops as sources of NO
3
−
(Fig. 11.8). Syswerda et al. (2012)
combined measured NO
3
−
concentrations in soil water beneath the root zone with
modeled soil water export to provide estimates of NO
3
−
leached from the root zone
over 11 years. Soil water export (drainage) is markedly higher in the Conventional,
No-till, and Deciduous Forest systems (Fig. 11.8A) than in the Reduced Input,
Biologically Based, Alfalfa, Poplar, and Early and Mid-successional systems, indi-
cating differences in evapotranspiration losses among these systems.
Nitrate leaching fluxes ranged from less than 1 kg N ha
−1
yr
−1
in unfertilized
Poplar to 62 kg N ha
−1
yr
−1
in the Conventional corn-soybean-wheat rotation
(Fig. 11.8B). Over 75% of the fertilizer added to the Conventional system was
lost as NO
3
−
over the 11-year period. Mean annual leaching losses were also high
in the No-till, Reduced Input, and Biologically Based annual cropping systems,
but progressively and significantly lower in each; the Biologically Based system
leached only 19 kg N ha
−1
yr
−1
, on average. Alfalfa leached even less N over this
period, and most of this occurred during a normal break year when a small unfer-
tilized grain crop was grown prior to reestablishing the alfalfa stand. The lowest
rates of NO
3
−
leaching were observed in the Early and Mid-successional systems,
and in Poplars, none of which were regularly fertilized (the Poplar system received
60 kg N ha
−1
in its first year). The relatively mature Deciduous Forest leached more
NO
3
−
than these systems, presumably reflecting low nutrient demand by the forests'
steady-state biomass in combination with high drainage rates. Over the 11-year
period, volume-weighted mean NO
3
−
concentrations in drainage water (Fig. 11.8C)
were highest in the Conventional system—above the threshold for acceptable
drinking water quality of 10 mg L
−1
as N, established by the U.S. Environmental
Protection Agency—and they approached this threshold in the No-till and Reduced
Input systems. Dissolved organic N and NH
4
+
were measured in these soil water
samples in earlier work and were found to be relatively minor constituents com-
pared to NO
3
−
(Syswerda et al. 2012). These findings corroborate results from other
studies (Power et al. 2001), although few studies have included such a diversity of
land cover and management regimes at a single location nor for this long a period.
Carbonate mineral dissolution and precipitation in soils can contribute to
soil-atmosphere carbon dioxide (CO
2
) exchanges (Hamilton et al. 2007). Dissolution
of carbonate minerals occurs in reaction with acidity, and can be either a source or
a sink for CO
2
depending on whether the reaction occurs with strong acids (e.g.,
nitric acid) or carbonic acid, respectively. Acidity in agricultural soils is mostly pro-
duced by two processes: (1) carbonic acid formed by CO
2
produced when roots and
microbes respire, and (2) nitric acid produced during nitrification of NH
4
+
to NO
3
−
.
Acidification can reduce soil fertility through several mechanisms; therefore, typi-
cally farmers of land free of native carbonate minerals periodically add carbonate
minerals (agricultural lime) to the soil to counteract acidifying processes.
Hamilton et al. (2007) analyzed the pathways by which carbonate minerals dis-
solve in soils and groundwater of the KBS region. In particular, they investigated
