Agriculture Reference
In-Depth Information
through these carbonate-rich soils on its way to groundwater, and because most
surface waters receive groundwater inputs that are alkaline. However, in surface
soils (0.5-2 m, or deeper in the coarsest soils) where carbonate minerals have been
leached since deglaciation and their acid neutralization capacity diminished, acid
precipitation could cause various biotic stresses including aluminum toxicity and
leaching of calcium to the point where plant growth may experience calcium defi-
ciency (Driscoll et al. 2001). In cropped soils, substantial acidity is generated by
nitrification stimulated by N fertilizers and by removal of base cations in harvest,
but much of this acidity is counteracted by the application of agricultural lime (cal-
cium carbonate or dolomite) at several year intervals (see below).
As indicated earlier, the hydrochemical changes that occur as water percolates
through the first few meters of soil are dramatic (Fig. 11.6). For example, as pre-
cipitation percolates through unfertilized soils at KBS, soil solute concentrations
increase considerably, primarily due to mineral dissolution (Kurzman 2006). In the
upper 1.2 m—the carbonate-leached zone in these soils—silicate mineral weather-
ing produces modest increases in solutes including base cations (calcium [Ca
2+
],
magnesium [Mg
2+
], sodium [Na
+
]), dissolved Si, and carbonate alkalinity (Jin
et al. 2008a, b). However, a much larger and abrupt increase in total solutes occurs
beneath about 1.5 m as percolating water contacts carbonate minerals. Within the
carbonate mineral zone, the soil water reacts with calcite and dolomite to sub-
stantially increase concentrations of dissolved Ca
2+
, Mg
2+
, and acid neutralizing
capacity (ANC, almost entirely due to bicarbonate at these pH values), with a con-
comitant increase in pH and specific conductance (Hamilton et al. 2007, Jin et al.
2008a). Nitrate also contributes significantly to the total anion composition in the
soil waters depicted in Fig. 11.6.
Effects of Agricultural Management on Water Quality
Land management practices—particularly fertilization and cultivation of N-fixing
crops—can profoundly influence the chemical composition of percolating waters
(Böhlke 2002, Chen and Driscoll 2009). Nitrogen often readily moves as NO
3
−
from cropping systems into groundwater, and in well-drained soils groundwater
NO
3
−
leaching rates beneath fertilized crops are commonly 10-50% of fertilizer
N application rates (Böhlke 2002, Raymond et al. 2012). In contrast, P and other
contaminants such as metals and pesticides that are less mobile tend to be retained
in upland soils, with some notable exceptions such as the herbicide atrazine and its
derivatives (Unterreiner and Kehew 2005, Bexfield 2008, Saad 2008). However,
P binding by soils eventually becomes saturated with high rates of application,
increasing the mobility of P in soils and potentially leading to its export into surface
waters (Domagalski and Johnson 2011, Kleinman et al. 2011, Sharpley et al. 2013).
The KBS Main Cropping System Experiment (MCSE; Table 11.2; Robertson
and Hamilton 2015, Chapter 1 in this volume) provides an opportunity to investi-
gate how conventional and alternative management of row crops affects the quality
of water percolating through soils in comparison with unmanaged (nonagricul-
tural) vegetation at various stages of ecological succession. Such investigations
commonly use tension samplers to collect soil water samples (using a vacuum)
