Agriculture Reference
In-Depth Information
how soil microbial processes influence the fate of the carbon in carbonate, dis-
solved from either native carbonate minerals or from agricultural liming materi-
als. The degree to which percolating water can dissolve carbonate minerals and
accumulate Ca
2+
and Mg
2+
is controlled by temperature and pH. The pH of soil
water is inversely proportional to dissolved free CO
2
from root and microbial res-
piration and to mineral acidity from precipitation and from internal soil processes.
Nitrification of fertilizer-derived N by soil bacteria is an important source of nitric
acid in the fertilized systems, and reaction of nitric acid with carbonate minerals
can cause their dissolution to switch from a net CO
2
sink to a source as NO
3
−
con-
centrations increase in infiltrating waters. Whether carbonate minerals dissolve in
reaction with dissolved free CO
2
or with nitric acid, either reaction yields dissolved
Ca
2+
and Mg
2+
. In glacial landscapes such as southern Michigan, carbonate mineral
dissolution is an important source of these cations when the sum of Ca
2+
and Mg
2+
concentrations exceeds ~2 meq L
−1
(Hamilton et al. 2007); dissolution of other
minerals (e.g., silicates) does not generate such high concentrations, although soils
elsewhere can contain other significant sources of these cations.
The charge equivalents of Ca
2+
, Mg
2+
, HCO
3
−
, and NO
3
−
in soil solutions yield
clues about the relative importance of carbonate mineral dissolution by reaction
with carbonic acid vs. nitric acid (Hamilton et al. 2007). Concentrations of Ca
2+
and Mg
2+
were significantly and positively correlated with NO
3
−
(
r
= 0.9 and 0.8,
respectively) across all MCSE systems (Kurzman 2006), reflecting the importance
of nitric acid reaction with carbonate minerals, as discussed above. And positive
correlations of Ca
2+
and Mg
2+
with NO
3
−
in soil water have also been observed in
other agricultural systems (Böhlke 2002). High NO
3
−
concentrations in soil water
beneath or emerging from fertilized agricultural fields are associated with unnatu-
rally high concentrations of Ca
2+
and Mg
2+
(i.e., well above the charge equivalent of
HCO
3
−
), likely as a result of the additional dissolution capacity of nitric acid (and
possibly sulfuric acid in some cases) compared with carbonic acid (Böhlke 2002,
Hamilton et al. 2007).
Groundwater Quality
Groundwater in the vicinity of KBS occurs in unconsolidated glacial deposits as an
upper unconfined aquifer, with an underlying semi-confined aquifer in some areas
(Allen et al. 1972). The depth of the unsaturated zone (i.e., from the land surface
to the water table) is generally <15 m. Groundwater surface gradients tend to fol-
low the land surface, but as noted above, the direction of groundwater low is not
always apparent from surface topography. Nonetheless, groundwater is generally
recharged in the upland areas and discharged to the lower-lying lakes, streams, and
wetlands. Lateral flow toward streams and rivers is likely to be more important in
the outwash plains, while the morainal systems are more likely to be dominated by
localized vertical flow systems (Kehew and Brewer 1992). Wetlands and lakes can
receive groundwater through-flow, in which groundwater inputs may enter on one
end and exit back to the groundwater on the other end (Kehew et al. 1998).
Groundwater flow paths can be complex in glacial deposits because of the pres-
ence of interbedded layers of coarse and fine materials, as shown by a detailed
