Agriculture Reference
In-Depth Information
groundwater emerges at the surface, it loses dissolved CO
2
by diffusion to the atmo-
sphere. This, as well as warmer summertime temperatures, often causes calcium
carbonate to precipitate on underwater surfaces and sediments. Carbonate precipi-
tates are frequently visible in local springs and streams as well as in wetlands and
lakes where groundwater inflow rates are high. The influence of carbonate precipi-
tation on streamwater concentrations of Ca
2+
and acid neutralizing capacity (car-
bonate alkalinity) is small unless the water resides in a lake or reservoir along the
stream network (Szramek and Walter 2004, Reid and Hamilton 2007, Baas 2009).
Most local streams receive considerable groundwater inputs as their valleys cut
downward through the glacial outwash plains, with groundwater entering via dif-
fuse seepage and occasional discrete springs along their channels. Stream riparian
zones with high rates of groundwater discharge are known to be hotspots of bio-
geochemical activity due to high biological productivity and the constant delivery
of reactive solutes by groundwater flow (McClain et al. 2003). Hedin et al. (1998)
analyzed biogeochemical processes at the soil-stream interface in a small tribu-
tary of Augusta Creek (Fig. 11.1) to show the changing importance of aerobic and
anaerobic microbial processes across a gradient of decreasing oxygen availability.
Denitrification, sulfate reduction, and methanogenesis occurred within the anaero-
bic zones.
The Lotic Intersite Nitrogen Experiment (LINX) investigated N cycling in head-
water streams across the United States, including streams in the vicinity of KBS.
This experiment used a coordinated set of whole-stream stable isotope additions to
reveal how streams act as N processors (Peterson et al. 2001; Webster et al. 2003;
Mulholland et al. 2008, 2009; Hall et al. 2009). The first set of LINX studies exam-
ined N cycling through food webs and included a stable isotope addition of
15
N in
NH
4
+
for 6 weeks. Using
15
N as a tracer enables the flow of N to be traced through
the stream ecosystem. This is because the minute amount of
15
N added has little
effect on N availability and
15
N behaves the same in the N cycle as does
14
N, the
much more abundant isotope in nature. Results for Eagle Creek, a second-order
tributary of the Kalamazoo River southeast of KBS (Fig. 11.1), showed a rapid
turnover of dissolved NH
4
+
and NO
3
−
in spite of relatively stable concentrations.
The distribution of the
15
N tracer in stream organic matter and organisms revealed
the importance of assimilative uptake of N by heterotrophic bacteria and fungi as
well as benthic algae (algae dwelling on submersed stream surfaces) for N uptake
into food webs (Hamilton et al. 2001, 2004; Raikow and Hamilton 2001).
A second set of LINX experiments focused on NO
3
−
dynamics in headwater
streams across the United States and included whole-stream
15
N tracer additions in
three predominant land-cover types: forest or other natural vegetation, agriculture,
and urban/suburban. The Rabbit River, a tributary of the Kalamazoo River (Fig. 11.1),
was included in the study. A survey of streams in the Rabbit River watershed
showed the highest NO
3
−
concentrations in watersheds with the most agriculture
(Arango and Tank 2008). And nationally, streams in agricultural watersheds tended
to have the highest NO
3
−
concentrations, followed by urban/suburban ones. Among
the 72 experiments conducted across eight biomes, there were positive relationships
between NO
3
−
concentrations and the rates of biotic N uptake and of denitrification.
However, at high NO
3
−
loading, stream N removal did not increase proportionately
