Agriculture Reference
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does not tend to precipitate from solution in wetlands or other surface waters. The
study found that the relative importance of groundwater varied from near zero to
near 100% of the water supplied to the wetlands, and that this explained much of
the variation in plant species composition across wetland sites. Statistical analyses
revealed seven wetland plant communities across the 24 wetlands: leatherleaf bogs,
bogs, a poor fen, fens, sedge meadow fens, wet swamps, and dry swamps. The envi-
ronmental variables most strongly related to these community classes were water
pH, fraction of groundwater, soil nutrient availability, hydrochemical variables that
covaried with pH and fraction of groundwater, and shading by tall woody species.
Water levels were of secondary importance.
Biogeochemical processes in aquatic sediments of wetlands and other shallow
waters have been studied in detail in connection with the KBS LTER as well as in
earlier work focused mainly on lakes (see below). Whitmire and Hamilton (2008)
compared rates of anaerobic metabolism (denitrification, iron reduction, sulfate reduc-
tion, and methanogenesis) in three groundwater- and three precipitation-fed wetlands.
Denitrification was not measurable in these wetlands, all of which had low NO
3
−
con-
centrations. Iron reduction was measurable mainly in precipitation-fed wetlands, while
sulfate reduction was only measurable in the groundwater-fed wetlands (groundwater
supplies SO
4
2−
; see Fig. 11.9). Methanogenesis was measurable in all wetlands, with no
differences between wetlands with contrasting water sources. In terms of total carbon
mineralization by anaerobic metabolism, sulfate reduction and methanogenesis were
the most important processes in groundwater- and precipitation-fed sites, respectively.
Whitmire and Hamilton (2005) examined NO
3
−
removal in wetland sediments
around KBS and found that N removal was linked to S cycling in many of the study
sites. Further research in the Hamilton lab revealed that the linkage is biologically medi-
ated and consistent with NO
3
−
utilization by S-oxidizing bacteria (Burgin and Hamilton
2008, Payne et al. 2009). While NO
3
−
evidently can either be denitrified or reduced
to NH
4
+
during S oxidation, only denitrification represents a permanent loss of reac-
tive N from the ecosystem. The factors governing the relative importance of these two
reactions remain incompletely understood (Burgin and Hamilton 2007), but we can
conclude that there is enough reduced S in these environments for S-oxidizing bacteria
to play a significant role in NO
3
−
removal via a form of chemolithoautotrophic denitrifi-
cation. This stands in contrast to what has long been thought (Robertson and Groffman
2015): heterotrophic denitrification (i.e., denitrification coupled to the oxidation of
organic matter) is not the only process that can permanently remove N in fresh waters.
Lakes
Lakes are abundant and diverse in the landscape around KBS, and they are an
important ecological, aesthetic, and recreational resource. Larger, deeper lakes
have been the subject of much study because of their importance to people (Wetzel
2001). Notably, most lakes in the area are shallow (i.e., <2 m deep) and become
entirely filled with emergent and floating-leaf aquatic vegetation during the sum-
mer; therefore, they would be considered wetlands in the National Wetlands
Inventory (Cowardin et al. 1979). Most local lakes are hydraulically connected to
