Agriculture Reference
In-Depth Information
These patterns suggest the potential for management intervention to signifi-
cantly reduce NO 3 leaching from cropping systems: (1) compared to conventional
management, no-till cultivation reduced NO 3 losses by 33%; (2)  the incorpo-
ration of cover crops as a substitute for N fertilizer reduced losses by 60-70%;
and (3) substituting perennial crops (alfalfa and hybrid poplars) for annual crops
reduced losses by 80-100%. In annual crops, most NO 3 loss takes place during
periods when plants are absent—the fall and spring for those systems without cover
crops (e.g., Syswerda et  al. 2012). Management to conserve NO 3 might thus be
best focused on reducing losses during these periods.
Nitrate lost to groundwater is later discharged from seeps, springs, and drains
into streams and rivers (Mueller and Helsel 1996, Crumpton et al. 2008), where it
can be further transformed or transported to lakes, estuaries, and marine systems
(Howarth et al. 1996, Alexander et al. 2000). In the U.S. Corn Belt, NO 3 concen-
trations in ground and surface waters often exceed the 10 mg N L −1 maximum con-
taminant level for drinking water set by the U.S. Environmental Protection Agency
(Jaynes et al. 1999, Mitchell et al. 2000).
Much of the NO 3 entering wetlands and small headwater streams is likely to be
transformed to the inert N 2 form (Hamilton 2015, Chapter 11 in this volume), albeit
with some production and emission of N 2 O (Paludan and Blicher-Mathiesen 1996,
Stadmark and Leonardson 2005, Beaulieu et al. 2011). Nitrate concentrations in
headwater streams often limit denitrification (Inwood et al. 2005) and are the best
predictor of N 2 O emissions rates from streams around KBS (Beaulieu et al. 2008).
On an areal basis, these streams have higher N 2 O emission rates (on average, 35.2
μg N 2 O-N m −2 hr −1 ; Beaulieu et al. 2008) than annual crops of the MCSE (on aver-
age, 14.5 μg N 2 O-N m −2 hr −1 ; Robertson et al. 2000), though their regional contribu-
tion is small because of a much smaller areal extent.
Losses via denitrification can be extremely high at the interface where emerg-
ing groundwater enters surface water bodies (e.g., Cooper 1990, Whitmire and
Hamilton 2005) as well as along subsurface flow paths (e.g., Pinay et al. 1995). In
a headwater stream at KBS, Hedin et al. (1998) and Ostrom et al. (2002) showed
that substantial amounts of NO 3 are removed from flow paths prior to stream entry
when sufficient dissolved organic carbon (DOC) is available to support denitrifica-
tion. Subsurface N chemistry and δ 15 N natural abundance analyses suggest that a
narrow near-stream region is functionally the most important location for denitrifier
NO 3 consumption. These studies suggest that managing these areas to provide suf-
ficient DOC—for example, by planting perennial vegetation streamside—could be
an effective mitigation strategy for reducing the impact of leached NO 3 on aquatic
systems.
Denitriication
Denitrification is the stepwise reduction of soil NO 3 to the N gases NO, N 2 O,
and N 2 . Four denitrification enzymes—nitrate reductase (Nar), nitrite reductase
(Nir), nitric oxide reductase (Nor), and nitrous oxide reductase (Nos)—are usually
induced sequentially under anaerobic conditions (Tiedje 1994, Robertson 2000). A
wide variety of mostly heterotrophic bacteria can denitrify (Schmidt and Waldron
Search WWH ::




Custom Search