Agriculture Reference
In-Depth Information
inputs. This agrees well with the estimate by Griffis et al. (2013) of 1.8% based
on tall tower measurements of atmospheric N
2
O at an agricultural landscape in
central Minnesota (USA). Millar et al. (2010, 2012) used this relationship to
develop a methodology for C markets to better incentivize N
2
O mitigation via
improved N fertilizer management.
Temporal variability severely challenges accurate estimates of annual N
2
O
fluxes from agricultural systems. As for denitrification, N
2
O emissions are often
stimulated by episodic agronomic and environmental events, including fertiliza-
tion, tillage, rainfall, and freeze-thaw cycles (e.g., Wagner-Riddle et al. 2007,
Halvorson et al. 2008). Episodic N
2
O fluxes can constitute a substantial portion
of long-term total N
2
O emissions (Parkin and Kaspar 2006), questioning the
validity of annual emission estimates based on infrequent sampling (Smith and
Dobbie 2001). Automated chamber methods where fluxes are measured several
times a day address the issue of the temporal variability, and at KBS as else-
where, such measurements reveal a substantial amount of diel and day-to-day
variation (Ambus and Robertson 1998).
Agronomic Nitrogen Balances in Annual MCSE Ecosystems
Although we do not have sufficient knowledge of all N fluxes in the MCSE to
construct complete N budgets for each system, we can construct simple agronomic
budgets (e.g., Vitousek et al. 2009) for the annual cropping systems that are infor-
mative. Estimates of N in agronomic inputs (from fertilizer additions and BNF)
less outputs (in harvest) provide a first-order measure of surplus N. The balance
(Table 9.6) is instructive: the Conventional system has an overall balance of +7 kg
N ha
−1
yr
−1
, followed by No-till (0 kg N ha
−1
yr
−1
), Reduced Input (-28 kg N ha
−1
yr
−1
), and Biologically Based (-34 kg N ha
−1
yr
−1
). The Conventional system closely
compares to the balance of +10 kg N ha
−1
yr
−1
(Table 9.6) for a generalized U.S.
Midwest (Illinois) corn-soybean rotation determined by Vitousek et al. (2009).
That the Conventional System is in near balance (only ~7% of estimated inputs
are not removed by harvest) and the No-till is in exact balance suggests conservative
N management in these systems (Table 9.6). As noted earlier, N fertilizer for corn is
applied at rates recommended by university extension based on an EONR approach.
The negative surpluses in the Reduced Input and Biologically Based systems are
striking, and likely indicate cover crop scavenging of N that would otherwise be
lost to the environment by leaching and denitrification. Alternative explanations are
that soil organic matter could be providing additional N or that BNF by the cover
crops could be underestimated. However, soil organic matter is accreting in these
systems rather than declining (Syswerda et al. 2011), and thus is a sink not a source
of N. And while rates of BNF in the Reduced Input and Biologically Based systems
are only 12-16 kg N ha
−1
yr
−1
greater than in the Conventional system, which seems
low, recall that leguminous cover crops are grown during only one of the MCSE's
three rotation phases (preceding wheat). For this phase, rates of BNF for red clover
are 31 and 45 kg N ha
−1
yr
−1
for the Reduced Input and Biologically Based systems,
respectively (Table 9.4), which is a reasonable range for red clover (Schipanski and
Drinkwater 2011) and for winter cover crops in general (Parr et al. 2011).
