Agriculture Reference
In-Depth Information
CO
2
than it gained until around Day 200, when net photosynthesis by the recently
planted soybeans exceeded the respiration of the herbicide-treated grasses. Once the
soybeans senesced around Day 260, respiration again dominated the system's CO
2
flux, and the cumulative NEP remained negative (i.e., net CO
2
release) until the end
of the year, by which time some 500 g CO
2
m
−2
had been emitted by the system.
Overall, during the first year of the conversion study, converted fields lost ~520
g CO
2
m
−2
, mostly from the decomposition of killed grasses and soil C oxidation.
This compares to a gain of ~300 g CO
2
m
−2
by the reference field, which sequestered
C into belowground biomass and SOM (Zenone et al. 2011, Gelfand et al. 2011).
Combining eddy covariance results with the other major sources of GWI in the
system—farming inputs and N
2
O and CH
4
fluxes, in particular—provides a measure
of net GWI analogous to other, less continuous methods. N
2
O emissions were also
substantially higher in the converted sites (Fig. 12.6B), contributing to a total GWI
or C debt of 68±7 Mg CO
2
e m
−2
(Gelfand et al. 2011). This measured C debt (from
no-till conversion of CRP fields to agricultural production) is substantial but stands
at the lower end of previously modeled estimates of 75-305 Mg CO
2
e m
−2
(Fargione
et al. 2006, Searchinger 2008). No-till continuous corn or corn-soybean rotations,
when used for grain ethanol production, could repay this C debt in 29-40 years,
which is somewhat shorter than previously estimated (Fargione et al. 2008).
Summary
Intensively managed crop production systems contribute substantially to anthro-
pogenic climate change, but changing how systems are managed could mitigate
their impact. GWI analyses provide a measure for comparing the climate benefits
and costs of different management practices and, by summation, of entire crop-
ping systems. Major components of GWI include land-use change (where appropri-
ate), farming inputs (fuel, fertilizers, pesticides), soil C change, and fluxes of the
non-CO
2
GHGs N
2
O and CH
4
. Nitrous oxide emissions represent the largest GWI
in the MCSE annual cropping systems, mainly stemming from high fertilizer inputs
but also from the cultivation of N-fixing crops. Improved N management thus rep-
resents one of the largest potentials for the mitigation of agricultural GHG emis-
sions. Soil organic C gain represents an equally large mitigation potential where
soils could be managed to sequester C via no-till management, cover crops, and the
cultivation of perennial crops. Perennial, cellulosic biofuel crops offer substantial
climate change mitigation potential so long as their production does not cause food
crops with a higher GWI to be planted elsewhere.
References
Ambus, P., and G. P. Robertson. 1998. Automated near-continuous measurement of carbon
dioxide and nitrous oxide fluxes from soil. Soil Science Society of America Journal
62:394-400.
Barker, T., I. Bashmakov, L. Bernstein, J. E. Bogner, P. R. Bosch, R. Dave, O. R. Davidson,
B. S. Fisher, S. Gupta, K. Halsnæs, G. J. Geij, S. Kahn Riveiro, S. Kobayashi,
