Agriculture Reference
In-Depth Information
implemented, on the other hand, provide a substantial opportunity for mitigating
anthropogenic GHG contributions as well as providing other environmental ben-
efits (Robertson et al. 2008, NRC 2009, Tilman et al. 2009).
In light of the expectation for worldwide expansion and intensification of
agriculture in the coming decades, it seems crucial to pursue opportunities for
reducing the GHG contributions of agricultural crop production. Many such
opportunities are available, particularly in the areas of soil C conservation (CAST
2011) and better N management (Robertson and Vitousek 2009). Through the
strategic adoption of agronomic practices known to attenuate GHG emissions
(e.g., Millar et al. 2010), agriculture could contribute significantly to climate
change mitigation.
Long-term research such as that conducted at the KBS LTER has a particu-
larly important contribution to make in climate change mitigation because of the
variable nature and slow rate of change for many agricultural GHG fluxes. While
some emissions are sudden, such as biomass burning during land clearing, and
others are episodic but easily quantified, such as fuel used during agronomic opera-
tions, others can be difficult to reliably estimate based on short-term observations
because they change very slowly or are temporally variable. Changes in soil C
sequestration, for example, are normally too gradual to detect on an annual basis: a
change of 50 g C m
−2
(a typical annual gain after conversion to no-till management)
cannot be distinguished in 1 year against a spatially variable background pool of
5000 g C m
−2
. Long-term research provides the time necessary to document such
changes; detecting an increase of 500 g C m
−2
over 10 years is much more tractable
(Kravchenko and Robertson 2011).
Similarly, changes in N
2
O emissions are difficult to detect against a background
of high temporal variability. Nitrous oxide emissions from soils are notoriously
variable and unpredictable: fluxes can change an order of magnitude within a single
day (e.g., Ambus and Robertson 1998, Barton et al. 2008) in response to a variety
of environmental drivers. Long-term N
2
O research provides the large set of mea-
surements and hence the statistical power needed to assess differences among agro-
nomic systems and practices against an otherwise confusing backdrop of short-term
variability.
Providing a Common Basis for Systemwide Comparisons
The Concept of Global Warming Impact (GWI)
Greenhouse gases vary greatly in radiative forcing and residence time in the atmo-
sphere, so it is not enough to know that one system stores more soil C but liberates
more N
2
O than another system that oxidizes more CH
4
: a reference is needed to
appropriately weight the effect of different gases on the atmosphere's capacity to
hold heat. The Global Warming Potential (GWP; IPCC 2001) index satisfies this
need. The GWP is a combined measure of the radiative forcing of a given GHG
based on its physical capacity to absorb infrared radiation, its current concentration
in the atmosphere, and its atmospheric lifetime.
