Agriculture Reference
In-Depth Information
produce different effects on GHG fluxes, it is especially important to consider them
together, that is, to take a systems approach toward their understanding and man-
agement (Robertson 2014).
In this chapter, we describe an ecosystems approach to documenting changes
in GHG fluxes in intensive row-crop agriculture. We draw, in particular, on results
from the Kellogg Biological Station Long-Term Ecological Research site (KBS
LTER), where GHG fluxes have been studied in the Main Cropping System
Experiment (MCSE; Table 12.1; Robertson and Hamilton 2015, Chapter 1 in this
volume) since 1989. We discuss the value of long-term comparisons of different
cropping systems in determining the potential for management practices to con-
tribute to or mitigate GHG fluxes. We end with consideration of the GHG implica-
tions of crop production not only for grain but also for cellulosic biomass, which
is anticipated to become increasingly important in a future that includes cellulosic
biofuels.
Row-Crop Agriculture and GHG Mitigation
Historically, agricultural impacts on atmospheric chemistry have been domi-
nated by land-use change. Since the late eighteenth century, conversion of
forests and grasslands to cropland has resulted in emissions of CO
2
to the
atmosphere on the order of 130 to 170 Pg C (Wilson 1978, Sauerbeck 2001),
mostly due to immediate biomass burning and subsequent soil carbon (C) oxi-
dation. Global CO
2
emissions from deforestation today amount to ~1.5 Pg C
yr
−1
(Canadell et al. 2007).
In few established croplands today are GHG emissions dominated by soil C
oxidation. Rather, emissions now are dominated by CO
2
from fossil fuel combus-
tion during farm operations; CO
2
produced during the manufacture and transport of
fertilizers, pesticides, and other agricultural inputs; N
2
O emitted when nitrogen (N)
fertilizers are applied to soil; and CH
4
emitted during flooded conditions in lowland
rice. In most of the world's established agricultural soils (except drained wetlands),
soil C is either stable or, if managed appropriately, increasing, though this trend
could be reversed by a warming climate (Senthilkumar et al. 2009; Paul et al. 2015,
Chapter 5 in this volume).
The need for mitigation of agricultural GHG emissions becomes especially
important in light of the agricultural intensification yet required to feed an increas-
ing and more affluent world population (Tilman et al. 2011, Mueller et al. 2012).
Although intensification to date has improved yields on existing farmland and
thereby fed more people at a lower per-capita GHG cost (i.e., at a lower GHG cost
per unit yield) (Burney et al. 2010), the efficiency gained has not been sufficient to
halt the increase in GHG emissions from agriculture.
Growing demands for biofuel feedstocks could further increase agriculture's
GHG footprint: over the next several decades, millions of hectares will likely
be converted to biofuel cropping systems that will consume fuel and fertilizer
and could—if not carefully managed—exacerbate rather than alleviate atmo-
spheric GHG loading (Melillo et al. 2009). Bioenergy cropping systems correctly
