Agriculture Reference
In-Depth Information
have been depleted to a lesser degree than
in croplands (Ogle et al ., 2004), and in
some regions biomass has increased due to
the suppression of disturbance and subse-
quent woody encroachment. Woody en-
croachment is potentially a significant sink
for atmospheric CO 2 , but the magnitude of the
sink is not well quantified (Houghton et al .,
1999; Pacala et al ., 2001). Disturbance-
induced increases in decomposition rates
of aboveground litter and harvest removal
of some ( 30- 50% of forage in grazing sys-
tems, 40- 50% in grain crops) or almost
all (e.g. maize for silage) of the aboveground
biomass have altered carbon cycling
drastically within agricultural lands, and
thus the sources and sinks of CO 2 to the
atmosphere.
Much of the carbon lost from agricul-
tural soil and biomass pools can be recovered
with changes in management practices that
increase carbon inputs, stabilize carbon
within the system or reduce carbon losses,
while still maintaining outputs of food,
fibre and forage (Eagle and Olander, 2012).
Increased production, increased residue C
inputs to the soil and increased organic
matter additions have reversed historic soil
C losses in long-term experimental plots
(e.g. Buyanovsky and Wagner, 1998). How-
ever, the management practices that pro-
mote soil carbon sequestration would need
to be maintained over time to avoid subse-
quent losses of sequestered carbon. Across
Canada and the USA, mineral soils have
been sequestering 2.5 and 17.0 ± 0.45 mil-
lion t of carbon (Mt C) per year (Smith et al .,
1997, 2001; Ogle et al ., 2003), respectively,
largely through increased production and
improved management practices on annual
cropland. Conversion of agricultural land to
grassland, like under the Conservation Re-
serve Program in the USA (7.6-11.5 Mt C
year - 1 on 12.5 million ha (Mha) of land),
and afforestation have also sequestered car-
bon in agricultural and grazing lands (Follett
et al ., 2001).
Increasing carbon stocks is accompan-
ied by increased soil fertility, enhanced
soil water balance, increased production
efficiency and reduced reliance on external
inputs. These factors tend to enhance the
resilience of production and yields in the
face of climate variability (Post et al .,
2012). Practices that build resilience to
current climate variability are also ex-
pected to ameliorate some of the effects of
the forecasted increase in future extreme
weather events (Easterling et al ., 2007;
Goklany, 2007), thus enhancing carbon
stocks and the rehabilitation of degraded
lands. In addition, the implementation of
sustainable land management practices
can foster adaptation to a changing climate
(Lal, 2009).
This chapter documents agronomic
practices in North America capable of enhan-
cing production, yield and income, reducing
greenhouse gas emissions and mitigating
greenhouse gas emissions from other sec-
tors, and enhancing resilience in the face of
inter-annual climate variability, and thus re-
silience to climate change ( Table 24.1 ). It is
possible to advance agricultural develop-
ment agendas by focusing on a single aspect
of agriculture (e.g. yield). However, expand-
ing the role of land management for green-
house gas mitigation presents us with an
opportunity to increase investments in land
management practices that will rebuild re-
silience - increasing current yields while
enhancing the ability to meet future needs.
Positive Exemplars: Soil Carbon
Storage in North America
Conservation tillage and no-tillage
Tillage has been used ubiquitously in agri-
culture to prepare the seedbed, to incorpor-
ate fertilizer, manure and residues into the
soil, to relieve compaction and to control
weeds (Phillips et al ., 1980; Leij et al .,
2002). However, tilling the soil is disruptive
and can promote soil erosion, high rates of
soil moisture loss, degradation of soil struc-
ture and depletion of soil nutrients and C
stocks. Following long-term tillage, soil C
stocks can be reduced by as much as 20- 50%
(Haas et al ., 1957; Davidson and Ackerman,
1993; Murty et al ., 2002; Ogle et al ., 2003).
Conservation tillage reduces the negative
 
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