Managing Soil Carbon for Multiple Benefits – Positive Exemplars: North America - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 290

impacts of tillage, preserves soil resources
and can lead to the accrual of much of the
soil carbon lost during tillage (Lal, 1997;
Paul et al ., 1997; Paustian et al ., 1997) and
to net reduction in greenhouse gas emis-
sions (e.g. Dendooven et al ., 2012). A
change from conventional tillage to no-till
can sequester carbon, although results vary
as a function of soil type, climate and land-
use history (West and Post, 2002). The accu-
mulation of SOC will continue (provided
the soil is not tilled), and SOC levels can be
expected to peak after 5-10 years, with SOC
reaching a new equilibrium in 15-20 years.
Conservation tillage is one of the largest po-
tential sources of greenhouse gas mitigation
within the agricultural sector (Smith et al .,
2008) and, coupled with associated de-
clines in fuel use, could make an immedi-
ate, substantial contribution to offsetting
and reducing greenhouse gas emissions
(Caldeira et al ., 2004; CAST, 2004).
No-tillage and minimum-tillage sys-
tems provide opportunities for increasing
soil water by trapping snow and reducing
evaporative losses during the early part of
the growing season; crop yields are signifi-
cantly higher for field peas, flax and spring
wheat (in stubble) in conservation tillage
systems (most likely due to increased soil
water) (Lafond et al ., 1992). Based on these
results and previous studies (e.g. Gupta and
Larson, 1979), conservation tillage could in-
crease soil water to the point where fallow
cropping could be eliminated without in-
creasing production (Lafond et al ., 1992).
Tillage experiments in several semi-arid re-
gions demonstrate consistently higher soil
water contents under no-tillage (Scopel et al .,
2005; De Vita et al ., 2007) and increased
rain-use efficiency (Kronen, 1994; Rock-
ström et al ., 2009).
No-tillage management increases resi-
due cover and decreases evaporative losses
from the soil profile, thus enhancing mois-
ture availability and reducing water stress
for plant production (Holland, 2004). Till-
age management can also reduce energy use
associated with irrigation. However, the
adoption of no-till can increase emissions
of N 2 O, particularly in the short term (Six
et al ., 2004). Reducing tillage intensity has

been found to minimize CO 2 losses from de-
composition in drained histosols due to less
aeration and drier conditions at the soil sur-
face in the absence of intensive tillage
(i.e. switch ploughing).
Conservation tillage systems were ori-
ginally developed to address problems of
water quality, soil erosion and agricultural
sustainability (Woodfine, 2009). Conserva-
tion tillage reduces evaporation from the
surface, increases infiltration and shades the
soil, decreasing soil temperature - all of
which alter the water balance. In semi-arid
environments, no-tillage can lead to en-
hanced water balance, enabling increased
cropping intensity and greater return per
unit land area (Peterson and Westfall, 2004).
The impacts of no-tillage on yields tend to be
greatest for the driest sites (Cantero-Martinez
et al ., 2007). By making more efficient use of
existing precipitation, conservation tillage
builds carbon stocks, pays dividends in
terms of increased yields and ensures
greater yields during dry years by conserv-
ing soil moisture (Wang et al ., 2006).

Cover crops, green manuring,
catch crops

Cover cropping, green manuring and catch
crops (crops that are not planted for harvest
but to take up and retain nitrogen) all in-
crease carbon inputs to the soil by extend-
ing the time over which plants are fixing
atmospheric CO 2 . Green manures and catch
crops have the benefit that they enhance
system nitrogen balance, which further in-
creases productivity. Growing cover crops
enhances soil protection and groundwater
quality, controls pests and increases C
stocks by enhancing carbon inputs when
the ground would otherwise lay fallow
(Bowman et al ., 1999; Govaerts et al ., 2009).
Using green manures can simultaneously
build soil C and nitrogen stocks (Vanden-
Bygaart et al ., 2003), enhancing soil fertility
and sequestering carbon in the soil, but
likely increasing N 2 O emissions. Use of
catch crops also tends to sequester carbon
in the soil (Friebauer et al ., 2004; Christensen
et al ., 2009).