Managing Soil Carbon for Multiple Benefits – Positive Exemplars: North America - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 292

few quantitative studies have been carried
out to date. One study did evaluate the im-
pacts of maize harvest residues on soil carbon
stocks, and found that under mulch tillage
50% of the aboveground residues could be
removed without impacting soil carbon
stocks negatively (top 20 cm) (Sheehan
et al ., 2004). Another (Corbeels et al ., 2006)
suggested that most of the carbon seques-
tration benefits of direct-seeding mulch-
based cropping systems in Brazil were real-
ized due to increased carbon inputs from
a mulch crop. Under no-tillage, the propor-
tion of surface residues that could be re-
moved increased to nearly three-quarters.
Maintaining vegetative cover increases
water infiltration, O 2 diffusion into the soil
(limiting denitrification) and CH 4 diffusion
into the soil (promoting CH 4 consumption);
it also promotes the uptake of mineral ni-
trogen, slows nitrogen loss and maintains soil
organic nitrogen stocks, all of which limit
denitrification.
In the US Southern Great Plains, sur-
face residues slow the movement of water
across the surface, providing more time for
infiltration, evaporation is reduced with
residues due to reduced wind speeds and
temperatures at the surface, and soil water
storage at many semi-arid locations increased
with increasing amounts of crop residue
maintained on the surface (Unger et al ., 1991).
In semi-arid areas, conservation tillage (espe-
cially no-tillage) can enhance production,
but may be limited on severely degraded
soils because of low organic matter con-
tents, low soil fertility, poor physical con-
dition, low water infiltration rates and poor
plant productivity. Because crop residues
are often a valuable resource for other pur-
poses (fodder, bedding, fuel, etc.), use of res-
idues for mulching may require a trade-off
with these other uses (Giller et al ., 2009).
Also, in some cases immobilization of min-
eral nitrogen in decomposing litter may detract
from soil fertility.
Retaining mulch and other crop res-
idues enhances water storage in soil, enab-
ling adaptation to extreme events (drought,
heat stress), which are expected to become
more frequent as the climate changes. Sur-
face residues absorb radiation, keeping the

soil temperature cooler (Steiner, 1989;
Moreno et al ., 1997). They also increase sur-
face roughness, decrease wind-driven dry-
ing of the soil surface and insulate the soil
(Lampurlanes and Cantero-Martinez, 2006).
In total, surface mulch reduces crop water
requirements by 30% (FAO, 2007). Surface
mulches also absorb energy from falling
raindrops, reducing soil erosion. In situ-
ations where precipitation is lower/more er-
ratic, the physical presence of crop residues
on the soil surface protects the upper soil
layer, reducing soil temperatures and hence
water loss, both important factors for the
adaptation of plant growth as the climate
warms (Maruthi et al ., 2008).

Positive Exemplars: Reducing N 2 O
Emissions in North America

Plant growth is enhanced by adding fertil-
izers - mineral or organic. However, while
several studies have documented soil car-
bon sequestration in response to added fer-
tilizers (Alvarez, 2005), even if C inputs to
the soil increase in response to fertilization,
added fertilizers can accelerate soil C outputs
(decomposition) offsetting any C sequestra-
tion (Russell et al ., 2009). Perhaps most im-
portantly, increased N 2 O emissions are likely
to offset most C sequestration in almost all
cases (Schlesinger, 1999).
Synchronization of nitrogen supply and
nitrogen demand limits the build-up of min-
eral nitrogen (either NH + or NO - ) in the soil,
thus limiting N 2 O production via either
nitrification or denitrification. Practices that
promote the synchronization of nitrogen
supply and demand - increasing the effi-
ciency of nitrogen use - are likely to reduce
N 2 O fluxes (Mosier et al ., 2004). For ex-
ample, autumn fertilizer applications lead
to greater emission than spring applications,
because new crops or re-emerging forage
grass seedlings in the spring remove mineral
nitrogen from the soil, limiting the availabil-
ity of nitrogen for microbially mediated nitri-
fication and denitrification. Plants typically
demand the greatest amount of nitrogen
early in the season, and thus application at