Managing Soil Carbon for Multiple Benefits – Positive Exemplars: North America - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 291

Enhancing inputs of nitrogen and nitro-
gen retention and minimizing soil erosion are
keys to maximizing production and ensur-
ing long-term sustainability of agricultural
systems (Greenland, 1975). Use of nitrogen-
fixing crops - green manures: nitrogen-fixing
plants that are not harvested - and non-crop
nitrogen fixers is an important component
of soil fertility. Sowing non-leguminous
cover crops between harvested crops can
enhance nitrogen inputs through fixation
and plough-under, while having a minimal
impact on crop growing season length
(Fageria, 2007). By enhancing soil fertility
and nitrogen supply to crops, green manur-
ing can have immediate impacts on the
growth of subsequent crops. Similarly, catch
crops sown following a crop take up re-
sidual nutrients in the soil; subsequently
ploughing those catch crops into the soil be-
fore the next crop increases the availability
of nutrients to subsequent crops (Hansen
et al ., 2007). Cover cropping can employ
nitrogen-fixing crops to bolster soil fertility,
but cover cropping with non-leguminous
crops also provides surface cover that min-
imizes soil erosion between crop growing
seasons.

2003; Erenstein and Laxmi, 2008; Govaerts
et al ., 2009). If production and carbon inputs
decrease with decreasing fallow frequency,
soil carbon stocks can decrease (Olsson and
Ardo, 2002).
Bare fallows can be beneficial for crops
growing in certain dryland situations,
where a period of bare fallow can increase
soil moisture storage, subsequently increas-
ing crop yields, enabling a wider variety of
crops, and reducing inter-annual yield vari-
ability. However, bare fallow periods re-
duce the period over which crops can be
grown; gross income is also reduced. New
tillage, crop rotation, fertilization and resi-
due management practices can be inte-
grated to conserve and use precipitation effi-
ciently, broadening the portion of semi-arid
areas acceptable for a given crop. Reducing
bare fallow can result in substantial in-
creases in income and food security in
semi-arid ecosystems (Peterson and West-
fall, 2004). Because bare ground is directly
exposed to intense heat, wind and rainfall,
erosion is greater for bare fallow than
cropped land; reducing bare fallow will re-
duce soil erosion.

Mulching/residue management

Reduced bare fallow frequency

Within annual croplands, much of the car-
bon in plant biomass is removed during
harvest. Although most carbon storage in
agricultural systems is in the soil, plant and
residue management are directly related to
soil carbon inputs and are important for ero-
sion prevention. Plants and plant residues
can also be used as a renewable energy
source, offsetting the use of fossil fuels.
Understanding how much crop residue
must remain on the soil surface to prevent
erosion is an important practical question.
The answer to the question will dictate
management strategies to prevent erosion
and maintain soils. An equivalent question
for soil carbon stocks is how much crop
residue carbon must be input into the soil to
maintain soil carbon stocks? Quantitative
answers to this question are very important
for understanding the impacts of harvest
and biomass removal practices, but very

Reduced carbon inputs associated with more
frequent bare summer fallow in semi-arid re-
gions reduces the level of soil organic matter
in dryland agricultural systems (Rasmussen
et al ., 1998). Bare fallows are increasingly
recognized in most agroecosystems as gener-
ally not beneficial, and indeed may be harm-
ful. Reducing the frequency of bare fallow
leads to carbon sequestration by increasing
the time over which carbon is taken up by
plants in input to the soil (VandenBygaart et al .,
2003). In semi-arid regions, alternate-year fal-
low is used to collect soil moisture; conser-
vation tillage is often required to enable the
reduction of fallow frequency (Peterson
et al ., 1996). Conservation tillage coupled
with reduced fallow frequencies have been
used successfully in a variety of semi-arid
environments to sequester carbon in soils
(Peterson et al ., 1996; VandenBygaart et al .,