Opportunities for Increasing Soil C Sequestration

A number of agricultural land management practices have shown potential for increasing C contentin agricultural soils (Desjardins et al., 2001b). Table I summarizes some of the results reported in the literature. These data are either based on measured or modeled values. As shown by Smith et al. (2001), a range of values are associated with most management practices. This range depends on soil texture, soil taxonomy, climatic conditions, and many other factors. The practices in Table I are shown independently, however, some practices are often adopted in unison and may or may not have an additive effect. For example, a reduction in bare fallow is usually accompanied by greater use of fertilizers and sometimes the adoption of conservation tillage. McConkey et al. (1999) discuss how several interacting effects can be calculated for cropping in various soil zones on the Canadian Prairies. There are probably several other practices that have the potential to sequester C in agricultural soils but their contribution to the net GHG budget still remains to be clarified. For example, improvements in manure and nutrient application could lead to increased C sequestration. Moldboard plowing in combination with the application of liquid dairy manure has been shown to increase soil C at a depth of 20-40 cm (Chantigny et al., 2001).

We will focus on some of the more promising agricultural practices that may be used to increase soil C sequestration.

3.1. CONVERTING CROPLAND TO GRASSLAND

Converting cropland into perennial forage production may result in a substantial increase in soil C sequestration. Conant et al. (2001), in a comprehensive literature review, reported rates of soil C sequestration of 1.01 Mg C ha-1 yr-1 for conversion from cultivation to grassland. This value is slightly larger than the value of 0.62 Mg C ha-1 yr-1 predicted by the CENTURY model for Canada (Smith et al., 2001). Conversion of cropland to forage production does not always lead to increased soil C. For example, Campbell et al. (2000) observed very little increase in soil C when cultivated land was cropped to crested wheat grass for 10 yr in Southwestern Saskatchewan. They ascribed this somewhat surprising result to poor weather conditions for growing grasses during the experimental period.

TABLE I

Amount of C sequestered in agricultural soils for various management practices

Practice

Amount C sequestered (Range) (Mg C/ha/yr)

Region

Investigators

Nutrient additions via

0.30

42 data points

Conant et al., 2001

fertilizer

0.05-0.15

U.S.A.

Lal et al., 1998

0.14-0.18

U.S.A.

Halvorson et al., 1999

0.16

Lee and Dodson, 1996

0.04

Canada

Smith et al., 2001

Nutrient additions via

0.2-0.5

U.S.A.

Follett, 2001

manure

Converting cultivated lands

1.01

23 data points

Conant et al., 2001

to grasslands

0.62

Canada

Smith et al., 2001

Including more forages

0.75

6 data points

Conant et al., 2001

(especially legumes) in

rotations

0.44

Canada

Smith et al., 2001

Adopting conservation

0.06

Canada

Smith et al., 2001

tillage

0.3-0.6

U.S.A.

Follett and McConkey, 2000

0.1-0.5

U.S.A.

Lal, 2001

Conversion from

0.13

Canada

Smith et al., 2001

conventional till to no-till

0.20-0.35

Canada

McConkey et al., 1999

0.14-0.56

U.S.A.

Lal, 1997

0.5-0.8

U.S.A.

Lal et al., 1998

0.17-0.56

Dev. Countries

Lal, 1997

0.12-0.29

Asia

Lal, 1997

0.09-0.29

Africa

Lal, 1997

0.12-0.29

L. America

Lal, 1997

Reduction of summer fallow

a0.123-0.185

Canada

Campbell et al., 2001a

b0.07

Canada

Campbell et al., 2001a

0.03-0.14

Canada

Smith et al., 2001

0.02-0.04

Canada

McConkey et al., 1999

0.05-0.4

U.S.A.

Lal, 2001

Improved grassland

0.5-0.6

Australia

Gifford et al., 1992

management

0.59

Canada/U.S.A.

Conant et al., 2001

0.28

Australia

Conant et al., 2001

0.05-0.3

U.S.A.

Lal, 2001

aChange from 50% fallow to continuous cropping. bChange from fallow-wheat to fallow-wheat-wheat.

3.2. REDUCED TILLAGE

No-till management is considered to be one of the most efficient practices for sequestering C in cropland. Currently, the rate of 0.2 Mg C ha-1 yr-1 has been suggested for use across Canada (Bruce et al., 1998). However, the net effect of tillage on C sequestration is complex. The increase in soil moisture associated with no-till often results in higher crop yield, especially in arid or semiarid environments; but this may also lead to more rapid soil C decomposition. The lower temperatures and more limited soil aeration at higher soil moisture may lead to less soil decomposition. Hence, it is no surprise that there are conflicting observations with respect to the impact of this practice on soil C, depending on where the measurements are carried out. No-till also has benefits such as minimizing C loss associated with soil erosion. In addition, there is a reduction in fossil fuel emissions because of reduced machinery and tractor use. Greater conservation of soil moisture by the surface residue layer under conservation tillage may also facilitate continuous cropping in semi-arid environments thereby resulting in less summer fallowing in certain regions and increased crop production and C inputs to soil.

Conversion of conventional tillage to no-till will increase SOC if it: (a) reduces the rate of SOC decomposition; or (b) increases yield and thus C inputs. The latter is often observed in some areas of the prairies but less often in the more humid regions of the country. Under arid or semiarid conditions, the yield increase from no-till is more a function of greater water conservation and the latter benefit may either not be significant in more humid regions or may even be a detriment on heavy textured soils. No-till may also increase SOC at a given site by preventing erosion, though this does not represent a net removal of C from the atmosphere. The results of Campbell et al. (2001a) indicate that without adequate fertilization the adoption of no-tillage will not necessarily increase soil C.

Conservation tillage appears to be more prevalent in North America than in Europe (Smith et al., 2000). In European countries, where no-tillage would yield great benefits in terms of water conservation (eg. in Spain), reduced tillage accounts for less than 5% of total land cultivated in 1995 (Costa, 1997).

3.3. REDUCTION OF BARE FALLOW

Greater cropping intensity by reducing the frequency of bare fallow in crop rotations will increase crop production and thus increase C inputs to soil and increase SOC (Campbell et al., 2001b). This will also increase water use, keeping soils dryer longer and thus reduce soil decomposition. On the Canadian Prairies, the area under summer fallow has decreased from 12 to 5 million ha in recent years and is expected to decrease to 3 million ha in 10-20 yr (Dumanski et al., 1998). It has been estimated that reduction of summer fallow by 1.8 million ha in the Canadian prairies would remove about 1.5 Tg CO2/yr by 2010 from the atmosphere (Desjardins et al., 2001a). This reduction in summer fallow has many implications on the GHG budget.

In Canada, conversion from summer fallow to cropped land takes place primarily on the Canadian Prairies. This conversion will result in increased soil C if it: (a) reduces the rate of SOC decomposition; (b) increases crop production and thus C inputs; (c) reduces erosion (though this is not truly C sequestration). The rate of increase in SOC due to this change is a function of climatic conditions. That is, if weather is consistently favorable, production (C inputs) will be greater than if weather is consistently unfavorable and hence, SOC gains will be greater in favorable weather situations.

3.4. INTRODUCING FORAGE IN CROP ROTATION

Most studies have shown a consistent positive contribution of grasses to soil carbon sequestration. Perennial grasses or legumes in rotation, high yielding varieties and soil management practices that permit the return of large amounts of crop residues to the soil can potentially increase SOC, thus increasing the likelihood for sequestering atmospheric CO2. Use of legumes in crop rotations can also appreciably reduce the requirements for N fertilizers for various cropping systems, thereby reducing net fossil fuel requirements and the C cost of manufacturing N fertilizers. Forage crops are, however, largely relied upon for food for animals. Hence, some of the benefits of C sequestration may be offset by increased CH4 emissions from livestock (Desjardins et al., 2001a). Further, forage crops grown in rotation with cereals and oilseeds in the semiarid prairies of Canada often negatively influence yields of the cereals thus resulting in a reduction in C inputs (Campbell et al., 1990). Thus such systems are most advantageous in subhumid regions such as the Black Chernozems and Gray and Dark Gray Luvisolic soils of western Canada.

3.5. NUTRIENT ADDITIONS VIA FERTILIZERS

Many agricultural ecosystems are nitrogen-limited. Adding N fertilizer usually results in increased crop production (i.e., C inputs) and may therefore increase C sequestration in soils. As expected, the addition of nutrients has been shown to increase carbon content in soil (Table I). In considering the net effect on the GHG budget we must take into account the fact that nutrient additions via fertilizer can lead to higher N2O emissions and may also tend to reduce the CH4 uptake by soils (Mosier et al., 1997). Further, there is about 1 kg C emitted in the manufacture and transportation of 1 kg N fertilizer that must also be accounted for in any C balance (Janzen et al., 1999). Still, in a long-term experiment conducted in a Black Chernozem in Saskatchewan, Campbell et al. (2001a) recorded significant increases in C sequestration even after allowing for the latter expenditures in C from fertilizer N manufacture and transportation.

When assessing the impact of management on GHG, there is often a tendency to ignore the impact of the various mitigating practices on the emissions of the other

GHG. For example, trace gas fluxes of CH4 and N2O may change the mitigation potentials of a land management option and hence, all GHGs should be considered when estimating the net C sequestration potential of a management practice. This is especially important when one considers the relative impact of these 3 gases in terms of their global warming potential.

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