Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 36

crop yield since it is a function of crop water use. Hatfield and Prueger (2011) found
there was variation among soils within the same field, with a corresponding dif-
ference in yield between these two soils caused by the difference in soil water use
throughout the growing season. The role of soil management on WUE was reviewed
by Hatfield et al. (2001); they proposed that practices that increased water availability
would lead to improved WUE. In a simple exercise using the WUE relationship devel-
oped for corn in the central United States, they observed that the additional amount of
required soil water to increase the current level of yields of 10,000 to 18,000 kg ha -1
would require an additional 200 mm of soil water to be transpired through the plant
during the growing season. If we follow the relationship developed by Hudson (1994)
between soil water holding capacity and SOM, achieving sufficient change in the soil
profile to result in this amount of extra water in the soil profile will require very aggres-
sive soil management to increase soil organic levels.
This analysis can be extended further with respect to biofuel production because
the observations from Hickman et al. (2010) and VanLoocke et al. (2010) suggest that
the water use of switchgrass ( Panicum virgatum L.) and Miscanthus ( Miscanthus
giganteus ) is greater than that of corn. To achieve the projected yields for these
crops to be viable bioenergy crops will require production systems with an increased
amount of soil water availability. Increasing available soil water for transpiration
will require a combination of techniques; these include increasing the soil water
holding capacity, reducing soil water evaporation, and reducing drainage through
the soil profile. The combination of strategies that will provide mitigation strategies
and enhance adaptation approaches will require a focus on combining soil water and
SOM dynamics. The role of conservation practices to enhance marginal soils is not
as simple as characterizing their effectiveness by the degree of tillage or manage-
ment of surface residues in order to improve SOC content. Interactions among soil
water balance, soil temperature, soil biological activity, and gas exchange between
the soil and the atmosphere are complex and vary by cropping system, soil, and
climate (West and Marland 2002). These interactions prevent the development of a
standard set of responses on how management of conservation practices will reduce
climate change through mitigation. The foundation for soil management changes
is related to the change in SOC. Follett (2001) attributed these changes to tillage
and soil management systems, management to increase the amount of crop cover,
and increased efficiency in the use of inputs (N and water) by the cropping system.
Martens et al. (2005) provided a review on the role of soil management practices
on the mitigation of greenhouse gas (GHG) emissions, and one of their conclusions
was the observation that agricultural impacts on C cycling in soil needed improved
understanding before the full potential of soil management practices as C mitigation
strategies could be quantified. West and Marland (2002) stated that changing tillage
practices could potentially cause a reduction in C emissions from agriculture of 368
kg C ha -1 year -1 , with variation among cropping systems and climates. Franzluebbers
(2005, 2010) concluded that reductions in tillage intensity would increase the SOC
content; however, the effects of changing tillage on other GHG emissions were less
well defined. These studies direct us toward the conclusion that the role of soil man-
agement practices on increasing C storage in the soil profile will be understood only
if we can quantify the effect on soil processes by altering soil management practices.