Marginality Principle - Principles of Sustainable Soil Management in Agroecosystems - page 37

2.6.2 a groforeStry
One family of management practices with great potential and increasing usage as a
practice to increase SOC is agroforestry. Agroforestry practices include field wind-
breaks, silvopastures, alley cropping, forest farming, and riparian buffers. These
practices are innately resilient to climatic extremes as they routinely involve mul-
tiple perennial species that provide greater plant diversity and less vulnerability to
climate stress than is provided by monocropping and annual species. The perennial
woody vegetation also modifies the local microclimate by influencing airflow and
light interception. Understory and adjacent plant species are thus protected from
extremes in temperature and from damaging winds (Stigter 1988; Brenner 1996;
Cleugh and Hughes 2002). Deeper root systems of the perennial vegetation afford
greater resilience to drought and increased exploitation of soil water and nutrients
from soil layers not accessible to more shallow rooted annual crops. Greater nutrient
efficiency and WUE of species within agroforestry systems are a key strength and
further enhance their potential as a mitigation practice under the uncertainties of
climate change (Wallace 1996; Kho 2008).
Agroforestry practices also sequester C in soils. In a review of several Midwestern
US sites, Paul et al. (2003) found SOC changes after tree planting of -0.07 to 0.55
Mg C ha -1 year -1 beneath deciduous trees and -0.85 to 0.58 Mg C ha -1 year -1 beneath
coniferous trees. In another review, Post and Kwon (2000) reported SOC changes
following tree planting, from small decreases in cool temperate pine plantings to
increases of 3.0 Mg C ha -1 year -1 in wet subtropical plantings. Sauer et al. (2007)
estimated an SOC change of 0.11 Mg C ha -1 year -1 for the surface 15 cm beneath
a 35-year-old eastern red cedar ( Juniperus virginiana )-Scotch pine ( Pinus sylves-
tris ) field windbreak in eastern Nebraska. Hernandez-Ramirez et al. (2011) used
stable C isotope techniques on soil samples from the Sauer et al. (2007) study and
a white pine ( Pinus strobus ) planting in Iowa to determine the source of the SOC
found beneath the trees. Their source-partitioning analysis indicated that 53.9% and
47.1% of the SOC in the 7.5- and 10-cm surface layers at the Nebraska and Iowa
locations, respectively, was tree derived. When the C sequestration in above- and
below-ground tree biomass is also considered (Schroeder 1994; Kort and Turnock
1999; Schoeneberger 2009), agroforestry practices are likely to have C sequestration
potential at least equivalent to the oft-cited practice of conversion from tilled to NT
crop production (e.g., West and Post 2002).
Although agroforestry practices are inherently more resilient to climate-related
stresses, rapidly increasing global food demand is creating intense pressure to pro-
duce more food, fiber, and fuel per unit of land area. To avoid losses in food produc-
tion, agroforestry plantings must either provide a food source themselves (fruit, nut,
or other edible produce) or provide ecosystem services (C sequestration, enhanced
hydrology, or improved water quality) to compensate for increasing intensification of
production practices on adjacent cropland.
Utilization of conservation practices for the mitigation of GHG in association
with agricultural systems can reduce CO 2 , CH 4 , or N 2 O emissions. CH 4 emissions
from soil systems and conservation practices are relatively small compared to ani-
mal production systems including manure storage, while CO 2 and N 2 O dynamics