Agriculture Reference
In-Depth Information
of biological N fixation. Nitrous oxide production, on the other hand, was as high
from the Alfalfa system as from any of the annual cropping systems (Gelfand and
Robertson 2015, Chapter 12 in this volume, Millar and Robertson 2015, Chapter
9 in this volume). Randall and colleagues proposed alfalfa plantings as a means to
reduce tile drainage nitrate pollution across the U.S. Midwest. A review by Ledgard
(2001) found that grazed grassland-legume mixed systems, with minimal fertilizer
inputs, were associated with modest N losses from denitrification (6 kg N ha
−1
) and
leaching (23 kg N ha
−1
).
Generally, perennial crops are confined to marginal farming areas with steep ter-
rain, variable topography, or shallow, infertile soils. In the Midwest—and through-
out temperate agricultural regions—alfalfa is the most important perennial crop.
In southwest Michigan alfalfa has been grown on ~15% of agricultural land since
the 1930s (Sylvester and Gutman 2008). This is presumably due to the species'
high-quality residues and high productivity, together with access to ready markets
provided by the state's dairy industry. Alfalfa is adapted to a broad range of envi-
ronments, and there are varieties that can be grown under intensive irrigated and
fertilized management.
Perennial vegetation is currently the fastest means for capturing C and reducing
farm nitrate losses on a significant scale, and it thus could profoundly improve the
delivery of ecosystem services generated from agriculture. Perennial crops grown
for cellulosic biofuel on lands now not suited for food crops offer a major opportu-
nity for agriculture to contribute to climate stabilization, soil and N conservation,
pest suppression, and other ecosystem services including societal benefits such as
national fuel security (Robertson et al. 2008, 2011). Planting perennial forages on
land now used for food crops would also provide benefits, but promoting this for
broad adaption would require radical changes in agricultural policies and marketing
systems.
Cover crop integration is a small step in the direction of perennializing grain
crops, and cover crop use could be promoted with existing agricultural policy
instruments. An example of wide-scale cover crop adoption is available from the
south of Sweden where row-crop farmers were paid to grow cover crops, resulting
in a reduction of nitrate leaching (Kirchmann et al. 2002). Another way forward
may be to develop grains such as wheat or sorghum that have a perennial life cycle
(DeHaan et al. 2005, Glover et al. 2010). Both of these approaches to perennializa-
tion deserve support through agricultural policies that promote cover crop integra-
tion for incremental improvements in existing systems and research to develop a
portfolio of perennial crops that could help to further diversify rural landscapes.
Summary
Row crops can be managed to deliver ecosystem services in addition to yield. KBS
LTER results illustrate the delivery of services by current farming systems as well
as the principles that can be used to design future systems to enhance the delivery
of these and other services. Documented services other than yield include (1) soil
C accretion with its positive effects on soil fertility, water-holding capacity, and
