Agriculture Reference
In-Depth Information
to loss with physical disturbance; Grandy and Robertson (2006) found rapid C
loss following a single initial tillage of plots in the MCSE Mown Grassland (never
tilled) community. That C gains under biologically based management occurred
despite tillage implicates that other factors, such as crop diversity, residue quality,
or a longer annual crop duration, may lead to more persistent soil C accumulation
than under no-till. Combining these factors with no-till is an intriguing possibility
for building soil C even faster than no-till or biologically based management alone.
Research on soil C sequestration in other long-term agroecosystem experiments
has been consistent with KBS MCSE findings. Biodiverse cropping systems have
been shown to be generally associated with soil C gains if N fertilizer inputs are nil
or minimal (Drinkwater et al. 1998, Russell et al. 2009). More important than diver-
sity may be the duration of living cover. The perennial crop systems in the MCSE
have accumulated more C than have the annual crop systems. Carbon gains in the
Alfalfa system, for example, had increased by 50% 12 years after establishment
(Grandy and Robertson 2007). This may be attributed, in part, to the long growing
season for C fixation in alfalfa, which has ~70 more growing days than the 115-day
corn growing season and ~30 more days than corn interseeded with a winter cover
crop. No tillage is conducted in the Alfalfa system, and the combination of C inputs
from roots, continuous cover, and lack of disturbance has led to substantial gains
in soil C (Fig. 15.1).
Energy Efficiency
Evaluating the performance of different management systems is a challenge when
inputs and outputs vary considerably. Substantial amounts of fossil energy are
consumed in common management inputs and practices, including fertilizer, pes-
ticides, and field operations conducted by labor-saving machinery. Organic farm-
ing is often assumed to require less energy because of the absence of synthetic
chemicals and fertilizers (Pimentel et al. 2005), even though field operations can
also be energy-intensive. Forage production and conservation tillage systems are
moderately intensive types of agriculture. High economic yields tend to be associ-
ated with energy-intensive, conventional agriculture, and questions arise regarding
the associated trade-offs. Is a system with low-energy input more efficient if outputs
are also low? An assessment of energy balance for the whole cropping system is
one way to evaluate these trade-offs (Hülsbergen et al. 2001, Gelfand et al. 2010).
In the MCSE, annual farming energy inputs varied from 4.8 GJ ha
−1
in the
Biologically Based system to 7.1 GJ ha
−1
in the Conventional system (Table 15.2;
Gelfand et al. 2010). Energy inputs were generally lower than previously reported
for conventional management in long-term row crop trials in Pennsylvania (Pimentel
et al. 2005) and in Central Europe (Maeder et al. 2002). This may be a reflection of
the recommended management practices for Michigan field crop production, which
do not rely on manure amendments or high fertilization rates (Gelfand et al. 2010).
Energy outputs were evaluated in terms of food produced for direct human con-
sumption, or indirect consumption in the case of alfalfa, where energy outputs were
based on meat produced when harvested biomass is used as ruminant livestock feed
(Table 15.2). The No-till system was the most efficient grain production system,
