Researcher have hypothesized that soils could be used
to sequester additional carbon and prompted researchers to investigate soil
carbon storage efficiencies and to evaluate if there is an upper limit to a
soil’s carbon stabilization capacity. This has typically been done by applying
differing C-inputs to field plots and then measuring C-stocks in the soil. The
results in many cases have shown a linear increase in soil carbon with
increasing carbon inputs (Huggins et al., 1998b; Kong et al., 2005; Paustian et
al., 1997); however, in some long-term agroecosystem experiments little to no
change in soil carbon stocks has been detected with changing carbon input
levels (Reicosky et al., 2002; Huggins et al., 1998a; Huggins and Fuchs, 1997;
Huggins et al., 1998). These investigations have lead researchers to propose
soil carbon saturation theory (Six et al., 2002; Stewart et al., 2008; Gulde et
al., 2008).
As proposed by Stewart et al. (2007), soil carbon
saturation is a soil’s unique limit to stabilize carbon, in other words the
maximum amount of organic carbon the soil can accumulate. This concept implies
that even with increasing levels of organic matter inputs, the amount of
organic matter within the soil would not accumulate. In addition to studying
soil organic C stocks under various carbon loading rates researchers have also
intensively investigated the dynamics of specific pools in relation to
saturation theory (Six et al., 2002; Stewart et al., 2008; Gulde et al., 2008).
Three main mechanisms, chemical stabilization, physical protection, and
biochemical stabilization (Christensen, 1996; Stevenson, 1994, Six et al.,
2002; Sollins et al., 1996; Baldock and Skjemstad, 2000), of carbon
stabilization have been proposed. Chemical stabilization refers to
intermolecular interactions between organic and inorganic substances
(Guggenberger and Kaiser, 2003), physical protection to the accumulation of
organic matter due to physical barriers or exclusion of microbes and their
enzymes from the organic matter (Jastrow et al., 1996; Six et al., 2004), and biological recalcitrance to preservation
of the organic matter due to structures inherently stable against biological
attack (Krull et al., 2003; Poirier et al., 2003). This theory is especially
significant for interpreting the results of experiments regarding soil organic
matter accumulation as a result of manure application.
When compared to commercial synthetic fertilizers
manure nutrient content is relatively dilute. Thus, to achieve a desired
nutrient mass application a greater amount of mass of manure than mineral
fertilizer needs to be applied. As an example, in Iowa approximately 112-168 kg
N/ha (100 to 150 lbs. N/acre) is recommended for corn after soybeans. If our
fertilizer source is anhydrous ammonia this translates to an application rate
of 136-204 kg anhydrous ammonia per hectare. If manure from a swine facility
using concrete storage structures is used to meet nitrogen requirements then an
application rate of 16,000-24,000 kg manure per hectare are required (based on
average nitrogen content of 58.1 lb N/1000 gallons from Lorimor and Kohl,
1997). At these application rates approximately 1100 and 1650 kg/ha of solids
will be applied, of which between half to three quarters (550 – 825 kg/ha)
would be organic in nature.
In terms of soil formation and developmen, the
application of this organic matter with the manure is most closely associated
with the vegetation component. By applying manure, we are adding to the amount
of organic residue the soil receives and also adjusting the array and quantity
of specific organic compounds that are processed by the soil microorganisms. In
general, the amount of land applied organic residue is small in comparison to
the amount of residue returned to the soil with a typical corn crop (roughly
18,000 kg/ha of above ground biomass) when applied at an agronomic rate, and
yet reports of manures impacts on soil tilth and organic matter levels persist
(Nowak et al., 2002). It is possible for small increases in carbon inputs to
cause large increases in soil organic carbon levels (see figure 2 diagramming
Stewart et al.’s model of soil carbon dynamics); however, this generally
requires that the mineral associated pool, i.e., the physio-chemically
protected pool to not be saturated. Although work in this area is far from
comprehensive, it generally appears that this pool is saturated in most
agricultural systems (see Hassink., 1997; Six et al., 2002).
Figure 2. Conceptual model the relationship between
annual carbon inputs and soil organic carbon content (based on Stewart et al.,
2007)
Despite the relatively low levels of organic matter
addition, manures may have the ability to improve soil aggregation, aggregate
stability and tilth. The work of Celik et al. (2004) showed that the mean
weighted diameter of water –stable aggregates was 65% greater for manure and
compost amended soils than in soils that received no organic amendment.
Similarly, Wortmann and Shapiro (2008) found that large aggregates were
increased by 200% or more by both manure and compost application within 15 days
after application, with the effect persisting for seven months. In their study Wortmann and Shapiro (2008)
used Bray extractable phosphorus levels to track the new inputs of compost and
manure. Using this technique, they noted that the manure and compost generally
served to consolidate smaller aggregates into macro-aggregates and that this
occurred to a greater extent in the compost amended soil than in the manure
amended soil. This indicates that the hierarchical storage structure proposed
by Six et al. (2002), who suggested that organic matter would first accumulate
in the physiochemical pools and then in aggregate protected fraction, is
correct.
This
hierarchical storage also supports the theory the layering model for the growth
of organic matter in soil of Sollins et al. (2009). In their conceptual model
Sollins et al. (2009) suggest that the innermost layer is protein rich as
proteins can for exceptionally strong bonds with mineral surfaces (Kleber et
al., 2007). Organic molecules can then interact with these surface coatings to
bind the particles together as aggregates. One argument working in favor of
this hypothesis is that the application of manure or compost is known to
increase microbial activity (Spiehs et al., 2010). These microbes produce
binding agents that anchor the cells and often coat them with enzymes. The
remaining organic matter from the manure or compost can then interact with
these enzymes and cement the soil particles together. Using this theory,
aggregates can be formed quickly if the surfaces of particles are conditions to
bind to the organic matter, would be relatively water stable as it is held
together by organic matter, but effects would break down as the organic
materials mineralize.
Extending
this theory, we’d hypothesize that this would imply that compost application
should have greater and longer lasting impacts than fresh manure at an equal
carbon loading as the compost would be more stabilized against microbial
brake-down than the fresh manure. This additional stability of aggregates in
compost amended soils was noted by Wortmann and Shapiro (2008) and provides
support for short term improvements in soil tilth and structure from manure
application argument. Additionally, we’d expect that tillage would reduce or
eliminate these impacts as it allows oxidation of the applied organic matter
and that including a cover crop in the rotation would further enhance aggregate
stabilization. Both of these practices interaction with manure application were
tested by Spiehs et al. (2010), although their survey of hydraulic properties
was limited, they did suggest that the benefits of manure application were
enhanced in no-till and cover cropping systems.
Overall,
these results paint a picture that manure, when managed correctly, can be a
beneficial fertilizer that not only supplies nutrients needed to support crop
production, but also can be part of a system to improve soil tilth, health, and
hydraulic properties.
