Agriculture Reference
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Soil Organic C (g kg -1 )
Figure  5.4 . Relationship between SOC and soil aggregate size (mean weight-diameter;
mm) in the MCSE. From A.S. Grandy (unpublished data).
(Sollins et  al. 1999). These methods have shown that the greatest C sequestra-
tion potential lies in the mineral-associated fractions and in fine, intraaggregate
particulate C. For example, Grandy and Robertson (2007) found that 82% of the
C increase in the 2000- to 8000-µm size class of the No-till system was due to
mineral-associated SOM. Incubation-based methods for separating fractions into
active, slow, and resistant pools demonstrated the greatest potential C accumulation
in the slow pool (6.5 to 21.4 g C m −2 yr −1 ) followed by the resistant (0.66 to 16.05 g
C m −2 yr −1 ) and active pools (0 to 0.24 g C m −2 yr −1 ).
Consistent with these results, De Gryze et  al. (2004) found that in the Early
Successional system, mineral-associated SOM accumulated C at 5- to 13-fold
higher rates than did other fractions, while in the Poplar stands the fastest C accu-
mulation rates occurred in the fine intraaggregate fraction. Soil organic matter
pools with higher turnover times, such as the interaggregate pool, are extremely
sensitive to disturbance and may show large, rapid changes in response to manage-
ment. However, these pools do not have the long-term sequestration potential of
mineral-associated pools with slower turnover times (Six et al. 2002a, b).
Sollins et al. (2009) used sequential density fractionation to separate KBS soil
particles collected from 0- to 25-cm depth into “light” predominately mineral-free
organic matter from the “heavy” particles associated with the soil minerals. Quartz,
alkali feldspars, vermiculite, and kaolinite are major components of the fraction
with density < 2.6 g cm −3 . Quartz dominates the 2.6-2.8 g cm −3 fraction, account-
ing for 73% of soil dry weight. The primary minerals hornblende, hematite, and
epidote account for 2% of soil dry weight. The density of the particles was found to
depend on the thickness of their SOM coating as well as the density of the mineral
particles themselves.
The <1.65 g cm −3 density fraction represented 16% of total soil C with a total
lignin-phenol content that accounted for 5.5% of total C.  The 1.65-1.85 g cm −3
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