Soil Organic Matter Dynamics - The Ecology of Agricultural Landscapes - page 117

density fraction accounted for 8% of soil C and the 1.85-2.00 g cm −3 fraction for
14%. Phenol and lignin concentrations were lower in the higher-density fractions
and the C:N ratio decreased with density. The 14 C-based MRTs measured by Sollins
et al. (2009) ranged from 108 years in the 2.00-2.30 g cm −3 density fraction to
165 years in the <1.65 g cm −3 fraction and 225 years in the 1.65-1.85 g cm −3 frac-
tion. The heaviest densities (> 2.80 g cm −3 ) had an average MRT of 1050 years. The
average MRT for all densities was similar to the MRT of 420 years shown for KBS
soil in Table 5.2.
Incubation-based measurements can also identify fractions enriched in SOM
and sensitive to changes in management. As noted earlier, Paul et al. (1999a) used
long-term incubations to show how only 5 years of MCSE management affected
SOM fractions. From 10 to 15% of total SOC was oxidized during their 365-day
incubations, with the highest CO 2 production occurring in soils with the higher
SOC contents (Table 5.5). The active fraction, as defined by curve fitting of the
CO 2 evolved early during incubation (Paul et al. 1995, 2001b; Horwath et al. 1996),
represented 1.8-3.3% of the SOC. This is equivalent to half the C content of the
interaggregate fraction of SOC isolated by density fractionation, agreeing with the
analysis of Haile-Mariam et al. (2008) that the interaggregate fraction contains
some nonlabile constituents. The slow pool represented 41-45% of the SOC and
serves as the primary basis for soil fertility and ecosystem SOM dynamics. It is
made up, in part, by materials in the intraaggregate fraction and by the more labile
components of the silt and clay fractions (18-21%) shown to be corn-derived, based
on stable carbon isotope ratios as explained in more detail below (Table 5.6). The
interaggregate fraction, measured by density gradient fractionation before aggre-
gate dispersion, is also referred to as the light fraction. The intraaggregate fraction,
measured by sieving the >53-μm soil fraction after dispersion, is also often referred
to as the particulate organic material.
The 6- M acid hydrolysis treatment used to measure the size of the resis-
tant (non-hydrolyzable) fraction of SOC does not remove modern lignin plant

Table 5.5. Analytically defined pools and mean residence time (MRT) of SOC
5 years after the initiation of the MCSE. a

System SOC Pool

(g kg −1 )

CO 2 -C
Produced
(% of
SOC)

Active C Pool Slow C Pool Resistant C Pool

(% of
SOC)

MRT
(days)

(% of
SOC)

MRT
(years)

(% of
SOC)

MRT
(years)

Conventional 8.7 10 2.0 45 42 13.1 56 1435

Poplar 9.1 13 3.3 66 41 11.0 ND ND

Early Successional 9.0 15 2.5 36 42 9.0 ND ND

Mown Grassland
(never tilled)

14.6 15 1.8 30 45 9.0 53 170

a Long-term incubations (365 days) and acid hydrolysis used to determine pools and residence times. See Table 5.1 for
a description of MCSE systems.
Note : CO 2 -C = carbon dioxide-C; ND = not determined.
Source: Paul et al. (1999a).