Agriculture Reference
In-Depth Information
Table 5.2. Soil organic carbon (SOC) characterization of total and
non-hydrolyzable fractions in forest and agricultural soils.
a
Soil
b
SOC (g kg
−1
)
MRT (years)
NHC (%)
MRT NHC (years)
KBS LTER, MI (Forest)
0-20 cm
10.7 (0.5)
422 (51)
56
977 (50)
25-50 cm
2.6 (0.4)
933 (67)
23
895 (54)
50-100 cm
1.3 (0.1)
1712 (50)
21
4406 (65)
Hoytville, OH (CT)
0-20 cm
17.8 (0.7)
920 (53)
46
1770 (45)
25-50 cm
8.6 (0.40)
2627 (55)
45
5660 (870)
50-100 cm
4.3 (0.1)
6607 (79)
44
9875 (75)
Lamberton, MN (CT)
0-20 cm
17.9 (1.0)
1100 (53)
49
1510 (45)
25-50 cm
8.7(0.7)
3100 (55)
45
3965 (65)
50-100 cm
4.3(0.2)
6107 (75)
48
7285 (90)
a
Includes SOC content, mean residence time (MRT), non-hydrolyzable C fraction (NHC), and MRT of the NHC.
Data are means (±SE, when applicable).
b
The KBS LTER site is the Deciduous Forest system of the MCSE (Table 5.1) and the other two sites are agricultural
row-crop production sites with conventional tillage (CT) and chemical use.
Source:
Paul et al. (2001a, b).
the surface layer (Table 5.2). Soil at 50- to 100-cm depth contains 4.3 g C kg
−1
, with
an MRT of 6607 years, and thus has higher SOC content and is older than KBS soil.
The long time required for SOM development is reflected in the MRT of 9875 years
for the oldest NHC fraction, which accounts for 44% of total SOC at 50-100 cm.
The grassland-derived soil from Lamberton, Minnesota, shows similar SOC levels
and proportions of NHC as the forest-derived Hoytville soil. The Lamberton SOC
has somewhat longer MRTs than Hoytville and substantially longer MRTs than
those of KBS. In all cases, a strong relationship exists between the MRT of the SOC
and the amount of SOC present, indicating the time it takes for SOM to accumulate.
Long-term incubations of these soils (Collins et al. 2000) showed that although
SOC from deeper depths in the profile had very long MRTs, when brought into the
laboratory and disturbed, the samples released nearly as much CO
2
per unit of SOC
as did soil from the surface horizons. This implies that the long MRTs were the
result of matrix interactions, such as those with silt and clay, and profile position
and not necessarily intrinsic chemistry. Labile constituents are decomposed early
in the incubation and thus largely control the size of the active SOC pool. This was
confirmed by the observation that the stable C isotope ratios of CO
2
produced in
incubations of soils obtained from cultivated sites moved toward the ratio produced
by native-site soils late in their long-term incubation. At that point in the incuba-
tion, microbes are decomposing the older, native-derived materials. The stable C
isotope ratio of the respired CO
2
of the cultivated sites' SOC did not become equal
to that of CO
2
from the native sites' SOC because the total CO
2
evolved was less
than 10% of the total SOC (Collins et al. 1999).
