Agriculture Reference
In-Depth Information
constituents or some microbial cell walls (Paul et al. 2006, Plante et al. 2006), and
thus, the non-hydrolyzable fraction contains some young C. However, it contains
mostly old C and provides a reasonable estimate of the size of the resistant fraction
required for modeling purposes. Carbon dating of this fraction in native KBS for-
est surface soils yields a MRT of 977 years (Table 5.2). The MRT of bulk soil C in
the upper 20 cm of cultivated KBS soil was 546 years (Paul et al. 2001a) relative
to 422 years for the native forest soil (Table 5.2), indicating that some labile C had
been lost on cultivation and not replaced by more recent plant inputs.
Plants with the C 4 photosynthetic pathway have higher 13 C: 12 C isotope ratios
(expressed as δ 13 C in parts per million or ‰) as compared with C 3 plants. Typically,
C 4 plants such as corn, sorghum, and switchgrass have δ 13 C values of -10 to -14‰
as compared to values of -22 to -30‰ in C 3 plants such as soybean, wheat, and
alfalfa. This difference has been used to measure the dynamics of the inter- and
intraaggregate particulate material and the SOM associated with silt and clay frac-
tions. The growth of corn for 10 years on a KBS site adjacent to the MCSE that
was formerly dominated by C 3 plants allowed Haile-Mariam et al. (2008) to exam-
ine SOM dynamics using δ 13 C analysis of soils from the field before and after
800  days of incubation. The interaggregate fraction, which represents partially
decomposed plant residues (as shown by C:N ratios of 21:1), still had 45% non-
corn C after 10  years of continuous corn, showing that even interaggregate frac-
tions contain some old C (Table 5.6). This increased to 72% noncorn C by the end
of the incubation, showing the difference in lability between the corn and non-
corn C.  Mid-infrared spectroscopic analysis of this fraction showed that a small
Table 5.6. Properties of SOC and δ 13 C analysis of soils from continuous corn sites
adjacent to the MCSE. a
Property
Interaggregate
Intraaggregate
Silt
Clay
Whole Soil
Field soil, day zero
% of soil C
4.5
20.4
17.5
48.5
100
C:N (by mass)
21
15.2
10.0
8.5
10.4
δ 13 C ‰
-18.2
-20.7
-23.5
-23.2
-23.4
Corn-derived %C
55.7
38.1
18.4
21.2
21.3
Incubated, day 800
% of soil C
2.9
25
18.4
47.4
100
C:N by mass
20.4
14.0
8.8
7.7
8.8
δ 13 C ‰
-22.2
-22.9
-24.4
-23.6
-23.2
Corn-derived %C
27.7
22.7
11.9
19.8
20.5
Corn C MRT (years)
3.9
11.4
10.9
16.5
Non-corn C MRT (years)
19.7
33.4
47.1
40
a SOC fractions determined by long-term incubation.
Note: MRT = mean residence time.
Source: Haile-Mariam et al. (2008).
 
Search WWH ::




Custom Search