Ea. Paul et al., Dynamics of resistant soil carbon of midwestern agricultural soils measured by naturally occurring C-14 abundance, GEODERMA, 104(3-4), 2001, pp. 239-256
Information on the mean residence time (MRT) of soil organic carbon (SOC) o
n different soil types and management regimes is required for pedo-geologic
al, agronomic, ecological and global change interpretations. This is best d
etermined by carbon dating the total soil together with acid hydrolysis and
carbon dating of the non-hydrolyzable residue (NHC). Midwestern US soils i
n a transect from Lamberton. MN to Kutztown. PA were found to contain from
33% to 65% of their SOC in the non-hydrolyzable fraction. Soils on lacustri
ne deposits had the most NHC; glacial till and shale soils, the least. The
MRTs of the SOC of surface horizons of soil ranged from modem to 1100 years
with an average of 560 years. The MRT increased to an average of 1700 year
s in the 25-50-cm depth increment and 2757 years at 50-100 cm. The NHC was
1340 years greater at the surface and 5584 years at depth. The MRTs of the
total SOC were inversely correlated to sand and directly related to clay co
ntent. Silt did not have a significant effect on the MRT of total SOC, but
was significantly correlated with the MRT of the NHC. A four-parameter mode
l described the relationship between the SOC content and MRT. The complexit
y of this equation reflected the strong effect of depth, which greatly decr
eased SOC while increasing the MRT. The MRT of these soils, as determined w
ith carbon dating of the naturally occurring C-14, was compared to that mea
sured with the C-13 signal produced by approximately 30 years of continuous
corn (Zea mays L.) (C-4) on soils with a known plant history of C-3-C-4 cr
opping. The equation of C-14 MRT = 176((CMRT)-C-13)(0.54) with an R-2 of 0.
70 showed that although short-term C-13 studies correlate well with the tot
al MRT, they reflect the dynamics of the active and slow pools, not the tot
al SOC. (C) 2001 Elsevier Science B.V. All rights reserved.