several small grains may have merged to form a single large grain, and this might
explain why the relationship between
14 C age and grain size was somewhat unclear
for larger grains in buried A horizons.
Although difficulties remain in comparing
14 C ages between humus extracts and
sclerotia grains, the average age of organic materials incorporated during formation
of sclerotia grains is thought to be older than that of humic substances (Watanabe
et al. 2007b). Most of the sclerotia grains examined here are thought to be “dead”
sclerotia of C. geophilum, which persist in the soil and can be differentiated from
“live” sclerotia by being hard and brittle, as described by Trappe (1969) and
Massicotte et al. (1992). In in vitro studies of sclerotium germination and formation
on agar, initial growth of a white hyphal tip from field-collected “live” sclerotia
occurred within 10 days (Trappe 1969) or 20 days at the most (Massicotte et al.
1992). In most sclerotia that proved to be viable, hyphal growth was detected within
30 days, but some sclerotia remained on the agar for up to 75 days before germinating, and at maturity, after formation of a dark rind, they resembled sclerotia
formed in soil (Massicotte et al. 1992). It therefore seems plausible to conclude that
sclerotium formation in soils may also terminate within several months. Humic
acids, continuously exposed to metabolism by microbes, are composed of heterogeneous material. On the other hand, sclerotia grains, as spore structures derived from
fungal cell walls, are likely to be composed of homogeneous material. Consequently,
the AMS14 C ages of sclerotia grains are considered to indicate the age of individual
grains and may be older than the soil profile they occupy, consistent with the age of
the beginning of soil formation, compared with humic acids, which may indicate the
average age of soil humus.
As the cell wall of ectomycorrhizae contains a high concentration of aluminum
(Brunner and Frey 2000; Brunner 2001), the homogeneous distribution of aluminum
in the transverse wall of ignited grains observed by Watanabe et al. (2004a) suggests
that the grains may be a biosynthetic product of the ectomycorrhizae, responsible for
aluminum retention. Furthermore, the mean weight of sclerotia grains is regulated by
the amount of exchangeable aluminum in soils, regardless of soil type (Watanabe
et al. 2002, 2004a). Watanabe et al. (2007a) performed a quantitative elemental
analysis of MYK sclerotia, which clarified that the concentrations of carbon and
oxygen, the major elements regulating the weight of grains, were 48 wt% and 30 wt
%, respectively, and that of aluminum was 1.4 wt%, with no significant differences
in element concentrations between grains of different size. Extending the investigation to various forest soils and comparing grain turnover in light of the relationship
between soil chemical properties and
14 C age of grains would be warranted.
Carbon content was measured by using a CO 2 pressure gauge during the process
of preparing graphite. The carbon content of sclerotia grains in surface A horizons
was 40–50%, whereas that of sclerotia grains in buried A soils was 38–46%. Older
grains showed a slight decrease in carbon content (Watanabe et al. 2007b). The
results from Myoko profile may indicate that sclerotia grains in buried humic soils
are undergoing decomposition.
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M. Watanabe et al.
explain why the relationship between
14 C age and grain size was somewhat unclear
for larger grains in buried A horizons.
Although difficulties remain in comparing
14 C ages between humus extracts and
sclerotia grains, the average age of organic materials incorporated during formation
of sclerotia grains is thought to be older than that of humic substances (Watanabe
et al. 2007b). Most of the sclerotia grains examined here are thought to be “dead”
sclerotia of C. geophilum, which persist in the soil and can be differentiated from
“live” sclerotia by being hard and brittle, as described by Trappe (1969) and
Massicotte et al. (1992). In in vitro studies of sclerotium germination and formation
on agar, initial growth of a white hyphal tip from field-collected “live” sclerotia
occurred within 10 days (Trappe 1969) or 20 days at the most (Massicotte et al.
1992). In most sclerotia that proved to be viable, hyphal growth was detected within
30 days, but some sclerotia remained on the agar for up to 75 days before germinating, and at maturity, after formation of a dark rind, they resembled sclerotia
formed in soil (Massicotte et al. 1992). It therefore seems plausible to conclude that
sclerotium formation in soils may also terminate within several months. Humic
acids, continuously exposed to metabolism by microbes, are composed of heterogeneous material. On the other hand, sclerotia grains, as spore structures derived from
fungal cell walls, are likely to be composed of homogeneous material. Consequently,
the AMS14 C ages of sclerotia grains are considered to indicate the age of individual
grains and may be older than the soil profile they occupy, consistent with the age of
the beginning of soil formation, compared with humic acids, which may indicate the
average age of soil humus.
As the cell wall of ectomycorrhizae contains a high concentration of aluminum
(Brunner and Frey 2000; Brunner 2001), the homogeneous distribution of aluminum
in the transverse wall of ignited grains observed by Watanabe et al. (2004a) suggests
that the grains may be a biosynthetic product of the ectomycorrhizae, responsible for
aluminum retention. Furthermore, the mean weight of sclerotia grains is regulated by
the amount of exchangeable aluminum in soils, regardless of soil type (Watanabe
et al. 2002, 2004a). Watanabe et al. (2007a) performed a quantitative elemental
analysis of MYK sclerotia, which clarified that the concentrations of carbon and
oxygen, the major elements regulating the weight of grains, were 48 wt% and 30 wt
%, respectively, and that of aluminum was 1.4 wt%, with no significant differences
in element concentrations between grains of different size. Extending the investigation to various forest soils and comparing grain turnover in light of the relationship
between soil chemical properties and
14 C age of grains would be warranted.
Carbon content was measured by using a CO 2 pressure gauge during the process
of preparing graphite. The carbon content of sclerotia grains in surface A horizons
was 40–50%, whereas that of sclerotia grains in buried A soils was 38–46%. Older
grains showed a slight decrease in carbon content (Watanabe et al. 2007b). The
results from Myoko profile may indicate that sclerotia grains in buried humic soils
are undergoing decomposition.
124
M. Watanabe et al.