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Organic-humic materials and soil particles as developed in a chronosequence of forest soil parent materials
0 ~ GeoUwm Vol. 8. No. 1, pp. 133-134,1985 Printed m Great Britain. All rights reserved
0146-6380/85 $03.00+00.00 Copyright (~) 1985 Pergamon Press Ltd
O r g a n i c - h u m i c materials and soil particles as developed in a chronosequence of forest soil parent materials G. SPYCHER, P. SOLLINSand J. L. YOUNG Departments of Forest Science and Soil Science, Oregon State University. Corvallis. OR q7331. U.S.A. (In cooperation with the Agricultural Research Service, USDA)
This study sought information about characteristics of organic-humic materials that have deveoped in association with mineral matter as soils have evolved from raw parent materials, with emphasis on the density, size and composition of resulting organic-mineral particles. Sod samples were collected from a chronosequence of forest soils derived from andesitic parent materials (volcanic-ash mud flows) at Mt Shasta in California, and from a lithologically similar but much older site at the H, J. Andrews (HJA) forest in the Oregon Cascades. Soil ages were about 50 yr on Flow A, 300 yr on Flow B, several thousand years on Flows D & E, and >14,000 years at the HJA forest site. Features of macro- and micro-aggregates were examined in petrographic thin sections and in scanning electron micrographs. Structure in the soils from the younger mud-flows (A & B) reflects original conditions shortly after flow events. The framework consists of relatively clean coarse sands with some partial crusts composed of finer rock fragments. Grains in the subsurface horizons appear cemented by mineral precipitates at points of lnterparticle contact. (The associations resemble "rudimentary chitonic'" fabric type.) Structure in soils on older flows D & E reflect completion and stabilization of coatings around grains by accumulation of organic-humic substances and mineral weathering products. Sand grains are encrusted with finer particles--thin coating of colloidal organic and mineral matter completely cover grain surfaces, but few microaggregates are present ("chalmydic" fabric type). Structure at the Oregon site shows a more advanced stage of weathering. Particles are much more complex: most are sand- and silt-size despite the considerably higher clay content. Large compound aggregates consist of numerous microaggregates and some coated sand/silt grains--all highly variable in size, and interlaced with microbial residues and fungal filaments ('~matrigranic" fabric type). Sub-samples of air-dry soil were separated with a series of density liquids (tetrabromoethanc;EtOH mixtures) into five fractions: FI <1.6 g/cm 3 F2 1.6 to 1.9 g/cm ~ F3 1.9 to 2.2 g/cm ~ F4 2.2 to 2.5 g/cm 3 F5 >2.5 g/cm 3 Light materials (F1) were mainly recent detrital fragments; all other fractions contained combination organic-mineral particles. The density-separated fractions were subjected to elemental analysis (C,N) and to Curie-point pyrolysis mass spectrometry (Py-MS). Carbon-14 was measured in the soil particles >1.6 g/cm 3 (light fraction removed), in some untreated particle-size fractions, and in their residues left after hyrolysis in 6N HC1. Proportions of lighter particles increased with soil age (Fig, 1). Concentrations of C and N in the fractions decreased and C/N ratios narrowed with increased particle density. Proportions of total soil C and N were 1oo
o~
-
/5
~ 50-
d
0
FI F2 F3 F4 if5 Fig. 1. Cumulative percentages of particles in five density fractions. 133
134
G. SPYCHERet al.
greatest in F2; proportions were less in both lighter and heavier particle-density fractions (Fig. 2). However, the gross distribution of C and N among the five density fractions and the C/N ratio of the organic materials in each fraction were essentially independent of soil age, texture, structure, fabric type or total soil carbon. Py-MS analyses provided good spectra on F1, F2, and F3 preparations; spectra from scarce amounts of organic-humic substances in the heavier F4 and F5 fractions showed some trends, but inadequate replication prevented definitive interpretation. Py-MS peaks indicating fragments from lignins and polysaccharides were characteristic of the lightest particles. Peaks for aromatic, aliphatic components (alkene, alkylbenzene, furan, pyrrole fragments), heterocyclic N and sulfur-containing compounds increased in intensity with particle density. Similar patterns were evident for organic-humic materials as influenced by soil age: trends were somewhat weaker but sufficed to distinguish the two younger soils from the three older soils. 50
--
"o
40
50
.T
o
/T'~\/i~.-~a r b°n
-
40
(3.
E c 30 Z {D
20
T/,' '..r ~ .~ ,~ \
l,'
T,,
3o _o
Nitrogen /
~',~,r /
Z
"3;,/
o lo 's
20
40
•.-....t I I
I
I
I
I
P4
F2
F3
F4
F5
Fig. 2. Proportion of C and N and C/N ratio of organic-humic materials in each density fraction. F1--Mainly plant debris; F2. F3--Contain clay and silt-size organic-mineral particles and non-dispersed microaggregates; F4, F5--Contain large organic-mineral grains.
Carbon dating failed to show significant amounts of "old" carbon in surface soil from the Mt Shasta and HJA forest sites--a surprise considering the several reports dating soil humic substances at > 1000 years B.P. At the Oregon site, about 63% of the surface soil carbon, excluding light-fraction C, had a ~C content of 106% modern. The remainder gave a calculated equivalent age of 390 yr. But, carbon in sand-size, very dispersion-resistant macroaggregates or concretions that survived 6N HCI hyrolysis dated 980 + 50 yr and accounted for - 1 0 % of the total soil carbon in the Oregon surface samples. Postulates from SEM studies and other data suggest these highly-stable residue-particles may have formed during recurrent forest fires. The organic-humic components in subsurface horizons were older; equivalent 14C ages were 380 yr in the 63-83 cm layer from the Mt Shasta E flow, and 2470 yr in the 40-60 cm layer of the Oregon soil. In general, either aggregation in surface horizons of these volcanic-origin forest soils has not provided long-term physical protection, or there has been more rapid than expected incorporation of bomb carbon into humic constituents in the surface horizons, or both. The nature and dynamics of various organic-mineral particles in soils continues to merit considerable reassessment, particularly where the less-studied X-ray "amorphous" rather than more crystalline components dominate the clay-size mineral phase. Acknowledgements--Thanks go to A. Fairhail for 14C dating assays, to J. Norgren for thin-section preparation and
interpretations and to H. Meuzelaar for pyrolysis-mass spectrometry charts.
0146-6380/85 $03.00+00.00 Copyright (~) 1985 Pergamon Press Ltd
O r g a n i c - h u m i c materials and soil particles as developed in a chronosequence of forest soil parent materials G. SPYCHER, P. SOLLINSand J. L. YOUNG Departments of Forest Science and Soil Science, Oregon State University. Corvallis. OR q7331. U.S.A. (In cooperation with the Agricultural Research Service, USDA)
This study sought information about characteristics of organic-humic materials that have deveoped in association with mineral matter as soils have evolved from raw parent materials, with emphasis on the density, size and composition of resulting organic-mineral particles. Sod samples were collected from a chronosequence of forest soils derived from andesitic parent materials (volcanic-ash mud flows) at Mt Shasta in California, and from a lithologically similar but much older site at the H, J. Andrews (HJA) forest in the Oregon Cascades. Soil ages were about 50 yr on Flow A, 300 yr on Flow B, several thousand years on Flows D & E, and >14,000 years at the HJA forest site. Features of macro- and micro-aggregates were examined in petrographic thin sections and in scanning electron micrographs. Structure in the soils from the younger mud-flows (A & B) reflects original conditions shortly after flow events. The framework consists of relatively clean coarse sands with some partial crusts composed of finer rock fragments. Grains in the subsurface horizons appear cemented by mineral precipitates at points of lnterparticle contact. (The associations resemble "rudimentary chitonic'" fabric type.) Structure in soils on older flows D & E reflect completion and stabilization of coatings around grains by accumulation of organic-humic substances and mineral weathering products. Sand grains are encrusted with finer particles--thin coating of colloidal organic and mineral matter completely cover grain surfaces, but few microaggregates are present ("chalmydic" fabric type). Structure at the Oregon site shows a more advanced stage of weathering. Particles are much more complex: most are sand- and silt-size despite the considerably higher clay content. Large compound aggregates consist of numerous microaggregates and some coated sand/silt grains--all highly variable in size, and interlaced with microbial residues and fungal filaments ('~matrigranic" fabric type). Sub-samples of air-dry soil were separated with a series of density liquids (tetrabromoethanc;EtOH mixtures) into five fractions: FI <1.6 g/cm 3 F2 1.6 to 1.9 g/cm ~ F3 1.9 to 2.2 g/cm ~ F4 2.2 to 2.5 g/cm 3 F5 >2.5 g/cm 3 Light materials (F1) were mainly recent detrital fragments; all other fractions contained combination organic-mineral particles. The density-separated fractions were subjected to elemental analysis (C,N) and to Curie-point pyrolysis mass spectrometry (Py-MS). Carbon-14 was measured in the soil particles >1.6 g/cm 3 (light fraction removed), in some untreated particle-size fractions, and in their residues left after hyrolysis in 6N HC1. Proportions of lighter particles increased with soil age (Fig, 1). Concentrations of C and N in the fractions decreased and C/N ratios narrowed with increased particle density. Proportions of total soil C and N were 1oo
o~
-
/5
~ 50-
d
0
FI F2 F3 F4 if5 Fig. 1. Cumulative percentages of particles in five density fractions. 133
134
G. SPYCHERet al.
greatest in F2; proportions were less in both lighter and heavier particle-density fractions (Fig. 2). However, the gross distribution of C and N among the five density fractions and the C/N ratio of the organic materials in each fraction were essentially independent of soil age, texture, structure, fabric type or total soil carbon. Py-MS analyses provided good spectra on F1, F2, and F3 preparations; spectra from scarce amounts of organic-humic substances in the heavier F4 and F5 fractions showed some trends, but inadequate replication prevented definitive interpretation. Py-MS peaks indicating fragments from lignins and polysaccharides were characteristic of the lightest particles. Peaks for aromatic, aliphatic components (alkene, alkylbenzene, furan, pyrrole fragments), heterocyclic N and sulfur-containing compounds increased in intensity with particle density. Similar patterns were evident for organic-humic materials as influenced by soil age: trends were somewhat weaker but sufficed to distinguish the two younger soils from the three older soils. 50
--
"o
40
50
.T
o
/T'~\/i~.-~a r b°n
-
40
(3.
E c 30 Z {D
20
T/,' '..r ~ .~ ,~ \
l,'
T,,
3o _o
Nitrogen /
~',~,r /
Z
"3;,/
o lo 's
20
40
•.-....t I I
I
I
I
I
P4
F2
F3
F4
F5
Fig. 2. Proportion of C and N and C/N ratio of organic-humic materials in each density fraction. F1--Mainly plant debris; F2. F3--Contain clay and silt-size organic-mineral particles and non-dispersed microaggregates; F4, F5--Contain large organic-mineral grains.
Carbon dating failed to show significant amounts of "old" carbon in surface soil from the Mt Shasta and HJA forest sites--a surprise considering the several reports dating soil humic substances at > 1000 years B.P. At the Oregon site, about 63% of the surface soil carbon, excluding light-fraction C, had a ~C content of 106% modern. The remainder gave a calculated equivalent age of 390 yr. But, carbon in sand-size, very dispersion-resistant macroaggregates or concretions that survived 6N HCI hyrolysis dated 980 + 50 yr and accounted for - 1 0 % of the total soil carbon in the Oregon surface samples. Postulates from SEM studies and other data suggest these highly-stable residue-particles may have formed during recurrent forest fires. The organic-humic components in subsurface horizons were older; equivalent 14C ages were 380 yr in the 63-83 cm layer from the Mt Shasta E flow, and 2470 yr in the 40-60 cm layer of the Oregon soil. In general, either aggregation in surface horizons of these volcanic-origin forest soils has not provided long-term physical protection, or there has been more rapid than expected incorporation of bomb carbon into humic constituents in the surface horizons, or both. The nature and dynamics of various organic-mineral particles in soils continues to merit considerable reassessment, particularly where the less-studied X-ray "amorphous" rather than more crystalline components dominate the clay-size mineral phase. Acknowledgements--Thanks go to A. Fairhail for 14C dating assays, to J. Norgren for thin-section preparation and
interpretations and to H. Meuzelaar for pyrolysis-mass spectrometry charts.