1. STABILIZATION MECHANISMS OF SOIL ORGANIC MATTER IN allophanic and non-allophanic VOLCANIC SOILs in the HIGHLAND area of Tenerife
NATALIA RODRÍGUEZ1*, JOSE A. GONZÁLEZ-PÉREZ2, CARMEN D. ARBELO1, ANTONIO RODRÍGUEZ-RODRÍGUEZ1
Department of Soil Science and Geology, University of La Laguna, Tenerife, Canary Islands(Spain).
Institute for Natural Resources and Agrobiology of Seville. IRNAS-CSIC. Seville (Spain).
INTRODUCTION
Andosols and andic soils are considered as efficient C-sinks in terms of C sequestration. These soils are usually developed from volcanic materials, and are characterized by a predominance of short-range ordered minerals like allophanes, imogolite and other Fe and Al oxyhydroxides. Such materials accumulate high quantities of soil organic matter (SOM), and it is supposed to be due to the formation of organic-metal complexes, when pH < 5, forming non-allophanic soils, or organic-mineral complexes, if pH > 5, in allophanic soils. Nonetheless, SOM protecting mechanisms acting in these soils remain uncertain, and it is no clear that organic-mineral/metal complexes are the main protection method.
The aim of this work is to elucidate what are the protection mechanisms acting in some volcanic soils from highland area of Tenerife (Canary Islands, Spain), using a density fractionation method of SOM (SPT δ = 1,9 g cm-3) to establish its distribution in soil structure and analytical pyrolysis-GC/MS to elucidate differences on its molecular structure.
SITE OF STUDY
Teide National Park present a subalpine continental climate.
The vegetation in the Park is included in three well- defined zonal ecosystems by climatic and ecologic characteristics: the pine forest, the highlands scrubland, dominated by Pinus canariensis and Spartocytisus supranubius- Pterocephalus lasiospermus, respectively, and barelands.
RESULTS
1. Soil properties
Four soil types are defined by its andic properties: non-allophanic, allophanic, intermediate and mineral. All studied soils show low pH, and a relatively scarce SOM content, except non-allophanic soils.
MATERIALS AND METHODS
Teide National Park present a subalpine continental climate (precipitation of 500 mm/year at 2,000 m.a.s.l., decreasing gradually with increasing altitude and a range of temperature between 10°C to 3°C). The vegetation in the Park is included in three well- defined zonal ecosystems by climatic and ecologic characteristics: the pine forest, the highlands scrubland, dominated by Pinus canariensis and Spartocytisus supranubius, respectively, and barelands.
The study were performed on 19 A soil horizons, covering the main different parent materials, plant formations and climatic conditions existing on Teide National Park. Soil physicochemical properties were analysed, as well as a physical fractionation by density (sodium politungstate solution with δ = 1,9 g cm-3) to isolate SOM fractions that differ in stability. Analytical pyrolysis (Py- GC/MS) at 500º C was made for bulk soil, decaying litter, light fractions (δ < 1,9 g cm-3) and heavy fractions (δ > 1,9 g cm-3).
Non-allophanic soils
Intermediate soils
Allophanic soils
Mineral soils
Non-crystalline minerals
Organic carbon complexed to mineral phase (g/kg)
Pine forest
Broom scrubland
Bareland
Rosalito scrubland
Fig. 1. Correlation between non-crystalline minerals (ratio Alo + ½ Feo) and organic carbon associated with organo-mineral complexes (Cp) of studied soils.
2. SOM pools
SOM density fractionation procedure shows a high proportion of free or weakly associated to soil particles organic matter in these soils (C-LF), but this SOM seems to correspond to organic carbon in organic-mineral complexes (Cp)  this fractionation procedure does not discriminate well between light volcanic materials from free SOM (Fig. 2).
Fig. 2. SOM pools of studied soils. C-LF: organic carbon associated to light fraction, C-HF: organic carbon associated to heavy fraction, Cp: organic carbon extracted with sodium pyrophosphate, associated to organo-mineral complexes.
3. Structural characteristics of SOM
Plant litter is characterised by aliphatic hydrocarbon of long chain and lignin- derived compounds (Fig.3).
Light fractions present characteristics of plant material, and fungal and microbial signs (Fig.4).
Heavy fractions show information about C-sequestering mechanisms. Non-allophanic soils have a selective preservation of hydrocarbon chains from plants, allophanic soils have less organic C, and it is derived from microorganisms (Fig.5).
Rich in lignin-derived
Rich in fatty acids and n-Alkane/alkenes
Rich in aromatic compounds
Rich in protein-derived compounds
PC 2 (7,1% explained variance)
PC 1 (68,6% explained variance)
PC 2 (10,6% explained variance)
PC 1 (65,5% explained variance)
Organic-metal complexes rich in hydrocarbon long chains from plants
Organic-metal and organic-mineral complexes rich in aromatic compounds
Organic-mineral complexes rich in polysaccharides and protein-derived compounds
Low C content, rich in polysaccharides and protein-derived compounds from microorganisms
Fig.3. TIC of litter samples with main peaks
Fig.4. PCA of light fractions based on compound contents
Fig.5. PCA of heavy fractions based on compound contents
CONCLUSIONS
Allophanic and non-allophanic andosols present different mechanisms for SOM protection. In allophanic soils SOM is associated to non-crystalline minerals forming organo-mineral complexes, with an structure rich in polysaccharides and N-compounds derived from microorganisms.
In non-allophanic soils exist a selective preservation of relatively unaltered plant components in organo-metal complexes, either inside stable micro- aggregates, by mineral coatings formation or in tri-dimensional nets of alkyl material and metal colloids.
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