Verbleib des organischen Kohlenstoffs in Bodenfraktionen nach Landnutzungswechsel in den humiden Tropen

Abstract

The soil organic matter pool represents a major part of the terrestrial carbon reservoir. Decomposition of soil organic matter contributes to important fluxes of the global carbon cycle. In order to understand, assess, and predict the impact of land use or climatic changes in the tropics, quantitative knowledge of stabilization and decomposition processes of soil organic matter is necessary. This thesis elucidated how soil properties and land use influence the carbon storage in bulk soil and in physically and chemically defined soil fractions. Furthermore, the stability of older forest-derived carbon and the stability of recently incorporated pasture-derived carbon in these fractions were investigated. For this purpose, fractionation procedures were combined with 13C isotope analyses on soil samples from sites that had undergone a land use change (natural forest > pasture, natural forest > pasture > secondary forest) which was connected with a vegetation change from C3 to C4 or from C3 to C4 to C3 plants. Soil sampling was conducted in the humid tropics in northwest Ecuador. Soils developed from different parent material (marine Tertiary sediments and volcanic ashes) differing in key factors which influence storage and stabilisation of soil organic matter.Besides parent material, soil texture was the main factor which controlled total soil organic carbon (soil C) contents in these soils. The major part of soil C was bound to the mineral phase. Higher soil C contents in the volcanic ash soils emphasise the higher carbon sequestration of Al-humus complexes and non-crystalline hydroxides in comparison to sedimentary soils dominated by smectite. Results of the chemical fractionation point to an enhanced stability of the mineral-bound soil C against oxidation with NaOCl in the volcanic ash soils. In addition, more soil C could be extracted by Na4P2O7 in these soils indicating the importance of Al-humus complexes. In contrast, treatment with HCl isolated a fixed proportion of soil C storage in both soil types indicating a similar composition of organic matter in the soils. The higher carbon sequestration capacity in the volcanic ash soils could be also related to higher carbon storage in the light fractions.Independent of soil parent material, deforestation followed by pasture establishment reduced soil C storage. By afforesting the former pasture, the amount soil C increased, but the values of the natural forest were not reached at all sites. Changes in soil C stocks were more pronounced in the light fractions compared to bulk soil. Thus, light fractions can be used as an early indicator for soil C changes induced by land use changes. The correlation of aggregation and the amount of occluded light fraction and the slow turnover support the hypothesis, that organic matter is stabilised by aggregation in both soil types.The recently incorporated pasture-derived carbon showed a faster turnover in the volcanic ash soils than in the sedimentary soils. Mineral surfaces of sedimentary soils form stronger interactions with the organic matter. In contrast to sedimentary soils, no pasture-derived carbon could be detected in any fraction of the volcanic ash soils under secondary forest. Thus, recently incorporated carbon was not stabilised in volcanic ash soils. However, pasture-derived carbon could be detected in the chemical isolated fractions under pasture. This indicates that none of the used chemical fractionation techniques isolated a soil organic matter pool that is under field conditions stable against microbial degradation.