| dc.description.abstract | Soil carbon (C) sequestration is a strategy to capture atmospheric CO2 and transfer it into stable soil organic carbon (SOC) forms. There are a numbers of mechanisms of C stabilisation, the processes that made SOC resistant to decomposition. For example: physical and chemical adsorption, physical protection by inaccessibility or spatial impedance to the substrate by decomposer, selective preservation of SOC by their recalcitrance and the environmental climatic conditions. The studies of C stabilisation and destabilisation (the processes that make organic substrate available for decomposition) across the soil profile are still scarce in volcanic soils. Few studies have been carried out to investigate those processes in Andisols despite the fact that the C storage potential of Andisols is higher than in any other soil type. Stable C occurs in this soil type due to its association with aluminum- and iron-oxides together with amorphous clay minerals like allophane and imogolite type materials. Recent investigations have shown that the lack of fresh C substrate in the subsoil limits the biological activity, and would influence preservation of native SOC in soil depth. This would regulate the mechanisms of the stabilisation of SOC in the subsoil if fresh C is not available. In this thesis, I investigated the factors and mechanisms that control the C stabilisation in the top- and subsoil horizons of volcanic temperate old growth rainforest soils in Southern Chile. This mountain rainforest of southern Chile still maintain pristine conditions and has been developed under a temperate climate, with low temperatures and high precipitation, where woody vegetation produces a litter with slow decomposition rates resulting in the accumulation of C on the surface soil, depending of climate conditions and soil texture. Therefore, these temperate rainforests provide a unique opportunity under pristine conditions to develop a baseline of the patterns and processes in the biogeochemical C cycling. The outline of this thesis begins with a general introduction. In Chapter I, we address the general objectives of our thesis with regards to SOC stabilisation/destabilisation processes operating in temperate rainforest Andisols of southern Chile. In Chapter II, we made a literature review for the factors and mechanisms involved in the stabilisation/destabilisation of SOC in volcanic soils. From this review emerge interesting results depending on the combination of several factors: Al and Fe activity, soil pH, allophane and imogolite type materials and the environmental conditions (vegetation type, precipitation and temperature). In this chapter, it ́s hypothesized that soil pH and the competition between Al- soil organic matter (SOM) complex and allophane formation is the most important control between stabilisation processes. Moreover, higher metal:Cp ratio in the subsoil might induce higher SOM saturation with Al and Fe than in the top soil, meaning both metals will not be able to stabilise more C and therefore, the Al and Fe in excess would be available to react with silica gel for the synthesis of allophane and imogolite type materials. In Chapter III, we show that the C stabilisation capacity evolves with soil age relative to the evolution of the mineral phase. Mineralogical changes during pedogenesis are related to the composition and of SOC in temperate old-growth forest of Southern Chile. The allophane formation at older stages of soil development leads to SOM stabilisation. In younger soils and at lower pH, metal–SOM complex formation may be more important. C stock distribution within the soil profile changes with soil age, with higher contribution of subsoil C to C stocks in younger Andisols. It is suggested that the combination of several factors (Al, Fe, and C extracted in Na-pyrophosphate, metal/Cp ratio, and pH of the soil) which change through pedogenesis control the SOM stabilisation. In Chapter IV, we focused on priming effect, the C released from native SOM from the fresh substrate addition at different soil depth to study the biological activity related to the stabilising capacity of these Andisols in two different sites with old-growth forest of Southern Chile. The limitation of the biological activity by lack of fresh C substrate is an important mechanism that would influence the preservation of native SOC in the subsoil if C substrate is not available. In this chapter, we hypothesized that SOM in these soils would be highly stable due to strong mineral interactions and therefore would not be possible the C release to upon priming effect. Moreover, the different stabilisation agents would induce contrasting potential SOC mineralisation and contrasting response to priming effect. Results showed a lower potential SOC mineralisation in subsoil of both Andisols compared to the surface soil in accordance with the increasing nutrients induced net negative priming in the topsoil, whereas priming of most subsoil SOC remained positive after 80 days of incubation. We suggest, that adsorption to the mineral phase is of greater importance for SOC stabilisation in Andisols than physical inaccessibility. Destabilisation of SOC by priming is possible and seems to be controlled by soil pH. Finally, In Chapter V we present a general discussion and conclusions where all factors and mechanisms we studied in SOC stabilisation/destabilisation in temperate old- growth rainforest Andisols of southern Chile are discussed. Also, we present the future directions concerning to the C stabilisation processes that operate in top- and subsoil. The main conclusion of this thesis are: (1) The pedogenic SOM stabilisation is relevant in these soils acting simultaneously by soil pH, non-crystalline clay and competition of metals (Al) and SOM for the formation of Al (and Fe)-SOM complexes or allophane type materials in the top and subsoils. However, the C complex formation it seems to be more important in the surface soils with low pH ( < 5.8 ) than the allophane formation in the subsoils with high pH (> 5.8) top, Then, the latter mechanism is the cause of C stabilisation the subsoils rather than C complex formation. (2) In general, the SOM in these soils type is highly stable due to strong mineral interactions, therefore, is not possible to release C upon priming. However, the different stabilisation agents would induce contrasting potential C mineralisation and contrasting response to priming in deeper horizons. | |
