C-Profor

It has recently been discovered that Amazonian podzols may store much larger amounts of carbon than previously thought, particularly in their deep Bh horizons (over 13.6 Pg for Amazonia alone, Montes et al., 2011). Similarly high carbon stocks are likely to exist in similar climate/soil areas, mainly in Africa and in Borneo. Such carbon stocks raise the problem of their stability in response to changes in land use or climate. Any significant changes in vegetation cover would significantly alter the soil water dynamics, which is likely to affect the organic matter turnover in soils. The direction of the change, however, is not clear and is likely to depend on the specific conditions and properties of the soils. Considering the Bh horizons of the equatorial podzols, it can be assumed that the drying of these generally saturated soils will lead to an increase in C mineralization, although the extent of this increase has not yet been determined. These unknowns resulted in research programs dedicated improving estimates of the Amazonian podzol carbon stock and to a rough estimate of its lability (ongoing projects FAPESP 2011/03350-2 & ARCUS PACA-Brasil 2010-2013). The first results issued from this study confirmed the high carbon stocks in podzols, as hypothesized in Montes et al., 2011. Although necessary, the final results expected from this study will still be insufficient to give a reliable estimation of podzol organic matter behaviour in a changing environment, whether it is due to local (land-use) or global (climate change) factors. In this context, the project objective is to quantify the rates of podzol organic matter mineralization under different scenarios of climate change towards drier or more variable moisture conditions. This requires (1) the quantification of podzol organic matter mineralization under alternating wet/dry cycles and (2) the modelling of water dynamics in podzols systems submitted to land use or climate changes. Point (1) will be addressed by quantifying the ability of soil microorganisms to metabolize the soil organic matter by monitoring mineralization at different matric potentials and under alternating wet/dry cycles. Point (2) will be addressed by modelling water dynamics (matric potential and groundwater level) under different land use / climate scenarios, using the knowledge of podzol system already obtained or being acquired by ongoing studies. Combining the results of points (1) and (2) will allow a refined modelling of the evolution of the podzol C stock. Additional data and validation of the long-term modelling will be sought by studying natural chronosequences, where Bh formed in the past under water-logging conditions and nowadays are exposed to alternate dry and humid pedoclimate. Such situation are found in Brazil when hydromorphic podzol area is incised by regressive erosion. Situation where the whole podzol system is now submitted to dryer climate, however, are not fund in Brazil because of its paleoclimate history, but exist in Africa. The objective there is to compare organic matter quantity, average age and characteristics between still active podzols and paleopodzols. The project is also designed to stimulate the development of appropriate techniques and instrumentation with regard to (1) organic matter carbon release under controlled water matric potential and (2) fast and low-cost organic matter characterization by Excitation-Emission Fluorescence Matrix Spectroscopy and Laser Induced Fluorescence Spectroscopy on solid samples. The objective is to identify the pertinent information contained in the spectroscopic data with regard to a given organic matter property or behaviour. Once known, this relationship will assess the feasibility of small, fast, low-cost instrumental systems dedicated to specific measurements. Meanwhile, the expected results relating to the sensitivity of deep carbon to mineralization will help to evaluate land-use recommendations and forest protection measures.