APPLICATION OF ISOTHERMAL CALORIMETRY TO MONITOR SOM TURNOVER IN AFFORESTED AND AGRICULTURAL SOIL CHRONOSEQUENCES N. Barros2*, M. Villanueva2, J. Proupín2, J. A. Rodríguez Añón2, M. T. Dell´Abate 1 1
Consiglio per la Ricerca e la sperimentazione in Agricoltura – Centro di ricerca per lo studio delle relazioni tra pianta e suolo (CRA-RPS); 2 University of Santiago de Compostela, Department of Applied Physics, Spain
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[email protected]
Modelling SOM dynamics requires highly reproducible and accurate methods to monitor the evolution and changes in SOM nature of mineral soils caused by land use change. This is especially important when measuring the impact of soil management on C sequestration capacity in mineral soils. C sequestration in mineral soils depends not only on the balances of C quantities but on the composition of SOM and its degradability. The most accurate way to evaluate SOM degradability is by direct measurement of microbial basal metabolism usually done by quantification of the CO2 associated to soil microbial respiration. But most of the CO2 released is associated to carbohydrates respiration and the last is only one of the fractions that usually takes part of the SOM macromolecule. The microbial degradation of the most recalcitrant SOM fractions release heat but not CO2. Therefore measurements performed in base on CO2 exclusively can yield limited information about the degradation of the SOM continuum. Isothermal calorimetry enables concomitant measurement of the heat and CO2 evolved by the SOM microbial degradation. Heat and CO2 are connected through the calorespirometric ratios. In living systems calorespirometric ratios depend on the nature of the substrate being degraded and on the carbon conversion efficiency of microorganisms when degrading carbohydrates. It constitutes an innovative approach in soil science that can contribute to improve the knowledge about SOM turnover by tracking the changes in the nature of SOM that is degraded instead of monitoring the evolution in C quantities. In this work, this method is applied to a set of soil chronosequences involving the conversion of pastures to forest in a humid tempered region in Spain, and to agriculture in the Arequipa desert, in Peru. It is aimed to follow the evolution of SOM with time after the land use change to see changes in the SOM nature with respect to those in undisturbed pastures. In the case of the afforested stands, calorespirometric ratios were sensitive to the tree species used, indicating degradation of SOM fractions considered more recalcitrant than carbohydrates in the samples afforested with Pinus radiata and degradation of carbohydrates in the stands with Eucaliptus nitens. In the agricultural chronosequence calorimetry provided enough sensitivity to detect microbial metabolism is soils with low C content. The calorespirometric ratios yield a SOM turnover based in very efficient metabolism of carbohydrates by the soil microbial population.
