EFFECT OF DRYING AND THAWING CYCLES ON SOIL CARBON SEQUESTRATION ALONG SOIL DEVELOPMENT GRADIENTS FORMED IN EXTREME ENVIRONMENTS OF CHILE.

Director: Francisco Matus

University: Universidad de La Frontera

Abstract

Climate change in Chile has increased the frequency and intensity of extreme events along the country and in extreme environments as well. For example, excessive rainfall at high elevation in the Andes in the northern regions or the ice melting and glacier retreat leading to a temporal gradient in landscape and new soil formation in subantarctic regions in the south. The Influence of these events on greenhouse gases (GHG), e.g. by freezing-thawing and drying-rewetting, soil C sequestration, temperature-sensitivity of soil organic matter (SOM) decomposition and priming effect (PE) (the acceleration of native SOM decay by fresh C input) have never been evaluated. We hypothesized that gradual SOM stabilization in presence of incipient clay formation, modulate the quantity and quality of organic C at relatively short time scale along temporal gradients of soil formed under different extreme environments such as found in the Altiplano in the extreme north and subantarctic climate in Cordillera Darwing in extreme south. The hypothesis also will be tested that microorganisms use exoenzymes (EXOMET) to decompose the SOM. Soil respiration occurs only in an intracellular environment, however CO₂ was detected in sterilized soil using other via for CO₂ production. We predict that drying thawing cycles will slow down the burst CO₂ in soils with more recalcitrant than labile SOM. Consequently, we expect a decrease of PE and increase in temperature-sensitivity of SOM decomposition according to Arrhenius law. This study aims to investigate the effects of (1) freeze-thaw and drying-rewetting (4 cycle) mainly focused on CO₂ at three temperatures 2/-18, 7/-18 and 12/-18 °C, (2) possible PE on decomposition of SOM in combination with temperature and moisture, 3) to examine the fraction of soil carbohydrates (neutral sugars) to know whether C is originated mainly from microbial or plant and 4) combination of temperature and moisture on CO₂ response on EXOMET. Soils from four pedogenic temporal gradients (Altiplano in the extremely north, Nahulebuta and Puyehue in central south and ice cover in Santa Inés island, Cordillera Darwin in extremely south) will be sampled during 3 campaigns. About 38 sampling points (152 samples) are regarded. Soil samples will be evaluated under controlled conditions where soil aggregation, temperature sensitivity, PE, microbial abundance and exoenzymatic activity depending on temperature and moisture will be measured. Neutral and amino sugars with stable isotope techniques will be used to evaluate microbial C and N utilization. We will explore the position-specific C labelling (compound-specific position 13C and 15N analysis) to study the metabolic pathway during SOM decomposition. We expect greater priming susceptibility, high temperature-sensitivity on recalcitrant SOM decomposition and adaptation of microbial community and exoenzymatic activity changes. The exchange and training of participating groups is the primary objective and will considerably broaden the analytical experience as well as the communication skills of PhD students. The combination of the expertise of participating institutions will allow broad development of multidisciplinary approaches to investigate the effects of freezing-thawing cycles on C sequestration in extreme environments.