Sunlight Doubles Aboveground Carbon Loss in a Seasonally Dry Woodland in Patagonia

Abstract

Photodegradation of aboveground senescent plant material (plant litter) due to exposure to solar radiation has been identified as a dominant control on carbon (C) loss in semiarid ecosystems [1], upturning traditional models of C cycling based only on available moisture and litter quality parameters. In addition to the photochemical mineralization of organic matter, sunlight alters the chemistry of cell walls in plant litter, making them more susceptible to subsequent biotic degradation. Nevertheless, the interactive effects of sunlight exposure, climate seasonality and biotic decomposition on C turnover remain unresolved in terrestrial ecosystems. We show here that accelerated litter decomposition in a Patagonian woodland is explained by the exposure of plant litter to sunlight, coupled with a marked dry summer season. The initial controls of decomposition varied seasonally from direct photochemical mineralization in the dry summer to biotic degradation in the wet winter. Nevertheless, manipulation of sunlight received by plant litter using spectral filters demonstrated that direct photodegradation and its legacy, associated with increased microbial access to labile carbohydrates, are responsible for the acceleration of aboveground C turnover in this mediterranean-type climate. Across plant species, exposure to solar radiation doubled rates of C loss to the atmosphere. Changes in vegetation cover or biodiversity due to projected increased drought and dry season length will likely exacerbate C losses from aboveground litter due to sunlight exposure, negatively impacting the C balance in ecosystems that are particularly vulnerable to global change.