Plant litter decomposition constitutes one of the largest fluxes in the global carbon cycle releasing ≈68Pg C y-1 into the atmosphere. In arid and semi-arid systems, which account for ≈43% of the world’s land area, litter decomposition rates are systematically underestimated by up to 30%, leading to a large part of the carbon budget being unaccounted for. Recent research has highlighted some of the potential mechanisms which lead to this underestimation but fails
to elucidate how the various mechanisms interact. I investigated this by utilizing a fully factorial experimental spanning both the dry and wet seasons, manipulating UV by filtering, temperature and humidity using Open Top Chambers and standardising wet season water input by spraying with deionised water. I demonstrate that, in the absence of precipitation, abiotic degradation (chiefly photodegradation, thermal decomposition and leaching) throughout the
dry season contribute significantly to litter decomposition with litter mass loss of 60%. Photodegradation forms both diurnal and seasonal feedback-loops with microbial activity which are either sustained by night-time humidity/dew adsorption, rainfall or (artificial) watering. I estimated that the main mechanisms of litter decomposition over the dry period are thermal degradation that contributed more than 50% to litter mass loss while photodegradation
contributed only 10%. The combined thermal and fungal degradation led to substantial decomposition of the soluble cell fraction (the most labile carbon) (59.9 ± 0.6% reduction). Despite the small contribution of photodegradation to overall decomposition, exposure to UV light led to a significant reduction in hemicellulose content by 26.30%, but had only a small, non-significant effect on mass loss (3.69%). The results indicate, that despite reduced dry season microbial decomposition in the filtered treatments there was a seasonal priming effect due to UV light exposure. Besides priming effects, dry season decomposition caused a shift in the fractions of the cell being decomposed. Due to the ubiquitous consumption of labile carbon throughout the dry season and the shift in dominant processes from abiotic to biotic, wet season decomposition preferentially degraded the more recalcitrant compounds such as hollocellulose.
Warming led to an increase in microbial decomposition by 26%, in photodegradation by 3% and a decrease in the relative influence of thermal decomposition by 28%. This study finds that the unexpectedly high dry season relative decomposition rate is a consequence of the strength of the feedback loops between abiotic and biotic mechanisms of decomposition. It highlights the necessity to approach dryland litter decomposition with a integrative view if we are to accurately predict litter decomposition rates and estimate carbon
budgets.