Litter decomposition plays a central role in carbon cycling in terrestrial ecosystems worldwide. In drylands, which cover 40% of the Earth’s land surface, photodegradation and biotic decomposition driven by non-rainfall moisture are important mechanisms of litter decay, though studies have only recently begun examining interactions between these two processes. We describe a novel priming mechanism in which photodegradation and biotic decay of the cuticle of plant litter increases litter absorption of non-rainfall moisture (fog, dew, and water vapor), supporting greater microbial decomposition.

We used several field experiments in a coastal fog desert and a series of in situ observations to demonstrate a relationship between solar radiation, cuticle integrity, water absorption rates, and mass loss.

Experimentally attenuating solar radiation for 36 months slowed mass loss, reduced cuticle degradation, and decreased litter moisture uptake relative to litter under ambient sunlight controls. In a separate field experiment, removing the cuticle of recently senesced grass tillers increased mass loss four-fold over 6 months relative to controls. Tillers with degraded cuticles also absorbed 3.8 times more water following an overnight dew event than did those with intact cuticles. Finally, fungal growth was consistently greater on the sun-facing side of in situ tillers than on the shaded side, coincident with greater cuticle degradation.

We present a conceptual model where the cuticle of plant litter acts as a water-resistant barrier that is first degraded by solar radiation and surficial microbes, increasing litter’s ability to absorb enough water during non-rainfall moisture events to support substantial biotic decomposition inside the tissue. Considering how photodegradation and non-rainfall moisture are both substantial drivers of litter decomposition in drylands, understanding how they interact under realistic field conditions will help us better predict how these systems are responding to changing climate regimes.