Sunlight can be an important driver of plant litter decomposition via photodegradation. Leaching from plant litter during precipitation events is another potential driver of decomposition. We explored whether exposure to sunlight primed litter such that it enhanced potential leaching losses from plant litter. In a previous study, leaf litter of 12 plant species (comprising 3 growth forms) was exposed to (1) full sunlight, (2) UV-filtered sunlight, or (3) UV- and blue-filtered sunlight for up to 21 months and mass loss was assessed. Litter exposed to full sunlight for 21 months lost the most mass, while litter filtered from UV and blue sunlight lost the least, illustrating that photodegradation by both UV and blue sunlight was a significant driver of litter mass loss. We used litter from that study to assess whether exposure to UV or UV and blue sunlight influenced subsequent leaching losses from litter. Litter was slowly stirred for 1 hour in 50 °C deionized water, and leaching losses were quantified by measuring mass loss. In addition to assessing how sunlight exposure influenced leaching losses, we also assessed how leaching losses changed with litter age or time in the field.
Leaching losses from litter were substantial and greater than anticipated, particularly from older litter. For example, leaching losses from litter that was not exposed to UV or blue sunlight for 20 months averaged 21% among litter types (range 7-35%). Leaching losses from litter exposed to full sunlight were even greater, averaging 29% (range 14-40%), illustrating that exposure to sunlight enhanced subsequent leaching losses. Leaching losses increased over time (from 2 to 21 months), especially in litter exposed to full sunlight. Hence, the view that soluble compounds are readily depleted, and not replaced during litter decay, may not apply in our arid system. Leaching losses from 21-month old litter were strongly correlated with the mass loss of that litter in the previous experiment; hence, the more mass lost from litter in the field, the more subsequently lost via leaching in our lab assay. In the context of predictions of more extreme precipitation events in the future, litter exposed to extended periods of sunlight punctuated by large precipitation events could experience faster decomposition via greater photodegradation as well as subsequent leaching losses.