Supplemental precipitation accelerates decay but only in photodegraded litter and implications that sunlight promotes leaching loss

While moisture availability exerts strong controls on biogeochemical processes in drylands, several studies have found no relationship between litter decay and precipitation. We examined whether interactions between precipitation and photodegradation could play a role in the inconsistent response of decay to precipitation. We assessed mass loss of 4 leaf litter types on the surface of the Sonoran Desert over 9.5 months in a full-factorial experiment consisting of 3 radiation treatments (full sunlight, filtered from photodegrading (UV/violet-blue) sunlight, complete shade) and 2 precipitation levels (ambient, supplemental (2.7 × ambient)). Exposure to photodegrading sunlight nearly doubled (i.e. 1.9) litter mass loss, and in combination with supplemental precipitation accelerated decay by 2.6 times. Surprisingly, supplemental precipitation had no influence on decay of litter that did not receive photodegrading sunlight. That photodegradation was required for supplemental precipitation to accelerate decay could explain past findings that decay is not related to precipitation, as most studies assessed litter shaded by plant canopies. Sunlight or precipitation treatments had no effect on litter microbial respiration, even though concentrations of water-soluble compounds were > 2 times greater in photodegraded litter. In a second experiment, we assessed the mass loss from litter attributable to a 30-min immersion in water. Immersion losses were 2.5 times greater from litter exposed to photodegrading sunlight for 20 months in the field than litter filtered from this sunlight, and these losses were strongly correlated with decay. Our findings demonstrate that photodegrading sunlight greatly accelerates litter decay, precipitation magnifies this effect, and leaching is likely a significant mechanism.