Short-term facilitation of microbial litter decomposition by ultraviolet radiation

Abstract:

In arid and semi-arid ecosystems, solar radiation plays an important role in C cycling by increasing the decomposition rate of plant litter and soil organic matter (i.e. photodegradation). Recent studies suggest that exposure to solar radiation can facilitate microbial decomposition of litter by altering litter chemistry and consequently litter degradability. However, it remains unclear at what time scale this facilitation mechanism operates. Does radiation exposure during the day facilitate microbial decomposition at night? In a laboratory jar incubation experiment, a common grass litter was exposed to either a cycle of alternating ultraviolet (UV) radiation and dark phases (2 days in UV phase and 2 days in dark phase) or a continuous dark treatment in order to examine the impacts of UV radiation on CO₂ production. The litter samples were also incubated under either wet or dry conditions to separate the contributions of abiotic photochemical and microbial processes to CO₂ production. Under wet conditions, radiation regime did not affect CO₂ production for the first half of the 4-month experiment; interestingly, in the second half of the experiment, wet litter in the alternating treatment produced at least 15% more CO₂, regardless of UV or dark phase, than wet litter in the continuous dark treatment. This difference in CO₂ production was significant even after accounting for higher temperature in the alternating treatment that may have stimulated microbial respiration or generated thermal CO₂ production. Under dry conditions, litter in the alternating treatment produced 1.6 times more CO₂ than litter in the continuous dark treatment, which we interpret to be abiotic photochemical CO₂ production. However, this abiotic flux was minimal compared to the microbial respiration flux, as cumulative CO₂ production from dry litter was less than 1% of that from wet litter. Overall, these results confirm that UV radiation does facilitate microbial decomposition at a diel scale. This diel facilitation mechanism is likely to be more important than abiotic processes in explaining the enhanced litter decay rate reported by previous photodegradation studies.