Canopy structure and phenology modulate the impacts of solar radiation on C and N dynamics during litter decomposition in a temperate forest

Decomposition of plant organic matter plays a key role in the terrestrial biogeochemical cycles. Sunlight has recently been identified as an important contributor to carbon [C] turnover through photodegradation, accelerating decomposition even in forest ecosystems where understorey solar irradiance remains relatively low. However, it is uncertain how C and nutrients dynamics respond to fluctuations in solar spectral irradiance caused by canopy structure (understorey vs. gaps) and season (open vs. closed canopy phenology). Spectral-attenuation treatments were used to compare litter decomposition over eight months, covering canopy phenology, in a temperate deciduous forest and an adjacent gap. Exposure to the full spectrum of sunlight increased the loss of litter C and lignin by 75% and 64% in the forest gap, and blue light was responsible for respectively 27% and 42% of that loss. Whereas in the understorey, C and lignin loss were similar among spectral-attenuation treatments over the experimental period, except prior to and during spring canopy flush when exposure to the full spectrum of sunlight promoted C loss by 15% overall, 80% of which was attributable to ultraviolet-B (UV-B) radiation. Nitrogen [N] was immobilized in the understorey during canopy flush before the canopy completely closed but N was swiftly released during canopy leaf-fall. Our study suggests that blue-driven photodegradation plays an important role in lignin decomposition and N dynamics in canopy gaps, whereas seasonal canopy phenology affecting sunlight reaching the forest floor drastically changes patterns of C and N in litter during decomposition. Hence, including sunlight dynamics driven by canopy structure and phenology would improve estimates of biogeochemical cycling in forests responding to changes in climate and land-use.

Automated summary

Decomposition of plant organic matter is crucial in terrestrial biogeochemical cycles. Sunlight has been found to accelerate carbon turnover through photodegradation in forest ecosystems. The impact of canopy structure and seasonal changes on solar irradiance remains uncertain, highlighting the need for improved estimates of biogeochemical cycling in forests.