Effects of UV radiation and rainfall reduction on leaf and soil parameters related to C and N cycles of a Mediterranean shrubland before and after a controlled fire

Abstract

In the Mediterranean basin, reduction in cloudiness owing to climate change is expected to enhance solar ultraviolet (UV) levels and to decrease rainfall over the coming years, which would be accompanied by more frequent and intense wildfires. The aim of the present study was to investigate the role of solar UV-A and UV-B radiation in C and N pools of a Mediterranean shrubland and whether drier conditions could alter this role before and after a fire. Methods: Over a three-year field experiment, 18 plots of 9 m 2 were subjected to three UV conditions (UV-A + UV-B exclusion, UV-B exclusion or near-ambient UV-A + UV-B exposure) combined with two rainfall regimes (natural or reduced rainfall). Several parameters related to C and N cycles in the soil and in the leaves and litter of two dominant plant species (Arbutus unedo and Phillyrea angustifolia) were measured before and after an experimental fire. Results: UV-A exposure increased soil moisture throughout the study period, as well as respiration before the fire. The additional presence of UV-B decreased β-glucosidase activity at 5–10 cm depth and soil respiration and pH. UV-B exposure also raised leaf C concentration in P. angustifolia and δ 15 N values in A. unedo. Reduced rainfall often emphasized the opposite effects of UV-A and UV-B on the studied parameters. After the fire, most of the UV and rainfall effects were lost. Conclusion: UV-A exposure seems to stimulate soil biological activity and, thus, C and N turn-over, while the effect of UV-B would be the opposite. At least in the short term, the “homogenizing influence” of fire would probably have a stronger effect on the C and N cycles than the expected changes in UV and rainfall levels