Quantifying soil moisture impacts on light use efficiency across biomes.

Terrestrial primary productivity and carbon cycle impacts of droughts are commonly quantified using vapour pressure deficit (VPD) data and remotely sensed greenness, without accounting for soil moisture. However, soil moisture limitation is known to strongly affect plant physiology. Here, we investigate light use efficiency, the ratio of gross primary productivity (GPP) to absorbed light. We derive its fractional reduction due to soil moisture (fLUE), separated from VPD and greenness changes, using artificial neural networks trained on eddy covariance data, multiple soil moisture datasets and remotely sensed greenness. This reveals substantial impacts of soil moisture alone that reduce GPP by up to 40% at sites located in sub-humid, semi-arid or arid regions. For sites in relatively moist climates, we find, paradoxically, a muted fLUE response to drying soil, but reduced fLUE under wet conditions. fLUE identifies substantial drought impacts that are not captured when relying solely on VPD and greenness changes and, when seasonally recurring, are missed by traditional, anomaly-based drought indices. Counter to common assumptions, fLUE reductions are largest in drought-deciduous vegetation, including grasslands. Our results highlight the necessity to account for soil moisture limitation in terrestrial primary productivity data products, especially for drought-related assessments

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

Background and aims 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 m2 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 δ15N 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 UVand 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 UVand rainfall levels