[1] Although land-water carbon (C) transport represents a critical link in the global C cycle, rare attempts have been made to compare hydrologic controls over storm pulses of dissolved organic C (DOC) and particulate organic C (POC) in mountainous watersheds. An immersible UV/Vis spectrophotometer was used to comparatively investigate the rapid storm responses of stream water DOC and POC in a small mountainous forested watershed in South Korea. High-frequency measurements at 5-min intervals during 42 hydrologic events, including monsoon storms and winter snowmelts, showed consistent patterns: POC concentrations were lower than DOC concentrations during base flow and small storm events but exceeded them during the peak flow periods of intense storm events. Although both the DOC and POC concentrations had hysteretic relationships with discharge, the POC concentrations showed larger increases and variations after crossing a threshold discharge on the rising limb of the storm hydrograph. Stronger responses to intense storms resulted in a disproportionately large export of POC at high flow, whereas a large portion of the total DOC flux was exported under prevailing low-flow conditions. The results demonstrate the potential of in situ optical measurements for investigating fine-resolution dynamics of the DOC and POC export during storm events. Stronger storm responses of the POC export compared to the limited response range of the DOC export suggest that erosion-induced POC export will become more important as a major pathway for the hydrologic soil C loss from mountainous watersheds in response to an increasing occurrence of extreme storm events.

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Abstract. At a local level, biogenic isoprene emissions can greatly affect the air quality of urban areas surrounded by large vegetation sources, such as in the Mediterranean region. The impacts of future warmer and drier conditions on isoprene emissions from Mediterranean emitters are still under debate. Seasonal variations of Quercus pubescens gas exchange and isoprene emission rates (ER) were studied from June 2012 to June 2013 at the O3HP site (French Mediterranean) under natural (ND) and amplified (AD, 32 %) drought. While AD significantly reduced stomatal conductance to water vapour throughout the research period excluding August, it did not significantly preclude CO2 net assimilation, which was lowest in summer (≈-1 µmolCO2 m−2 s−1). ER followed a significant seasonal pattern regardless of drought intensity, with mean ER maxima of 78.5 and 104.8 µgC gDM-1 h−1 in July (ND) and August (AD) respectively and minima of 6 and < 2 µgC gDM-1 h−1 in October and April respectively. The isoprene emission factor increased significantly by a factor of 2 in August and September under AD (137.8 and 74.3 µgC gDM-1 h−1) compared with ND (75.3 and 40.21 µgC gDM-1 h−1), but no significant changes occurred on ER. Aside from the June 2012 and 2013 measurements, the MEGAN2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) model was able to assess the observed ER variability only when its soil moisture activity factor γSM was not operating and regardless of the drought intensity; in this case more than 80 % and 50 % of ER seasonal variability was assessed in the ND and AD respectively. We suggest that a specific formulation of γSM be developed for the drought-adapted isoprene emitter, according to that obtained for Q. pubescens in this study (γSM= 0.192e51.93 SW with SW the soil water content). An isoprene algorithm (G14) was developed using an optimised artificial neural network (ANN) trained on our experimental dataset (ER + O3HP climatic and edaphic parameters cumulated over 0 to 21 days prior to the measurements). G14 assessed more than 80 % of the observed ER seasonal variations, regardless of the drought intensity. ERG14 was more sensitive to higher (0 to −7 days) frequency environmental changes under AD in comparison to ND. Using IPCC RCP2.6 and RCP8.5 climate scenarios, and SW and temperature as calculated by the ORCHIDEE land surface model, ERG14 was found to be mostly sensitive to future temperature and nearly insensitive to precipitation decrease (an annual increase of up to 240 % and at the most 10 % respectively in the most severe scenario). The main impact of future drier conditions in the Mediterranean was found to be an enhancement (+40 %) of isoprene emissions sensitivity to thermal stress.