Separating drought effects from roof artifacts on ecosystem processes in a grassland drought experiment

1: Given the predictions of increased drought probabilities under various climate change scenarios, there have been numerous experimental field studies simulating drought using transparent roofs in different ecosystems and regions. Such roofs may, however, have unknown side effects, called artifacts, on the measured variables potentially confounding the experimental results. A roofed control allows the quantification of potential artifacts, which is lacking in most experiments. 2: We conducted a drought experiment in experimental grasslands to study artifacts of transparent roofs and the resulting effects of artifacts on ecosystems relative to drought on three response variables (aboveground biomass, litter decomposition and plant metabolite profiles). We established three drought treatments, using (1) transparent roofs to exclude rainfall, (2) an unroofed control treatment receiving natural rainfall and (3) a roofed control, nested in the drought treatment but with rain water reapplied according to ambient conditions. 3: Roofs had a slight impact on air (+0.14uC during night) and soil temperatures (20.45uC on warm days, +0.25uC on cold nights), while photosynthetically active radiation was decreased significantly (216%). Aboveground plant community biomass was reduced in the drought treatment (241%), but there was no significant difference between the roofed and unroofed control, i.e., there were no measurable roof artifact effects. 4: Compared to the unroofed control, litter decomposition was decreased significantly both in the drought treatment (226%) and in the roofed control treatment (218%), suggesting artifact effects of the transparent roofs. Moreover, aboveground metabolite profiles in the model plant species Medicago x varia were different from the unroofed control in both the drought and roofed control treatments, and roof artifact effects were of comparable magnitude as drought effects. 5: Our results stress the need for roofed control treatments when using transparent roofs for studying drought effects, because roofs can cause significant side effects

Nutrient dynamics in river bed sediments: effects of hydrological disturbances using experimental flow manipulations

International audiencePurpose River sediments play a crucial role in the storage and transformation of organic matter (OM). Nutrient dy- namics are controlled by the interaction of several key parameters, i.e. river discharge, channel geometry and ver- tical exchanges of water (upwelling vs. downwelling zones). The main aim of this study was to evaluate the effect of channel forms and discharge variation on nutrient spiralling in the hyporheic zone (HZ) of streams. Materials and methods Four experimental flow manipula- tions (EFM) were carried out at two reaches with different channel forms (straight vs. sinuous) in an oligotrophic sub- tropical river in Australia. Flow manipulation consisted of reducing the river width with a temporary dam, diverting and concentrating the main water flux on two different geomorphological units (riffle vs. gravel bar), in order to simulate flooding conditions. Hyporheic waters were ana- lysed for their physicochemical characteristics and nutrient (nitrates + nitrites 0 NOx and soluble reactive phosphorus [SRP]) and OM contents at two depths (10 and 50 cm) within the bed sediments, both upstream and downstream of the geomorphological units. Results and discussion The physicochemical parameters clearly demonstrated the existence of hyporheic flow paths, characterized by the alternation of downwelling and upwell- ing areas, with more consistent gradients in gravel bars than in riffles. The HZ acted as source for NOx and SRP, but this role varied between geomorphological units and reaches. The effect of EFM differed between sampling points, irre- spective of the type of geomorphological unit. In gravel bars, a flush out during high discharge was observed for NOx, SRP and particulate organic matter (POM) at the sinuous channel, whereas storage and removal were recorded at the straight channel for SRP and NOx, respec- tively. At the riffle of the sinuous channel, very fine POM accumulated, while removal was noticed for POM. In con- trast, at the riffle of the straight channel, SRP accumulated in the HZ and NOx was removed out of the HZ. Conclusions Nutrient dynamics in the HZ and the response to flow increases were not governed by the geomorphological unit type. Other parameters that determine water residence time in the sediments, such as local heterogeneity in sediment characteristics (grain size, porosity and hydraulic conductivi- ty), channel sinuosity, reach slope and the size and form of the gravel bar, may be more significant explanatory variables for understanding OM and nutrient dynamics in the HZ. This study emphasizes the need for caution in making generalisa- tions about the role of river sediment in nutrient storage and the impact of floods on nutrient dynamics