Redox potential (Eh) describes the electrical state of a matrix. In soils, Eh is an important parameter controlling the persistence of many organic and inorganic compounds. A popular, but also criticized, manual measuring method makes use of a small tip of Pt placed on a copper wire that is placed in the soil; a reference electrode is placed in the same soil at a fixed distance. Fluctuations in redox potential values measured in the soil can be very large and depth-dependent. This will be overlooked when making single-point measurements. We developed the datalogger Hypnos 2.0 for continuous redox potential and temperature measurements at various depths in the soil and without disturbance of the site. Hypnos is field-deployable, relatively cheap, and runs on batteries. The datalogger can use a "sleep mode" between sampling events. In sleep mode, there is no constant voltage on the Pt wire or the reference electrode, but there is only a short pulse during sampling. We did not measure an effect of this short pulse on the measured redox potential. In sandy soils in mesocosms and in a salt marsh soil we measured changes in the Eh as large as from -400 to +100 mV within 4 d, and daily cycles of 200 mV. Both absolute redox potential values and their diurnal variations were depth-dependent. Because single redox measurements are insufficient in describing redox conditions in some soil systems, Hypnos can be a powerful tool when studying the effects of fluctuating redox conditions on metal availability and pollutant degradation.
The concentrations of polycyclic aromatic hydrocarbons (PAHs) in the leaf wax of three Plantago species were determined weekly for 3 weeks. The almost glabrous, free-standing leaves of Plantago major and the sparsely hairy Plantago lanceolata leaves were more heavily contaminated with low molecular weight (MW) PAHs (MW < 228) than the densely hairy, partly overlapping Plantago media leaves. This may be caused by the lower canopy roughness (higher aerodynamic resistance), the higher amount of leaf hairs (higher boundary resistance), and/or the higher leaf overlap (smaller accessible leaf area) of P. media. On the other hand, PAHs with MW ≥ 252 tended to show higher concentrations in P. media than in the other two species. This is likely caused by the dense layer of hairs on P. media leaves, which can efficiently intercept the largely particle-bound high MW PAHs. When the PAH concentrations were normalized to projected leaf surface area, the differences between P. media and the other two species became significant (p < 0.05) for the high MW PAHs, while the differences for the low MW PAHs decreased. Although the differences in PAH concentrations between species are relatively small (factor 2-5), this study clearly shows that plant architecture and leaf hairs influence the dry deposition of PAHs.
A B S T R A C TFor the conservation and restoration of biodiverse rich fens, base-rich and nutrient-poor conditions are vital. In wetlands with artificially stable surface water levels, the re-introduction of temporary water level fluctuations has been postulated to restore the acid neutralizing capacity (ANC) of fens during inundation and to reduce surface water P-input during episodes with drought. This is the first study testing this hypothesis in large-scale field manipulation experiments in rich fens with threatened rich fen mosses, rich fens with Calliergonella, and poor fens with Sphagnum. Five different experiments were conducted: 2 weeks of raised levels (+10 cm) in a floating and a non-floating fen during winter, 2 weeks of high levels in a non-floating fen during summer, and 2 weeks of lowered levels (À15 cm) in a floating and a non-floating fen during summer. For floating fens, both lowered and raised surface water levels in adjacent ditches did not show any effect on water tables, soil ANC or nutrient levels in fens. For nonfloating fens, raised surface water levels led to inundation in all vegetation types, without affecting nutrient concentrations or vegetation. Although redox potentials decreased immediately in upper soils, ANC was generally not enhanced in winter due to limited infiltration into the waterlogged soils. In summer, in contrast, ANC increased because accelerated evapotranspiration led to enhanced infiltration of inundation water and higher temperatures resulted in microbial alkalinity generation. Short-term lowering of surface water levels in summer led to lower water tables in non-floating fens, but only when precipitation rates were low. Vegetation, ANC and nutrient concentrations were, however, not affected. The effectiveness of short-term surface water level fluctuation to restore ANC strongly depends on peat buoyancy, water saturation of soils, season and weather conditions. This explains why short-term inundation in winter is often inadequate, while short-term inundation in summer does increase ANC. Short-term droughts do not affect the ANC or nutrient availability. Our results are not only important for the hydrological management of fens, but also have implications for future management since shortterm extreme weather events will occur more frequently due to climate change.
The concentrations of polycyclic aromatic hydrocarbons (PAHs) in the leaf wax of three Plantago species were determined weekly for 3 weeks. The almost glabrous, free-standing leaves of Plantago major and the sparsely hairy Plantago lanceolata leaves were more heavily contaminated with low molecular weight (MW) PAHs (MW < 228) than the densely hairy, partly overlapping Plantago media leaves. This may be caused by the lower canopy roughness (higher aerodynamic resistance), the higher amount of leaf hairs (higher boundary resistance), and/or the higher leaf overlap (smaller accessible leaf area) of P. media. On the other hand, PAHs with MW ≥ 252 tended to show higher concentrations in P. media than in the other two species. This is likely caused by the dense layer of hairs on P. media leaves, which can efficiently intercept the largely particle-bound high MW PAHs. When the PAH concentrations were normalized to projected leaf surface area, the differences between P. media and the other two species became significant (p < 0.05) for the high MW PAHs, while the differences for the low MW PAHs decreased. Although the differences in PAH concentrations between species are relatively small (factor 2-5), this study clearly shows that plant architecture and leaf hairs influence the dry deposition of PAHs.
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