Temperature-induced diurnal redox potential in soil.

With the capabilities to measure redox potentials (EH) at a high temporal resolution, scientists have observed diurnal EH that occur in a distinct periodicity in soils and sediments. These patterns have been disregarded for a long time because minor fluctuations of the EH in the tens of mV range are difficult to interpret. Various explanations have been proposed for the origin of diel EH but a cohesive assessment of the temperature-dependency for field- and laboratory-based investigations is missing at present. In this study, we investigated spatiotemporal diel EH of previous long-term (up to 10 years) field- and lab-based monitoring data collected at high-temporal (every hour) and spatial (up to 6 depths) resolution. In addition, we set up a redox experiment where we manipulated the soil temperature (ST) by diel temperature cycles to assess the EH response. Diel fluctuations were absent for laboratory experiments with ΔEH of a few mV (daily EH-max - daily EH-min), but we found pronounced fluctuations up to ∼100 mV for field investigations. The spatiotemporal pattern in EH fluctuations was amplified in the topsoil during the summer months concomitant with ST. We showed for the first time that changes in ST during an incubation experiment altered the EH by -3.3 mV °C-1 and inferred that the diel EH were driven by the thermal conditions of the soil itself. This is particularly important when EH is measured close to the soil surface and underlines that minor fluctuations of the EH with a recurring periodicity should be carefully checked for its dependency with the soil and reference electrode temperature. Redox measurements should not be considered a routine determination and cautious handling of EH data by physical sound corrections is urgently needed in order to link ΔEH to daily biogeochemical cycling in soils.

Impact of Spatial Soil and Climate Input Data Aggregation on Regional Yield Simulations.

We show the error in water-limited yields simulated by crop models which is associated with spatially aggregated soil and climate input data. Crop simulations at large scales (regional, national, continental) frequently use input data of low resolution. Therefore, climate and soil data are often generated via averaging and sampling by area majority. This may bias simulated yields at large scales, varying largely across models. Thus, we evaluated the error associated with spatially aggregated soil and climate data for 14 crop models. Yields of winter wheat and silage maize were simulated under water-limited production conditions. We calculated this error from crop yields simulated at spatial resolutions from 1 to 100 km for the state of North Rhine-Westphalia, Germany. Most models showed yields biased by <15% when aggregating only soil data. The relative mean absolute error (rMAE) of most models using aggregated soil data was in the range or larger than the inter-annual or inter-model variability in yields. This error increased further when both climate and soil data were aggregated. Distinct error patterns indicate that the rMAE may be estimated from few soil variables. Illustrating the range of these aggregation effects across models, this study is a first step towards an ex-ante assessment of aggregation errors in large-scale simulations.

Field scale boscalid residues and dissipation half-life estimation in a sandy soil.

The aim of this study was to analyze the environmental fate of the fungicide boscalid in a sandy soil. Boscalid was applied in spring 2010/11 to a cropland site in western Germany. Three years after second application 65 undisturbed soil samples were taken. Boscalid was extracted using accelerated solvent extraction (ASE). Boscalid contents in the plough horizon ranged between 0.12 and 0.53 with a field mean of 0.20 ± 0.09 µg kg(-1). These contents were considerably lower compared to calculation using literature DT50 values, whereby a concentration of 16.89 µg kg(-1) was expected assuming a literature DT50 value of 345 days. Therefore, the measured field boscalid concentration only yields 1.2% of the expected value. To test whether the unknown extraction efficiency, losses from spray drift and interception can explain the mismatch between calculated and measured concentrations all these uncertainties were taken into account into calculations, but field concentrations and DT50 were still lower as expected. Leaching to deeper horizons was also studied but could not explain the discrepancy either. Moreover, a short-term incubation experiment using (14)C labelled boscalid revealed also shorter DT50 values of 297-337 compared to the 345 days taken from literature. However, this DT50 value is still considerably larger compared to the 104-224 days that were calculated based on the field experiment. Our results indicate that boscalid dissipation under field conditions is much faster at agricultural sites with sandy soil type as expected from laboratory incubation experiments.

Comment on "Global convergence in the temperature sensitivity of respiration at ecosystem level".

Mahecha et al. (Reports, 13 August 2010, p. 838) estimated the temperature sensitivity of ecosystem respiration (Q(10)) and showed that temperature sensitivity and its site-to-site variability are lower than previously reported. We demonstrate that their Q(10) value of 1.4 is an underestimate if interpreted as the averaged sensitivities of all ecosystem components, because fast temperature fluctuations penetrate poorly into the soil.

Imaging water fluxes in porous media by magnetic resonance imaging using D(2)O as a tracer.

In this study, we investigate the usefulness of D(2)O as a conservative tracer for monitoring water flux by MRI in a heterogeneous sand column. The column consisted of a cylindrical 3x9-cm packing of fine sand in which an 8-mm diameter cylindrical obstacle was placed. Constant steady-state flux densities between J(w)=0.07 and 0.28 cm min(-1) corresponding to mean pore flow velocities between 0.20 and 0.79 cm min(-1) were imposed at the top of the sand column, and a constant hydraulic head of -39 cm was maintained at the lower boundary. We injected pulses of 0.01 M NiCl(2) and 55% D(2)O and monitored the motion of the tracer plumes by MRI using a fast spin echo sequence over a period of 20 min. We observed that the center of gravity of all plumes moved with the mean pore flow velocity, which showed that D(2)O behaves as a conservative tracer. The motion of the tracer plume at J(w)=0.14 cm min(-1) was validated by a numerical simulation using HYDRUS2D, which reproduced the experimentally observed behavior very satisfactorily.