Effects of simulated drought and nitrogen fertilizer on plant productivity and nitrous oxide (N2O) emissions of two pastures

Aims: As a consequence of global climate change, increases in the frequencies and severities of drought are anticipated for many parts of the world. Soil moisture and nitrogen (N) are among the major factors limiting grassland productivity. In pastures, N fertilizer returns by grazing animals are spatially and temporally heterogenous, and we therefore hypothesized that responses of plants and soil processes to drought may differ at the patch level. Methods: Using rain-exclusion roofs, we simulated severe summer drought in a three-year field experiment replicated at two grassland sites contrasting in climate and management intensity. The study included a factorial N application treatment encompassing the application of cattle urine and mineral nitrogen. Responses of plants, soil microbes, and soil organic matter were assessed (carbon and nitrogen pools). N2O emissions were measured on 72 dates, and soil N2O concentration profiles on 44 dates. Results: Plant productivity responded negatively to drought and positively to N application. Interestingly, no or only small drought-effect were found on plant productivity when cumulated over the entire experimental duration, despite large effects during and shortly after the period when rain-exclusion roofs were installed. We further did not find evidence for compensatory growth after drought, and drought-effects did not differ between fertilizer hot spots and unaffected areas. In the short-term, soil microbial biomass responded positively to drought, but no long-term effects were detected. Nitrous oxide (N2O) emissions originated primarily from fertilizer hot spots, and these emissions were massively reduced under drought, with effects lasting throughout most of the growing season. On a growing season basis, N2O emissions were estimated to be 1 to 2 orders of magnitude lower under drought. Conclusions: Overall, our data suggest that even severe summer drought may have relatively little effect on plant productivity in the type of grassland and climate investigated, at least when considered on an annual basis. In contrast, drought may result in a large and sustained reduction of N2O emission

A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization

Oxidation by soil bacteria is the only biological sink for atmospheric methane (CH4). There are substantial uncertainties regarding the global size of this sink, in part because the ecological controls of the involved processes are not well understood to date. We have investigated effects of severe summer drought and of nitrogen inputs (ammonium nitrate or cattle urine) on soil CH4 fluxes in a field experiment. Soil moisture was the most important factor regulating the temporal dynamics of CH4 fluxes. Simulated drought episodes altered the soil's water balance throughout the year, increasing CH4 oxidation by 50% on an annual basis. N fertilizers exerted only small and transient effects at the ecosystem level. Laboratory incubations suggested that effects differed between soil layers, with larger effects of drought and N application in the top soil than in deeper layers. With soil moisture being the primary controlling factor of methanotrophy, a detailed understanding of the ecosystem's water balance is required to predict CH4 budgets under future climatic condition

Erratum to: Effects of drought and N-fertilization on N cycling in two grassland soils

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH4NO3 application increased soil NH4 + and NO3 − concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N2O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH4NO3 reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N2O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N2O emissions in a changing climat

Quasi-homogenous photocatalysis of quantum-sized Fe-doped TiO2_{2} in optically transparent aqueous dispersions

In this study, the preparation of anatase TiO2 nanocrystals via a facile non-aqueous sol–gel route and their characterization are reported. The 3–4 nm particles are readily dispersable in aqueous media and show excellent photoreactivity in terms of rhodamine B degradation. The catalytic performance can be further increased considerably by doping with iron and UV-light irradiation as a pre-treatment. The effect of surface ligands (blocked adsorption sites, surface defects etc.) on the photoreactivity was thoroughly probed using thermogravimetric analysis combined with mass spectrometry. Photoelectrochemical characterization of thin-film electrodes made from the same TiO2 nanocrystals showed the opposite trend to the catalytic experiments, that is, a strong decrease in photocurrent and quantum efficiency upon doping due to introduction of shallow defect states

Interpolation in the Nevanlinna and Smirnov classes and harmonic majorants

31 pagesInternational audienceWe consider a free interpolation problem in Nevanlinna and Smirnov classes and find a characterization of the corresponding interpolating sequences in terms of the existence of harmonic majorants of certain functions. We also consider the related problem of characterizing positive functions in the disk having a harmonic majorant. An answer is given in terms of a dual relation which involves positive measures in the disk with bounded Poisson balayage. We deduce necessary and sufficient geometric conditions, both expressed in terms of certain maximal functions

The Sound of Batteries: An Operando Acoustic Emission Study of the LiNiO2_{2} Cathode in Li–Ion Cells

The development of advanced Li‐ion batteries relies on the implementation of high‐capacity Ni‐rich layered oxide cathode materials, such as NCM and NCA, among others. However, fast performance decay because of intrinsic chemical and structural instabilities hampers their practical application. Hence, thoroughly understanding degradation processes is crucial to overcome current limitations. To monitor instabilities of electrode materials under realistic operating conditions, the application of nondestructive operando techniques is required. While structural changes of crystalline phases can be studied by X‐ray diffraction, microstructural changes (e. g., particle fracture) cannot be easily accessed in situ and are therefore mostly investigated ex situ. Here, we use acoustic emission (AE) measurements to probe a potential next‐generation cathode material in real‐time. Specifically, we focus on LiNiO$_{2}$(LNO) and demonstrate that AE events in different frequency ranges can be correlated with the formation of the cathode solid‐electrolyte interphase and the mechanical degradation during electrochemical cycling

Do temporal and spatial heterogeneity modulate biodiversity–functioning relationships in com-munities of methanotrophic bacteria?

Positive relationships between biodiversity functioning have been found in communities of plants but also of soil microbes. The beneficial effects of diversity are thought to be driven by niche partitioning among community members, which leads to more complete or more efficient community-level resource use through various mechanisms. An intriguing related question is whether environmentally more heterogeneous habitats provide a larger total niche space and support stronger diversity—functioning relationships because they harbor more species or allow species to partition the available niche space more efficiently. Here, we tested this hypothesis by assembling communities of 1, 2 or 4 methanotrophic isolates and exposing them to temporally (constant or diurnal temperature cycling) and structurally (one or two aggregate size classes) more heterogeneous conditions. In total, we incubated 396 microcosms for 41 days and found that more biodiverse communities consumed more methane (CH4) and tended to have a larger community size (higher pmoA copy numbers). Diurnal temperature cycling strongly reduced CH4 oxidation and growth, whereas soil aggregate composition and diversity had no detectable effect. Biodiversity effects varied greatly with the identity of the community members that were combined. With respect to community level CH4 consumption, strain interactions were positive or neutral but never negative, and could neither be explained by 14 structural and function traits we collected or by the observed competitive hierarchy among the strains. Overall, our results indicate that methanotrophic diversity promotes methanotrophic community functioning. The strains that performed best varied with environmental conditions, suggesting that a high biodiversity is important for maintaining methanotrophic functioning as environmental conditions fluctuate over time