The role of the organic layer for phosphorus nutrition of young beech trees (Fagus sylvatica L.) revealed by multi-isotopic labelling (P-33; H2O-18) at two sites differing in soil phosphorus availability

The accumulation of an organic layer in forests is linked to the ratio between litterfall and decomposition rates, the latter being decelerated due to acidification and associated nutrient depletion with proceeding ecosystem development. Nevertheless, the nutrient pool in the organic layer might still represent an important interim storage and source for phosphorus (P) nutrition of forests on nutrient-poor soils. Due to the retention of P in soil e.g., by sorption to sesquioxides, P-poor ecosystems tend to show P recycling by organic matter decomposition. Our objective was to assess the importance of the organic layer to P nutrition of young beech trees. We established a mesocosm experiment including plants and soil from two forest sites differing in P availability. In half of all pots comprising both sites, the organic layer was present while the organic layer was lacking in the other half. We applied P-33 and H2O-18 to the pots. After 0h, 24h, 48h, 96h and 192h we destructively harvested the young beech trees, sampled the organic layer and mineral soil. P-33 activity was measured for every compartment in soil and plant (xylem, leaves, branches, stems) whereas δO-18 values in phosphate(δO-18P) were assessed for soil only. For both sites, δO-18P values in resin-extractable P in soil were close to those expected if isotope fractionation during intracellular pyrophosphate storage and subsequent release takes place. Therefore, δO-18P values indicate that bioavailable P in both soils has been cycled through microorganisms. However, the absence of an organic layer at the P-poor site resulted in a considerable shift of δO-18P values from those to be expected if P has been cycled through microorganisms. For both sites, the presence of the organic layer increased P-33 activity in xylem sap compared to the treatment without (104% P-poor site, 700% P-rich site). The total P-33 activity in plant tissue in pots from the P-rich site was not affected by the presence or absence of an organic layer after 192h, whereas a strong increase of 155 kBq/g DM was recorded for the P-poor site if an organic layer was present. Therefore, the key role of the organic layer for plant P nutrition on a P-depleted site was highlighted by our multi-isotopic labelling approach. In conclusion, our results suggest that P mobilization strategies differ among sites i.e., a P recycling vs. a P acquiring strategy

Stable hydrogen isotope ratios in soil organic matter

Stable H isotope ratios are a promising indicator of OM transformation processes (Schimmelmann et al., 2006). δ2H values of bulk organic matter and of specific organic compounds can be used as ecological tracer and forensic tool if the proportion of H that readily exchanges with ambient moisture is accounted for (Wassenaar & Hobson, 1998). There are a few reports about the H isotope ratios in plant-soil systems illustrating that there is little knowledge of the controls of the isotopic composition of the non-exchangeable H fraction of bulk OM (Schimmelmann et al., 2006; Ruppenthal et al., 2015). The increasingly closer relationship between δ2H values of rainfall and of non-exchangeable H in OM (δ2Hn) in the order, plant – plant litter (above- and belowground) – soil along a climatic gradient (Ruppenthal et al., 2015) suggests that decomposition influences δ2Hn values in OM in a systematic way. However, there are knowledge gaps concerning the fractionation factors and the extent of incorporation of ambient water-H into the nonexchangeable fraction of H in OM during decomposition. Our research will focus on the mechanisms responsible for the strong correlation between δ2H values in rainfall and δ2Hn values of SOM. Therefore, our study aims to investigate (1) the incorporation of ambient water-H into the nonexchangeable H fraction in OM during decomposition by heterotrophic bacteria as model organisms and quantify apparent fractionation factors, (2) the extent of incorporation of ambient water into the nonexchangeable H fraction of OM by the soil microbial community under laboratory conditions, and (3) the extent to which H is incorporated into nonexchangeable OM pool from ambient water during decomposition of aboveground litter under field conditions. We will work with microcosms using two bacteria species and determine decomposition rates of litter. Steam equilibration (Ruppenthal et al., 2015) and TC/EA-IRMS are used as analytical tools. We expect that different decomposition rates because of differences in litter quality will be reflected by the extent of H incorporation from ambient water into the nonexchangeable H fraction of the products. Additionally, different litter types enriched in 2H will be buried in soil of forest stands. We hypothesize that the incorporation of 2H-depleted ambient water into 2H-enriched nonexchangeable H fraction of OM will depend on litter type, soil moisture/ temperature, and the heterotrophic activity during the experiment

Stable hydrogen isotope ratios in crystal water of clay minerals

Hydrogen is the most abundant element in the Universe. But the utilization of the H isotopic composition (δH-2 value) of soil to elucidate biogeochemical processes or to serve as a palaeo climate proxy is still in its infancy. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. The δH-2 value of structural H in clay minerals – mainly from C-poor subsoils - has been studied since the 1970s. The δH-2 value of clay minerals mainly depends on (a) the average δH-2 value of ambient water at the site and time of formation, and on (b) the size of the equilibrium isotopic fractionation factor between water and clay mineral at the temperature of formation. In our research, we will focus on the δH-2 value of nonexchangeable H in the clay fraction of soils. Only nonexchangeable H in in structural water of minerals preserves its inherited δH-2 value and does not exchange with water at temperatures usually occurring in soil environments at the Earth’s surface. Nonexchangeable H is bound in crystal water, which integrates the δH-2 value of soil water over several millennia. This is in turn determined by palaeoclimatic variations of the precipitation’s δH-2 signal with distinguishable shifts e.g., from Pleistocene to Holocene. For a global data set, Ruppenthal (2014) reported a close correlation of bulk soil δH-2 values with those of the mean local precipitation and confirmed this for organic matter, while the clay fraction of soils was up to now not studied. We will adapt a steam equilibration method with water vapor of known H isotopic composition – formerly applied by Ruppenthal (2014) on SOM and bulk soil – to clay fractions and compare our results to the hitherto used heating treatments (200-250°C) under vacuum. We expect that the δH-2 signal of the clay fraction of Bt horizons will serve to differentiate soils developed in different climatic epochs (e.g., Holocene, last interstadial, last interglacial) by analyzing dated palaeo soil samples. To test the hypothesis that there is a similar global regression line of the δH-2 values in structural water of clay as up to now reported for bulk soils and soil organic matter, we will analyze the clay fraction in a global set of soil samples

15N tracing to elucidate links between biodiversity and nitrogen cycling in a grassland experiment

Nitrogen (N) cycling is a fundamental ecosystem function of high complexity because N undergoes many transformations in soil and vegetation. The effect of biodiversity loss on ecosystem functions in general, and on N cycling in particular, was studied in several manipulative field experiments. To generate a comprehensive view of the influence of species richness on all major N transformations, we conducted laboratory incubations, in which we added 15N-labeled ammonium and nitrate to soil samples of the “Jena Experiment”, a manipulative large scale, long-term biodiversity experiment in grassland. The experimental site is located in Jena, Germany. The design consists of 4 blocks and 82 plots with 1-60 species and 1-4 functional groups (grasses, legumes, small herbs, tall herbs). Approx. 400 g of field-fresh soil was sampled from each plot of one of the 4 blocks and divided into three aliquots of 100 g each. In order to trace N turnover, we amended the incubations (in triplicate) either with 15N-labelled (98 at%) ammonium, nitrate, or with a mixture of both. The samples were incubated for two months at 20°C. Soil solution was extracted 1, 2, 4, 9 and 16 days after 15N application by percolating 100 mL of nutrient solution through each vessel. Concentrations of NH4-N, NO3-N and total N in the extracts were determined with colorimetric methods. The N-isotopic composition in nitrate was analyzed by isotope ratio mass spectrometry (IRMS) using the denitrifier method. Ammonium N isotope ratios were determined using the “hypobromite oxidation” method, in which ammonium-N is converted to nitrite followed by azide reaction to nitrous oxide and IRMS analysis. The results will be comprehensively evaluated in a quantitative context using the modelling approach of Müller et al. (2007) to determine the size of six N pools and the rates of nine N transformations. Links between N transformation rates, N-pool size and plant species richness will be verified with the help of ANOVA

Controlling molecular weight and polymer architecture during the Passerini three component step-growth polymerization

A new approach to control the molecular weight and polymer architecture using the Passerini three-component step-growth polymerization is described. Starting from an AB-type monomer, linear homopolymers, diblock copolymers, as well as star-shaped polymers were synthesized in an efficient manner. By varying the ratio of the AB-type monomer and a suitable irreversible chain transfer agent (ICTA), different polymer architectures with specific molecular weights and high end-group fidelity were obtained

Synthesis of Passerini-3CR Polymers and Assembly into Cytocompatible Polymersomes

The versatility of the Passerini three component reaction (Passerini‐3CR) is herein exploited for the synthesis of an amphiphilic diblock copolymer, which self‐assembles into polymersomes. Carboxy‐functionalized poly(ethylene glycol) methyl ether is reacted with AB‐type bifunctional monomers and tert‐butyl isocyanide in a single process via Passerini‐3CR. The resultant diblock copolymer (P1) is obtained in good yield and molar mass dispersity and is well tolerated in model cell lines. The Passerini‐3CR versatility and reproducibility are shown by the synthesis of P2, P3, and P4 copolymers. The ability of the Passerini P1 polymersomes to incorporate hydrophilic molecules is verified by loading doxorubicin hydrochloride in P1DOX polymersomes. The flexibility of the synthesis is further demonstrated by simple post‐functionalization with a dye, Cyanine‐5 (Cy5). The obtained P1‐Cy5 polymersomes rapidly internalize in 2D cell monolayers and penetrate deep into 3D spheroids of MDA‐MB‐231 triple‐negative breast cancer cells. P1‐Cy5 polymersomes injected systemically in healthy mice are well tolerated and no visible adverse effects are seen under the conditions tested. These data demonstrate that new, biodegradable, biocompatible polymersomes having properties suitable for future use in drug delivery can be easily synthesized by the Passerini‐3CR

Historicising Material Agency: from Relations to Relational Constellations

Relational approaches have gradually been changing the face of archaeology over the last decade: analytically, through formal network analysis; and interpretively, with various frameworks of human-thing relations. Their popularity has been such, however, that it threatens to undermine their relevance. If everyone agrees that we should understand past worlds by tracing relations, then ‘finding relations’ in the past becomes a self-fulfilling prophecy. Focusing primarily on the interpretive approaches of material culture studies, this article proposes to counter the threat of irrelevance by not just tracing human-thing relations, but characterising how sets of relations were ordered. Such ordered sets are termed ‘relational constellations’. The article describes three relational constellations and their consequences based on practices of fine ware production in the Western Roman provinces (first century BC – third century AD): the fluid, the categorical, and the rooted constellation. Specifying relational constellations allows reconnecting material culture to specific historical trajectories, and offers scope for meaningful cross-cultural comparisons. As such a small theoretical addition based on the existing toolbox of practice-based approaches and relational thought can impact on historical narratives, and can save relational frameworks from the danger of triviality.This is the author accepted manuscript. The final version is available from Springer via http://dx.doi.org/10.1007/s10816-015-9244-

Temporal and small-scale spatial variation in grassland productivity, biomass quality, and nutrient limitation

Characterization of spatial and temporal variation in grassland productivity and nutrition is crucial for a comprehensive understanding of ecosystem function. Although within-site heterogeneity in soil and plant properties has been shown to be relevant for plant community stability, spatiotemporal variability in these factors is still understudied in temperate grasslands. Our study aimed to detect if soil characteristics and plant diversity could explain observed small-scale spatial and temporal variability in grassland productivity, biomass nutrient concentrations, and nutrient limitation. Therefore, we sampled 360 plots of 20 cm × 20 cm each at six consecutive dates in an unfertilized grassland in Southern Germany. Nutrient limitation was estimated using nutrient ratios in plant biomass. Absolute values of, and spatial variability in, productivity, biomass nutrient concentrations, and nutrient limitation were strongly associated with sampling date. In April, spatial heterogeneity was high and most plots showed phosphorous deficiency, while later in the season nitrogen was the major limiting nutrient. Additionally, a small significant positive association between plant diversity and biomass phosphorus concentrations was observed, but should be tested in more detail. We discuss how low biological activity e.g., of soil microbial organisms might have influenced observed heterogeneity of plant nutrition in early spring in combination with reduced active acquisition of soil resources by plants. These early-season conditions are particularly relevant for future studies as they differ substantially from more thoroughly studied later season conditions. Our study underlines the importance of considering small spatial scales and temporal variability to better elucidate mechanisms of ecosystem functioning and plant community assembly