Nicolas Brüggemann

Does photosynthesis affect grassland soil‐respired CO2 and its carbon isotope composition on a diurnal timescale?

Soil respiration is the largest flux of carbon (C) from terrestrial ecosystems to the atmosphere. Here, we tested the hypothesis that photosynthesis affects the diurnal pattern of grassland soil-respired CO(2) and its C isotope composition (delta(13)C(SR)). A combined shading and pulse-labelling experiment was carried out in a mountain grassland. delta(13)C(SR) was monitored at a high time resolution with a tunable diode laser absorption spectrometer. In unlabelled plots a diurnal pattern of delta(13)C(SR) was observed, which was not explained by soil temperature, moisture or flux rates and contained a component that was also independent of assimilate supply. In labelled plots delta(13)C(SR) reflected a rapid transfer and respiratory use of freshly plant-assimilated C and a diurnal shift in the predominant respiratory C source from recent (i.e. at least 1 d old) to fresh (i.e. photoassimilates produced on the same day). We conclude that in grasslands the plant-derived substrates used for soil respiratory processes vary during the day, and that photosynthesis provides an important and immediate C source. These findings indicate a tight coupling in the plant-soil system and the importance of plant metabolism for soil CO(2) fluxes.

Metabolic flux analysis of plastidic isoprenoid biosynthesis in poplar leaves emitting and nonemitting isoprene.

Isoprene biosynthesis demands a huge carbon flux through the plastidic isoprenoid pathway, and the concentration of its immediate precursor modulates this flux. The plastidic 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway is one of the most important pathways in plants and produces a large variety of essential isoprenoids. Its regulation, however, is still not well understood. Using the stable isotope 13C-labeling technique, we analyzed the carbon fluxes through the MEP pathway and into the major plastidic isoprenoid products in isoprene-emitting and transgenic isoprene-nonemitting (NE) gray poplar (Populus × canescens). We assessed the dependence on temperature, light intensity, and atmospheric [CO2]. Isoprene biosynthesis was by far (99%) the main carbon sink of MEP pathway intermediates in mature gray poplar leaves, and its production required severalfold higher carbon fluxes compared with NE leaves with almost zero isoprene emission. To compensate for the much lower demand for carbon, NE leaves drastically reduced the overall carbon flux within the MEP pathway. Feedback inhibition of 1-deoxy-d-xylulose-5-phosphate synthase activity by accumulated plastidic dimethylallyl diphosphate almost completely explained this reduction in carbon flux. Our data demonstrate that short-term biochemical feedback regulation of 1-deoxy-d-xylulose-5-phosphate synthase activity by plastidic dimethylallyl diphosphate is an important regulatory mechanism of the MEP pathway. Despite being relieved from the large carbon demand of isoprene biosynthesis, NE plants redirected only approximately 0.5% of this saved carbon toward essential nonvolatile isoprenoids, i.e. β-carotene and lutein, most probably to compensate for the absence of isoprene and its antioxidant properties.

Soil hydrology: Recent methodological advances, challenges, and perspectives

Technological and methodological progress is essential to improve our understanding of fundamental processes in natural and engineering sciences. In this paper, we will address the potential of new technological and methodological advancements in soil hydrology to move forward our understanding of soil water related processes across a broad range of scales. We will focus on advancements made in quantifying root water uptake processes, subsurface lateral flow, and deep drainage at the field and catchment scale, respectively. We will elaborate on the value of establishing a science-driven network of hydrological observatories to test fundamental hypotheses, to study organizational principles of soil hydrologic processes at catchment scale, and to provide data for the development and validation of models. Finally, we discuss recent developments in data assimilation methods, which provide new opportunities to better integrate observations and models and to improve predictions of the short-term evolution of hydrological processes.

Responses of belowground carbon allocation dynamics to extended shading in mountain grassland

Carbon (C) allocation strongly influences plant and soil processes. Short-term C allocation dynamics in ecosystems and their responses to environmental changes are still poorly understood. Using in situ 13CO2 pulse labeling, we studied the effects of 1 wk of shading on the transfer of recent photoassimilates between sugars and starch of above- and belowground plant organs and to soil microbial communities of a mountain meadow. C allocation to roots and microbial communities was rapid. Shading strongly reduced sucrose and starch concentrations in shoots, but not roots, and affected tracer dynamics in sucrose and starch of shoots, but not roots: recent C was slowly incorporated into root starch irrespective of the shading treatment. Shading reduced leaf respiration more strongly than root respiration. It caused no reduction in the amount of 13C incorporated into fungi and Gram-negative bacteria, but increased its residence time. These findings suggest that, under interrupted C supply, belowground C allocation (as reflected by the amount of tracer allocated to root starch, soil microbial communities and belowground respiration) was maintained at the expense of aboveground C status, and that C source strength may affect the turnover of recent plant-derived C in soil microbial communities.

A terrestrial observatory approach to the integrated investigation of the effects of deforestation on water, energy, and matter fluxes

Integrated observation platforms have been set up to investigate consequences of global change within a terrestrial network of observatories (TERENO) in Germany. The aim of TERENO is to foster the understanding of water, energy, and matter fluxes in terrestrial systems, as well as their biological and physical drivers. Part of the Lower Rhine Valley-Eifel observatory of TERENO is located within the Eifel National Park. Recently, the National Park forest management started to promote the natural regeneration of near-natural beech forest by removing a significant proportion of the spruce forest that was established for timber production after World War II. Within this context, the effects of such a disturbance on forest ecosystem functioning are currently investigated in a deforestation experiment in the Wüstebach catchment, which is one of the key experimental research sites within the Lower Rhine Valley-Eifel observatory. Here, we present the integrated observation system of the Wüstebach test site to exemplarily demonstrate the terrestrial observatory concept of TERENO that allows for a detailed monitoring of changes in hydrological and biogeochemical states and fluxes triggered by environmental disturbances. We present the observation platforms and the soil sampling campaign, as well as preliminary results including an analysis of data consistency. We specifically highlight the capability of integrated datasets to enable improved process understanding of the post-deforestation changes in ecosystem functioning.

Isoprene emission‐free poplars – a chance to reduce the impact from poplar plantations on the atmosphere

• Depending on the atmospheric composition, isoprene emissions from plants can have a severe impact on air quality and regional climate. For the plant itself, isoprene can enhance stress tolerance and also interfere with the attraction of herbivores and parasitoids. • Here, we tested the growth performance and fitness of Populus × canescens in which isoprene emission had been knocked down by RNA interference technology (PcISPS-RNAi plants) for two growing seasons under outdoor conditions. • Neither the growth nor biomass yield of the PcISPS-RNAi poplars was impaired, and they were even temporarily enhanced compared with control poplars. Modelling of the annual carbon balances revealed a reduced carbon loss of 2.2% of the total gross primary production by the absence of isoprene emission, and a 6.9% enhanced net growth of PcISPS-RNAi poplars. However, the knock down in isoprene emission resulted in reduced susceptibility to fungal infection, whereas the attractiveness for herbivores was enhanced. • The present study promises potential for the use of non- or low-isoprene-emitting poplars for more sustainable and environmentally friendly biomass production, as reducing isoprene emission will presumably have positive effects on regional climate and air quality.

Interlaboratory assessment of nitrous oxide isotopomer analysis by isotope ratio mass spectrometry and laser spectroscopy: current status and perspectives.

RATIONALE In recent years, research and applications of the N2O site-specific nitrogen isotope composition have advanced, reflecting awareness of the contribution of N2O to the anthropogenic greenhouse effect, and leading to significant progress in instrument development. Further dissemination of N2O isotopomer analysis, however, is hampered by a lack of internationally agreed gaseous N2O reference materials and an uncertain compatibility of different laboratories and analytical techniques. METHODS In a first comparison approach, eleven laboratories were each provided with N2O at tropospheric mole fractions (target gas T) and two reference gases (REF1 and REF2). The laboratories analysed all gases, applying their specific analytical routines. Compatibility of laboratories was assessed based on N2O isotopocule data for T, REF1 and REF2. Results for T were then standardised using REF1 and REF2 to evaluate the potential of N2O reference materials for improving compatibility between laboratories. RESULTS Compatibility between laboratories depended on the analytical technique: isotope ratio mass spectrometry (IRMS) results showed better compatibility for δ(15)N values, while the performance of laser spectroscopy was superior with respect to N2O site preference. This comparison, however, is restricted by the small number of participating laboratories applying laser spectroscopy. Offset and two-point calibration correction of the N2O isotopomer data significantly improved the consistency of position-dependent nitrogen isotope data while the effect on δ(15)N values was only minor. CONCLUSIONS The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor.

Long-term effects of clear-cutting and selective cutting on soil methane fluxes in a temperate spruce forest in southern Germany

Based on multi-year measurements of CH(4) exchange in sub-daily resolution we show that clear-cutting of a forest in Southern Germany increased soil temperature and moisture and decreased CH(4) uptake. CH(4) uptake in the first year after clear-cutting (-4.5 ± 0.2 μg C m(-2) h(-1)) was three times lower than during the pre-harvest period (-14.2 ± 1.3 μg C m(-2) h(-1)). In contrast, selective cutting did not significantly reduce CH(4) uptake. Annual mean uptake rates were -1.18 kg C ha(-1) yr(-1) (spruce control), -1.16 kg C ha(-1) yr(-1) (selective cut site) and -0.44 kg C ha(-1) yr(-1) (clear-cut site), respectively. Substantial seasonal and inter-annual variations in CH(4) fluxes were observed as a result of significant variability of weather conditions, demonstrating the need for long-term measurements. Our findings imply that a stepwise selective cutting instead of clear-cutting may contribute to mitigating global warming by maintaining a high CH(4) uptake capacity of the soil.

Nitrate leaching, direct and indirect nitrous oxide fluxes from sloping cropland in the purple soil area, southwestern China

This study provides a combined dataset on N loss pathways and fluxes from sloping cropland in the purple soil area, southwestern China. A lysimeter experiment was conducted to quantify nitrate leaching (May 2004-May 2010) and N(2)O emission (May 2009-May 2010) losses. Nitrate leaching was the dominant N loss pathway and annual leaching fluxes ranged from 19.2 to 53.4 kg N ha(-1), with significant differences between individual observation years (P < 0.05). Direct N(2)O emissions due to N fertilizer use were 1.72 ± 0.34 kg N ha(-1) yr(-1), which corresponds to an emission factor of 0.58 ± 0.12%. However, indirect N(2)O emissions caused by nitrate leaching and surface runoff N losses, may contribute another 0.15-0.42 kg N ha(-1) yr(-1). Our study shows that nitrate leaching lowered direct N(2)O emissions, highlighting the importance for a better understanding of the tradeoff between direct and indirect N(2)O emissions for the development of meaningful N(2)O emission strategies.

N2O source partitioning in soils using 15N site preference values corrected for the N2O reduction effect

RATIONALE The aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning. METHODS The N2O SP values (n = 431) were derived from six soil incubation studies in N2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance. RESULTS The average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 ‰ (±9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 ‰ (±6.3) and 1.7 ‰ (±6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (±21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (±14.0) and 28.3% (±14.0), respectively. CONCLUSIONS Using a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account.