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Zürich, Switzerland

Borjesson G.,Swedish University of Agricultural Sciences | Menichetti L.,Swedish University of Agricultural Sciences | Menichetti L.,Climate Air Pollution Group | Thornton B.,James Hutton Institute | And 3 more authors.
European Journal of Soil Science | Year: 2016

Microbial biomass is a small part of the total soil organic carbon (SOC) pool, but plays a major role in its turnover. A field experiment in Sweden, amended with different mineral and organic materials since 1956, was changed from continuous C3 plants to C4 vegetation (silage maize) in 2000. In 2012, soil samples from three fertilizer treatments (calcium nitrate, calcium nitrate plus straw and sewage sludge) and two controls (bare fallow and cropped unfertilized) were taken on three occasions (before, during and after cropping). Phospholipid fatty acids (PLFAs) were extracted from all soil samples and analysed for 12C and 13C contents in individual PLFAs. Seasonal variation in total PLFAs was small except for the most SOC-rich treatment (sewage sludge). Weighted means of δ13C in PLFAs showed that the plots fertilized with calcium nitrate only had the largest δ13C values in PLFAs before (-20.24‰) and after the vegetation period (-20.37‰). However, during the vegetation period the values were much smaller (-21.85‰). This coincided with a strong increase in the PLFA 18:2ω6,9, indicating the use of old organic matter by fungi. Monounsaturated PLFAs indicative of Gram-negative bacteria were more frequent before and after the growing season. This observed 'rebound' effect of the δ13C PLFA values during the vegetation period indicates that seasonal turnover of the microbial biomass can be substantial. © 2016 British Society of Soil Science. Source

Bassin S.,Institute of Sustainability science ISS | Bassin S.,Climate Air Pollution Group | Kach D.,Institute of Sustainability science ISS | Valsangiacomo A.,Institute of Sustainability science ISS | And 4 more authors.
Environmental Pollution | Year: 2015

In a free-air fumigation experiment with subalpine grassland, we studied long-term effects of elevated ozone (O3) and nitrogen (N) deposition on ecosystem N pools and on the fate of anthropogenic N. At three times during the seventh year of exposure, N pools and recovery of a stable isotope tracer (15N) were determined in above- and belowground plant parts, and in the soil. Plants were much better competitors for 15N than soil microorganisms. Plant N pools increased by 30-40% after N addition, while soil pools remained unaffected, suggesting that most of the extra N was taken up and stored in plant biomass, thus preventing the ecosystem from acquiring characteristics of eutrophication. Elevated O3 caused an increase of N in microbial biomass and in stabilized soil N, probably resulting from increased litter input and lower litter quality. Different from individual effects, the interaction between the pollutants remained partly unexplained © 2015 Published by Elsevier Ltd.. Source

Leifeld J.,Climate Air Pollution Group | Heiling M.,International Atomic Energy Agency | Hajdas I.,ETH Zurich
Radiocarbon | Year: 2015

Black carbon (BC) from incomplete combustion of organic materials is abundant in many soils. Its age is often higher than that of thermally unaltered soil organic carbon (SOC) owing to the presence of BC from fossil sources or to a high recalcitrance against microbial decomposition compared to that of plant residues. For a meaningful application of radiocarbon as an indicator for soil carbon age and turnover, the relative contribution of BC needs to be quantifed, but BC is diffcult to separate physically from soil. However, BC is thermally more stable than SOC, and hence thermal stability may provide a quantitative BC indicator. Here, we analyzed 30 light particulate organic carbon (POC) soil fractions for their thermal stability and for their14C signature. POC is particularly sensitive to “contamination” with BC, because it is obtained by combined size and density fractionation. A steady-state “bomb”14C model was used to derive mean POC ages. Soils from four sample sets, each consisting of six to eight individual POC samples and representing different feld sites and POC types, were analyzed. Samples from one of the sets were virtually BC free, and their mean POC ages ranged from 60 to 100 yr. The14C signature of samples from the other three sets indicated the presence of very old carbon, with mean POC ages of several hundred and up to 3500 yr. Two indicators for thermal stability—(1) the amount of heat released at temperatures >450°C and (2) the amount of heat released at 500°C (the latter representing the peak temperature of heat released from charcoal isolated from soil)—correlated both signifcantly and nonlinearly with POC age, indicating that samples with high BC content were older than those with low BC content. It can be concluded that at an individual site with increasing abundance of BC, both the age and the thermal stability of POC increase. However, thermal stability proved to be a reliable predictor for BC in only one sample set, whereas thermal signals of the other two BC-containing sample sets were not signifcantly different from those of BC-free samples. Thermal stability thus gives no unequivocal indication for the presence of BC in POC across different sites. © 2015 by the Arizona Board of Regents on behalf of the University of Arizona. Source

De Boeck H.J.,University of Antwerp | Bassin S.,Climate Air Pollution Group | Verlinden M.,University of Antwerp | Zeiter M.,Bern University of Applied Sciences | And 2 more authors.
New Phytologist | Year: 2016

The Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes. We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting monoliths to four different elevations (2440-660 m above sea level) for 17 d. Half of these were regularly irrigated while the other half were deprived of irrigation to additionally induce a drought at each site. Heat waves had no significant impacts on fluorescence (Fv/Fm, a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration. The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery. © 2015 New Phytologist Trust. Source

Kerre B.,Catholic University of Leuven | Bravo C.T.,University of Valladolid | Leifeld J.,Climate Air Pollution Group | Cornelissen G.,Norwegian Geotechnical Institute | And 3 more authors.
European Journal of Soil Science | Year: 2016

We have shown previously that soil with historical (> 150 years) applications of charcoal had larger recent (C4-maize derived) carbon content than adjacent soil; however, we could not determine whether there was an effect on older, C3-plant-derived, soil organic carbon (SOC). Therefore, we assessed the effect of historical additions of charcoal on the sequestration of recent and older SOC with a combination of δ13C analysis and different quantification techniques for black carbon (BC): dichromate oxidation (Cr2O7), chemo-thermal oxidation (CTO-285) and differential scanning calorimetry (DSC). Topsoils cropped with maize (Zea mays) under former charcoal production sites (N = 12) were identified in the field as black spots and had a larger (3.5%, P < 0.05) percentage of organic carbon (OC) contents than adjacent soil outside these spots (2.0%). The charcoal content varied with the detection technique used as follows: CTO-285 > DSC > Cr2O7. Black spots contained 1.6-1.7 times more (P < 0.05) maize-derived OC content than adjacent soil, irrespective of the BC quantification technique. The content of non-charcoal OC was 1.0-1.4 times larger in black spots than in adjacent soil, but differences were significant only for the Cr2O7 method. Soil physicochemical fractionation showed that at charcoal production sites more OC was recovered in the particulate organic matter and silt and clay fractions. The δ13C analysis suggested that additional maize-OC in black spots was in the physically more protected silt and clay fraction. Overall, this study shows that historical charcoal amendment in soil enhances the accumulation of recent maize-derived OC in a temperate climate without replacing the older C stocks. Highlights: We assessed the effect of historical additions of charcoal on the sequestration of recent and older SOC. Black spots contained 1.6-1.7 times more (P < 0.05) maize-derived OC content than adjacent soil. Additional maize-OC in black spots was in the physically more protected silt and clay fraction. Historical charcoal amendment in soil enhances the accumulation of recent maize-derived OC. © 2016 British Society of Soil Science. Source

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