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Future increase in atmospheric CO 2 concentrations will potentially enhance grassland biomass production and shift the functional group composition with consequences for ecosystem functioning. In the "GiFACE" experiment (Giessen Free Air Carbon dioxide Enrichment), fertilized grassland plots were fumigated with elevated CO 2 (eCO 2 ) year-round during daylight hours since 1998, at a level of +20% relative to ambient concentrations (in 1998, aCO 2 was 364 ppm and eCO 2 399 ppm; in 2014, aCO 2 was 397 ppm and eCO 2 518 ppm). Harvests were conducted twice annually through 23 years including 17 years with eCO 2 (1998 to 2014). Biomass consisted of C3 grasses and forbs, with a small proportion of legumes. The total aboveground biomass (TAB) was significantly increased under eCO 2 (p = .045 and .025, at first and second harvest). The dominant plant functional group grasses responded positively at the start, but for forbs, the effect of eCO 2 started out as a negative response. The increase in TAB in response to eCO 2 was approximately 15% during the period from 2006 to 2014, suggesting that there was no attenuation of eCO 2 effects over time, tentatively a consequence of the fertilization management. Biomass and soil moisture responses were closely linked. The soil moisture surplus (c. 3%) in eCO 2 manifested in the latter years was associated with a positive biomass response of both functional groups. The direction of the biomass response of the functional group forbs changed over the experimental duration, intensified by extreme weather conditions, pointing to the need of long-term field studies for obtaining reliable responses of perennial ecosystems to eCO 2 and as a basis for model development., (© 2017 John Wiley & Sons Ltd.)

Rising atmospheric CO 2 concentrations are expected to increase nitrous oxide (N 2 O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N 2 O emission increases under elevated atmospheric CO 2 (eCO 2 ), but the underlying processes are not yet fully understood. Here, we present results showing changes in soil N transformation dynamics from the Giessen Free Air CO 2 Enrichment (GiFACE): a permanent grassland that has been exposed to eCO 2 , +20% relative to ambient concentrations (aCO 2 ), for 15 years. We applied in the field an ammonium-nitrate fertilizer solution, in which either ammonium ( NH 4 + ) or nitrate ( NO 3 - ) was labelled with 15 N. The simultaneous gross N transformation rates were analysed with a 15 N tracing model and a solver method. The results confirmed that after 15 years of eCO 2 the N 2 O emissions under eCO 2 were still more than twofold higher than under aCO 2 . The tracing model results indicated that plant uptake of NH 4 + did not differ between treatments, but uptake of NO 3 - was significantly reduced under eCO 2 . However, the NH 4 + and NO 3 - availability increased slightly under eCO 2 . The N 2 O isotopic signature indicated that under eCO 2 the sources of the additional emissions, 8,407 μg N 2 O-N/m 2 during the first 58 days after labelling, were associated with NO 3 - reduction (+2.0%), NH 4 + oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO 2 provided an additional source of bioavailable supply of energy that triggered as a priming effect the stimulation of microbial soil organic matter (SOM) mineralization and fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete denitrification and therefore an increased N 2 O:N 2 emission ratio, explains the doubling of N 2 O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change., (© 2018 John Wiley & Sons Ltd.)

Biochar and hydrochar application to soil holds promise for climate change mitigation. This study provides first insights into the nutrient concentration and removal of grassland vegetation after addition of various carbon compounds together with pig slurry. Four treatments: control (no carbon application), feedstock, hydrochar and biochar from Miscanthus x giganteus were applied at a permanent grassland site near Giessen, Germany. Changes in plant functional groups, biomass production and nutrition status were monitored over 2 years. Total biomass production was not affected by the carbon amendments. However, biochar favoured growth of forbs over grasses, while legume growth was increased by all carbon amendments. The initial nutrient concentrations of the carbon compounds were enriched according to their degree of carbonization, potentially providing nutrients to plants. The plant biomass from hydro- and biochar amended plots, added up over 2 years, exhibited higher potassium concentrations compared to biomass from feedstock and control plots. All carbon amendments led to lower sodium concentrations in total biomass, compared to the control. Uncarbonized feedstock led to increased manganese concentrations in total biomass, while the concentrations of all other heavy metals were not influenced by any carbon amendment, compared to the control. From a plant and animal nutritional point of view, none of the carbon amendments reduced grassland yield or fodder quality. The study suggests that hydrochar and, even more so, biochar may provide a source of potassium to plants. [ABSTRACT FROM AUTHOR]

The growing world-wide use of N fertilizers (mineral or biological, i.e. legume growth) fuels nitrous oxide (N 2 O) emissions from agricultural soils, increasing the global atmospheric N 2 O concentrations by 18% since pre-industrial times. In addition to increasing N inputs, rising atmospheric CO 2 concentrations can increase the emissions of N 2 O. However, most studies were short-term (<5 years) experiments, from intensively fertilized agricultural sites; or measurements were only carried out during the vegetative period, neglecting freeze-thaw N 2 O emissions. Grasslands occupy one third of the earths’ surface and may represent a considerable positive feedback loop when N 2 O emissions increase under future CO 2 levels, accelerating global warming. We investigated N 2 O emissions under elevated CO 2 in a wet, species-rich grassland ecosystem over 11 years (Giessen-FACE: 1998-2009). The Giessen-FACE consists of 3 elevated (eCO 2 ) and 3 ambient (aCO 2 ) ring plots with three permanently installed maxi-chamber measurement places per ring. Measurements were carried out 1-3 times weekly starting one year prior to the CO 2 enrichment onset (May 1998) up to now. Elevated CO 2 more than doubled N 2 O emissions, causing 5.4 t CO 2 -eq. ha -1 of surplus N 2 O emissions over 11 years. Surprisingly, the largest stimulation did not occur after the spring N-fertilizer application (40 kg N ha -1 ), but over the vegetation period where the soil was warm and dry. After N fertilization in spring, and during the off-season, N 2 O emissions were moderately stimulated with eCO 2 . A carbon balance assessment revealed that 62% of the surplus eCO 2 -biomass yield carbon (sum: 8.7 t CO 2 -eq. ha -1 over 11 years) is already “consumed” by the surplus N 2 O emissions. Thus, even when the surplus biomass is pyrolyzed into biochar, a recalcitrant form of carbon for C sequestration in soils, the stable C amount would be insufficient to counterbalance the increased N 2 O emissions under elevated CO 2 . [ABSTRACT FROM AUTHOR]

Nutrient-poor organic soils under maritime grassland are often located in remote wet locations in the landscape. Leaving these soils without drainage maintenance often raise the water table but continuous management (grazing) means they could remain a source of carbon dioxide (CO 2 ) while also turning into a small source of methane (CH 4 ). Due to geographical and socio-economic reasons, removing these sites from agricultural production may be an option to mitigate greenhouse gas (GHG) emissions. To test this hypothesis we measured GHG fluxes over a four year period, at a drained and a rewetted organic soil under grassland, which were both grazed for the first two years and not grazed for the following two years. Statistical response functions estimated for gross primary production (GPP) and ecosystem respiration ( R eco ) were used to reconstruct annual CO 2 –C balances using site-specific models driven by soil temperature, solar radiation, soil water table (WT) and leaf area index (LAI). Annually, soil CO 2 emissions were comparable when grazed, although the rewetted site had a lower net ecosystem carbon balance (NECB) despite displaying higher CH 4 emissions. Both sites have lower CO 2 emissions than typical drained organic soils due to management practices: extensive grazing, no fertilisation and mean annual water tables above −25 cm. When grazing stopped, GPP and R eco increased dramatically driven by vigorous growth of vegetation at both sites. The shallow drained site remained a source of CO 2 and small source of CH 4 while the rewetted site became either neutral or a small sink of CO 2 with decreased CH 4 emissions compared to the grazing period. Nitrous oxide (N 2 O) emissions were negligible at either site. Removing grazing significantly reduced the NECB at both sites but in terms of global warming potential (GWP), the greatest GHG mitigation was in the rewetted site which exerted a cooling effect in the second year after the management shift. [ABSTRACT FROM AUTHOR]

Provisioning services from grassland ecosystems are strongly linked to physical and chemical grassland traits, which are affected by atmospheric CO 2 concentrations ([CO 2 ]s). The influences of increased [CO 2 ]s ([eCO 2 ]s) are typically investigated in Free Air Carbon dioxide Enrichment (FACE) studies via destructive sampling methods. This traditional approach is restricted to sampling plots and harvest dates, while hyperspectral approaches provide new opportunities as they are rapid, non-destructive and cost-effective. They further allow a high temporal resolution including spatially explicit information. In this study we investigated the hyperspectral predictability of 14 grassland traits linked to forage quality and quantity within a FACE experiment in central Germany with three plots under ambient atmospheric [CO 2 ]s, and three plots at [eCO 2 ]s (∼20% above ambient [CO 2 ]s). We analysed the suitability of various normalisation and feature selection techniques to link comprehensive laboratory analyses with two years of hyperspectral measurements (spectral range 600–1600 nm). We applied partial least squares regression and found good to excellent predictive performances (0.49 ≤ leave one out cross-validation R 2≤ 0.94), which depended on the normalisation method applied to the hyperspectral data prior to model training. Noteworthy, the models' predictive performances were not affected by the different [CO 2 ]s, which was anticipated due to the altered plant physiology under [eCO 2 ]s. Thus, an accurate monitoring of grassland traits under different [CO 2 ]s (present-day versus future, or within a FACE facility) is promising, if appropriate predictors are selected. Moreover, we show how hyperspectral predictions can be used e.g., within a future phenotyping approach, to monitor the grassland on a spatially explicit level and on a higher temporal resolution compared to conventional destructive sampling techniques. Based on the information during the vegetation period we show how hyperspectral monitoring might be used e.g., to adapt harvest practices or gain deeper insights into physiological plant alterations under [eCO 2 ]s. • First hyperspectral analysis within a free air carbon dioxide enrichment study • Performances of different hyperspectral normalisation techniques were investigated. • Good to excellent predictive performances for 14 traits related to ecosystem services. • Accurate monitoring of plant traits even under varying carbon dioxide concentrations. • Highlighting of capabilities to derive high resolution spatio-temporal canopy data. [ABSTRACT FROM AUTHOR]