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The rising atmospheric CO 2 concentrations have effects on the worldwide ecosystems such as an increase in biomass production as well as changing soil processes and conditions. Since this affects the ecosystem's net balance of greenhouse gas emissions, reliable projections about the CO 2 impact are required. Deterministic models can capture the interrelated biological, hydrological, and biogeochemical processes under changing CO 2 concentrations if long-term observations for model testing are provided. We used 13 years of data on above-ground biomass production, soil moisture, and emissions of CO 2 and N 2 O from the Free Air Carbon dioxide Enrichment (FACE) grassland experiment in Giessen, Germany. Then, the LandscapeDNDC ecosystem model was calibrated with data measured under current CO 2 concentrations and validated under elevated CO 2 . Depending on the hydrological conditions, different CO 2 effects were observed and captured well for all ecosystem variables but N 2 O emissions. Confidence intervals of ensemble simulations covered up to 96% of measured biomass and CO 2 emission values, while soil water content was well simulated in terms of annual cycle and location-specific CO 2 effects. N 2 O emissions under elevated CO 2 could not be reproduced, presumably due to a rarely considered mineralization process of organic nitrogen, which is not yet included in LandscapeDNDC.Agronomy 2020, 10, 50 2 of 17 climatic changes such as rising temperatures, shifting precipitation patterns, and unpredictable extreme events [2]. To assess the interaction of elevated CO 2 with the carbon (C) cycle outside of controlled laboratory environments, Free Air Carbon dioxide Enrichment (FACE) experiments were established to observe the reaction of whole ecosystems to enhanced CO 2 levels. During these FACE experiments, elevated CO 2 was found to fertilize plant primary production, leading, for example, to yield increases for cereals [3] and grapevine [4] as well as increased litter production in forests [5]. Due to the usually short duration of FACE experiments, it remains unclear whether these effects are permanent. Nutrients such as nitrogen (N), for example, have been hypothesized to become progressively limited in relation to increased C input via CO 2 fertilization [6]. Reliable predictions are made difficult both by a lack of process understanding of the C-N interactions [7] and by the low general validity of hypotheses on the effect of increased CO 2 [8]. Process-based ecosystem models used for hypothesis testing are therefore required to include a range of effects, e.g., on decomposition by soil bacteria [9], soil respiration and root biomass [10], root exudation [11], and root-associated mycorrhizal fungi [12].However, translating these processes into a reliable projection of the ecosystem response to enhanced CO 2 by means of a set of mathematical equations is challenging. The various approaches include models that concentrate, for example, on the factors directly relevant to greenhouse gas emissions [13][14][15] or tr...

European groundwater reservoirs are frequently subject to reactive nitrogen pollution (Nr) owing to the intensive use of nitrogen (N) fertilizer and animal manure in agriculture. Besides its risk on human health, groundwater Nr loading also affects the carbon (C) and N cycle of associated ecosystems. For a temperate grassland in Germany, the long-term (12 years) annual average exports of Nr in form of harvest exceeded Nr inputs via fertilization and deposition by more than 50 kgN ha−1. We hypothesize that the resulting deficit in the N budget of the plant-soil system could be closed by Nr input via the groundwater. To test this hypothesis, the ecosystem model LandscapeDNDC was used to simulate the C and N cycle of the respective grassland under different model setups, i.e., with and without additional Nr inputs via groundwater transport. Simulated plant nitrate uptake compensated the measured N deficit for 2 of 3 plots and lead to substantial improvements regarding the match between simulated and observed plant biomass and CO2 emission. This suggests that the C and N cycle of the investigated grassland were influenced by Nr inputs via groundwater transport. We also found that inputs of nitrate-rich groundwater increased the modelled nitrous oxide (N2O) emissions, while soil water content was not affected.

<p>The world's oceans have historically made a significant contribution in mitigating global warming by storing both large amounts of anthropogenic CO<sub>2</sub> emissions and a significant portion of the heat generated by the enhanced greenhouse effect. However, precisely because of this buffering function, they are themselves subject to massive chemical and physical regime shifts that are suspected to continue long after anthropogenic CO<sub>2</sub> emissions have ceased. For this reason, within the HORIZON2020-COMFORT project, we are studying the long-term effects that different scenarios of temporarily increasing atmospheric CO<sub>2</sub> concentrations could have on marine biogeochemistry. To this end, we use CLIMBER3alpha+C, an Earth system model of intermediate complexity, to study the response of the ocean carbon cycle and associated nutrients during and after the period of elevated atmospheric pCO<sub>2</sub> levels. Preliminary results show sustained changes in marine primary production, export of CaCO<sub>3</sub>, extent of hypoxic zones and production of dimethyl sulfide (DMS), with DMS acting as a condensation nucleus in cloud formation. This raises the possibility that the effects of elevated CO<sub>2</sub> on the oceans will cause a change in both the Earth's radiative balance and the marine carbon pump long after atmospheric CO<sub>2</sub> concentrations have returned to preindustrial levels.</p><p> </p><p> </p><p>Acknowledgements:</p><p>“This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820989 (project COMFORT, Our common future ocean in the Earth system – quantifying coupled cycles of carbon, oxygen, and nutrients for determining and achieving safe operating spaces with respect to tipping points).”</p><p> </p><p>Disclaimer:</p><p>“This [project/poster/presentation/etc.], reflects only the author’s/authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.”</p><p> </p>