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Nitrous oxide (N2O) is an important long-lived greenhouse gas and precursor of stratospheric ozone depleting mono-nitrogen oxides. The atmospheric concentration of N2O is persistently increasing; however, large uncertainties are associated with the distinct source strengths. Here we investigate for the first time N2O emission from terrestrial vegetation in response to natural solar ultra violet radiation. We conducted field site measurements to investigate N2O atmosphere exchange from grass vegetation exposed to solar irradiance with and without UV-screening. Further laboratory tests were conducted with a range of species to study the controls and possible loci of UV-induced N2O emission from plants. Plants released N2O in response to natural sunlight at rates of c. 20-50 nmol m-2 h-1, mostly due to the UV component. The emission rate is temperature dependent with a rather high activation energy indicative for an abiotic process. The prevailing zone for the N2O formation appears to be at the very surface of leaves. However, only c. 26% of the UV-induced N2O appears to originate from plant-N. Further, the process is dependent on atmospheric oxygen concentration. Our work demonstrates that ecosystem emission of the important greenhouse gas, N2O, may be up to c. 30% higher than hitherto assumed.

CLiMA!TE - backgroundThe concentration of CO2 in the atmosphere is increasing, globaltemperatures are increasing, and local precipitation patterns arechanging with increases in the intensity of rain events and droughtperiods. This is expected to affect the structure and functioning ofterrestrial ecosystems (IPCC, 2013) with major impacts on naturalenvironments as well as ecosystems used for agriculture or forestry. Over the past three decades, major efforts have been devoted to understandand predict such impacts of climate change on ecosystemprocesses and functioning in order to understand the urgency of the changes as well as the possibilities for ecosystem adaptation or climatechange mitigation. These efforts have included observationsof past changes, monitoring of ongoing changes, observations acrossenvironmental gradients (space for time substitution), ecosystem manipulationexperiments mimicking future climate changes, and dynamicecosystem modelling (Beier, 2004; Rustad, 2008). Each of theseapproaches has their forces and drawbacks, but across all a generallimitation is that observations and experiments have focused on onesingle climate factor. For example, observations across gradients canhardly combine simultaneous and ideal differences in two or eventhree climate factors at the same time to provide a multi factor responsepicture. Ecosystem experiments, which could do it, often limits themselves to one factor for practical reasons or because of lackof resources, since inclusion of one extra factor doubles the numberof experimental units and the demand for resources in a classic experimentaldesign. Therefore, very few multi factor climate change experiments exist. Instead the underlying assumption has been thatif the individual responses are known based on single factor experiments,then dynamic ecosystem or global models can predict theresponses of the combined factors. This approach may seem reasonablebut is constrained by at least two problems, which CLiMA!TE specifically aimed to overcome:1. When several factors act together, they may interact, and theseinteractions among the different climate change factors may not belinear and/or predictable. Computer models may predict some ofthese interactions relatively well (e.g. resource limitations due toincreased growth), while other interactions may be unpredictable.8 beier, c., et al.The assumption that the impact of the "climate change cocktail"may be predicted from an understanding of the individual factorsmay therefore be erroneous.2. Even when models do predict the interactions, we still need multifactor experiments to train and test the models in order to knowif the predictions are correct. Another inherent problem related to climate change and experimentationis the time scale. Climate change acts over decades, meaningthat climate change experiments running for 2-4 years only highlightshort term and transient effects on the ecosystems, while lackingthe ability to inform about long term and more stable effects.The "long term" perspective of climate change was therefore anotherimportant rationale for the CLiMA!TE experiment.The "long term" perspective of climate change calls for long termexperiments, which for decades has been argued from the scientificcommunity, was therefore another important rationale for the CLiMA!TE experiment. The VILLUM FOUNDATION provided avery rare opportunity to pursue this in reality.In summary, the CLiMA!TE experiment was driven by two major rationales:1) a need for realistic experiments involving combinations of the mainclimate change drivers and 2) the long term perspective of climate change.

International audience

t is vital to understand responses of soil microorganisms to predicted climate changes, as these directly control soil carbon (C) dynamics. The rate of turnover of soil organic carbon is mediated by soil microorganisms whose activity may be affected by climate change. After one year of multifactorial climate change treatments, at an undisturbed temperate heathland, soil microbial community dynamics were investigated by injection of a very small concentration (5.12 m gCg 2 1 soil) of 13 C- labeled glycine ( 13 C 2 , 99 atom %) to soils in situ . Plots were treated with elevated temperature ( + 1 u C, T), summer drought (D) and elevated atmospheric carbon dioxide (510 ppm [CO2]), as well as combined treatments (TD, TCO2, DCO2 and TDCO2). The 13 C enrichment of respired CO 2 and of phospholipid fatty acids (PLFAs) was determined after 24 h. 13 C-glycine incorporation into the biomarker PLFAs for specific microbial groups (Gram positive bacteria, Gram negative bacteria, actinobacteria and fungi) was quantified using gas chromatography-combustion-stable isotope ratio mass spectrometry (GC-C-IRMS). Gram positive bacteria opportunistically utilized the freshly added glycine substrate, i.e. incorporated 13 Cin all treatments, whereas fungi had minor or no glycine derived 13 C-enrichment, hence slowly reacting to a new substrate. The effects of elevated CO 2 did suggest increased direct incorporation of glycine in microbial biomass, in particular in G + bacteria, in an ecosystem subjected to elevated CO 2 . Warming decreased the concentration of PLFAs in general. The FACE CO 2 was 13 C-depleted ( d 13 C=12.2 % ) compared to ambient ( d 13 C= , 2 8 % ), and this enabled observation of the integrated longer term responses of soil microorganisms to the FACE over one year. All together, the bacterial (and not fungal) utilization of glycine indicates substrate preference and resource partitioning in the microbial community, and therefore suggests a diversified response pattern to future changes in substrate availability and climatic factors

Organic arable farming faces challenges with low crop yields, partly due to inefficient use of green manure-derived nitrogen (N). Under current farming practices, green manure leys are often cut and mulched during the growing season with the associated risk of environmental N losses, leading to eutrophication and global warming. In this 3-month incubation experiment, we tested a new green manure management strategy as part of the ICROFS project HighCrop. With the new strategy, green manure leys are instead harvested and preserved until the following spring either as compost mixed with straw (grass-clover:straw, 4:1, w:w) or as silage of harvested ley biomass. In spring, these two green manure materials can then be used for targeted fertilization of spring sown crops. The objectives of the study were to: • Assess how storage methods (compost vs. silage) affect N2O fluxes and soil respiratory CO2 emissions after soil application of preserved grass-clover green manure. • Determine whether the greenhouse gas fluxes are influenced by the incorporation method, more specifically harrowing (simulated by mixing the material into the top 5 cm soil layer) and ploughing (the material placed at 15 cm depth). • Compare composted and ensiled green manures concerning their abilities to provide plant-available N during a 3-month period. During the experiment, gas fluxes were measured at nine occasion followed by eight destructive soil harvests. In total, the study included 192 soil units that were incubated at 15 °C in darkness. Each unit consisted of a packed soil core (26 cm high × 10 cm diameter) with bulk density of 1.07 g cm-3 and gravimetric soil moisture of 20 %. The addition of compost and silage corresponded to a fertilization rate of 120 kg total N ha-1. A mineral fertilizer treatment was included as a reference and received 80 kg NH4-N ha-1. Compared to the more degraded compost, the silage material had a high content of labile compound. In addition, incorporation of green manure by harrowing was expected to improve soil microbes’ access to the materials, and thereby increase the decomposition rate. In line with this, cumulative CO2 emissions from the green manure treatments was lowest for compost incorporated by ploughing and highest for silage incorporated by harrowing. Between 32 and 54 % of the added green manure carbon was respired as CO2 during the 3-month experiment. Interestingly, mineral fertilizer suppressed soil respiratory CO2 emission. Generally, N2O emissions were higher from the silage-amended soils than from soils fertilized with compost. Especially, silage incorporated by ploughing gave rise to increased N2O effluxes, corresponding to 0.3 % of applied total N during the 3-month period. This could partly result from denitrification of initial soil nitrate, stimulated by high local oxygen consumption in the labile silage layer. In contrast, compost incorporated by harrowing caused a downwards N2O flux into the soil, presumably an effect of lacking mineral N availability in this treatment. Overall, our study showed that emissions of N2O can be reduced by incorporating green manure using harrowing instead of ploughing. Net mineralization of green manure-derived N was absent until more than three weeks after incorporation of the materials. Over the 3-month experiment, grass-clover silage provided the highest net release of inorganic N with preliminary results corresponding to 38-43 kg N ha-1, irrespective of the incorporation method used. In contrast, no increase in soil mineral N was observed for the composted grass-clover and straw mixture compared to the unfertilized control soil. In fact, soil incorporation of compost by harrowing caused immobilization of soil mineral nitrogen 1-2 months after experimental set-up.

Anaerobic digestion of animal manure and crop residues may be employed to produce biogas as a climate-neutral source of energy to provide fertilizers which allow recycling of plant nutrients on the farm. However, compared to fertilizing with the pristine input materials (e.g. raw animal slurry or plant residues), the effect on soil microbiota and soil fertility may be impacted due to the increased content of mineral nitrogen (N) and decreased amount of organic carbon (C); an issue of concern in especially organic farming systems. An incubation study was performed where 1) water, 2) raw cattle slurry, 3) anaerobically digested cattle slurry + maize, 4) anaerobically digested cattle slurry + grass-clover, or 5) fresh grass-clover was applied to soil at arable realistic rates. During the following 9 days experimental unites were sequentially sampled destructively and the soil assayed for content of mineral N, available organic C, microbial phospholipid fatty acids (community composition), catabolic response profiling (functional diversity) and emission of CO2 and N2O. Fertilizing with the anaerobic digested materials increased the soil concentration of NO3- ca. 30-40% compared to when raw slurry was applied. Grass-clover contributed with four times more readily degradable organic C than the other materials, causing an increased microbial biomass which depleted the soil for mineral N and probably also O2. Consequently, grass-clover also caused a ~10 times increase in emissions of both CO2 and N2O compared to any of the other treatments. Besides increasing the total microbial biomass, grass-clover also induced the largest changes in microbial diversity measures. Comparing to this, digested materials and raw cattle slurry only had little effect on the soil microbiota.

A field experiment was carried out including alternating perennial ryegrass (Lolium perenne L.) – clover (Trifolium repens + Trifolium pretense L.) pasture mix with annual maize (Zea mays L.). Maize was established after soil incorporation of a 1st-year grass-clover in a 6-m wide strip, whereas grass-clover was established without incorporating the 1st-year grass-clover in an equivalent 6-m wide strip. This resulted in an early interspecific competitive advantage for the perennial strip and when growing maize in close proximity to grass-clover total dry matter production was reduced with about 50% compared to >150 cm distance. However, it was partly compensated by greater yields by especially the 50-100 cm harvest. In contrast there was no significant difference in grass-clover yields according to distance to adjacent maize. Across years and harvest time there was less clover when grown in close proximity to maize (0-25 cm; 30%) compared to with > 25 cm distance (40%) indicating potential soil mineral interspecific competitive interactions. This was supported by a 50% maize soil N uptake reduction when grown in close proximity to grass-clover. Additional fertilizer application increased maize yields significantly averaging 1250 g DM m-2, with significant differences were found between the use of green manure (950 g DM m-2) and anaerobic digestate (1400 g DM m-2). The distance towards the grass-clover strip was still influencing the maize yields significantly despite the additional plant available N in the order of 50-100 cm > 150 cm = 100-150 cm > 0-50 cm distance. At the final harvest maize aboveground biomass was equal to 250 g carbon (C) m-2 when grown in close proximity to grass-clover (0-50 cm) and increased to 500 g C m-2 grown with > 50 cm distance. During establishment there was an inner row (50-100 cm distance) effect of 26% compared to traditional maize cropping which was not found at the later growth stages. The maize energy density was 17 megajoule (MJ) kg-1 independent of year, harvest time and distance to the grass-clover strip, with no effect of additional N application either. The maize energy yield was 0.8, 13 and 18 MJ m-2 at establishment, the vegetative growth stage and ripening, respectively. When comparing the ”occupation” of land by the present maize strip cropping system as compared to the traditional maize and grass-clover single field cropping during 1 full year the relative yield advantages was 0.96 and 1.01 in 2008 and 2009, respectively. Thus, the total land area required under traditional cropping attaining the yields achieved when dividing the field in strips are the same. It is concluded that the combination of maize with grass-clover does not possess the right level of co-existence and complementarity. To fulfill the society needs for increased used of biomass for bioenergy more emphasis on practices including rotational principles of dissimilar types of crops is required when developing cropping systems for the future.

The combination of perennials and annuals in a strip cropping system is challenging primarily because the interspecific competitive ability of the perennials towards the annuals seems to be too dominating. Especially at the first harvest (tillering) closest to the adjacent grass-clover strip severe total dry matter production reductions was found ranging from 25%, 5% and 20% in the vetch SC, rye SC and vetch-rye IC, respectively. Rye in particular was suffering from the grass-clover interspecific competition with a SC yield of 700 g DM m-2 (0-25 cm) at maturity as compared to 1025 g DM m-2 when grown > 150 cm away. When grown >150 cm away from the grass-clover strip rye accumulate 8 g soil mineral N m-2,which is in the range of the expected according to organic yield potentials, but only 5 g soil mineral N m-2 when grown 0-25 cm from the adjacent grass-clover. Grass-clover is clearly more competitive towards soil mineral N than adjacent rye plants. The ability of vetch to fix its own N might be an advantage if there is no other limiting growth parameters, like e.g. soil water, indicated by no significant difference between final harvest of vetch SC with 0-25 cm (480 g DM m-2) and >150 cm (550 g DM m-2) distance to grass-clover. The Land Equivalent Ratio (LER = LP+LB) based on aboveground DM production and N accumulation varied between 0.85-1.25 comparing harvests and distance to adjacent grass-clover. At maturity LER values show that available plant growth resources was used 25% more efficiently in intercrops than sole crops when grown with >25 cm and less than 75cm from the adjacent grass-clover strip. Ecosystem services like biomass for energy, functions like leguminous N2-fixation and the capacity of e.g. perennial species to act as carbon sinks should gain more attention when addressing climate change mitigation tools within the framework of organic farming principles and values before these more multipurpose cropping systems like the present strip cropping concept can be fully evaluated. Furthermore, the value of subdividing fields into strips to avoid the buildup of pathogens and pests, depletion of soil nutrients and to improve soil structure and fertility by e.g. alternating deep-rooted and shallow-rooted plants needs further research.

The background for this research activity is that the 2020 goals for reduction of the CO2 emissions to the atmosphere are so challenging that exorbitant amounts of biomass and other renewable sources of energy must be mobilised in order to – maybe – fulfil the ambitious 2020 goals. The macroalgae is an unexploited, not researched, not developed source of biomass and is at the same time an enormous resource by mass. It is therefore obvious to look into this vast biomass resource and by this report give some of the first suggestions of how this new and promising biomass resource can be exploited.

International audience; Field-scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a semi-natural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO2), warming, and extended summer drought in all combinations. Here, we report on the short-term responses of the nitrogen (N) cycle after two years of treatments. Elevated CO2 significantly affected above-ground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co-dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Below-ground, elevated CO2 had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH4+-N consumption, gross mineralization, potential nitrification, denitrification and N2O emissions. Drought reduced below-ground gross N mineralization and decreased fauna N mass and N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue, decreased litter N production, and decreased fauna N mineralization prevailed in the full treatment combination indicating reduced N turnover in the simulated future climate scenario, which could act to reduce the potential growth response of plants to elevated atmospheric CO2 concentration. In conclusion, effects observed in single-factor studies should be evaluated with caution. Multi-factor climate experiments are needed for improving realistic estimation of the combined ecosystem responses to future climate changes.

Global change is a reality. Atmospheric CO2 levels are rising as well as mean global temperature and precipitation patterns are changing. These three environmental factors have separately and in combination effect on ecosystem processes. Terrestrial ecosystems hold large amounts of carbon, why understanding plant and soil responses to such changes are necessary, as ecosystems potentially can ameliorate or accelerate global change. To predict the feedback of ecosystems to the atmospheric CO2 concentrations experiments imitating global change effects are therefore an important tool. This work on ecosystem-atmosphere exchange of carbon in a heathland under future climatic conditions, shows that extended summer drought in combination with elevated temperature will ensure permanent dryer soil conditions, which decreases carbon turnover, while elevated atmospheric CO2 concentrations will increase carbon turnover. In the full future climate scenario, carbon turnover is over all expected to increase and the heathland to become a source of atmospheric CO2. The methodology of static chamber CO2 flux measurements and applying the technology in a FACE (free air CO2 enrichment) facility is a challenge. Fluxes of CO2 from soil to atmosphere depend on a physical equilibrium between those two medias, why it is important to keep the CO2 gradient between soil and atmosphere unchanged during measurement. Uptake to plants via photosynthesis depends on a physiological process, which depends strongly on the atmospheric CO2 concentration. Photosynthesis and respiration run in parallel during measurements of net ecosystem exchange, and these measurements should therefore be performed with care to both the atmospheric CO2 concentration and the CO2 soil-atmosphere gradient.

One way of reducing the emissions of fossil fuel‐derived CO2 is to replace fossil fuels with biofuels. However, cultivation of soils results in emission of other greenhouse gasses, especially nitrous oxide (N2O). In this study we relate measured field emissions of N2O to the reduction in fossil fuel‐derived CO2, which is obtained when energy crops are used for biofuel production. The analysis includes five organically managed crops (viz. maize, rye, rye‐vetch, vetch and grass‐clover) and three scenarios for conversion of biomass to biofuel. The scenarios are 1) bioethanol production, 2) biogas production and 3) co‐production of bioethanol and biogas, where the energy crops are first used for bioethanol fermentation and subsequently the residues from this process are utilized for biogas production. The net reduction in greenhouse gas missions is calculated as the avoided fossil fuel‐derived CO2, where the N2O emission has been subtracted. This value does not account for farm machinery CO2 emissions and fuel consumption during biofuel production. We obtained the greatest net reduction in greenhouse gas emissions by co‐production of bioethanol and biogas or by biogas alone produced from either fresh grass‐clover or whole crop maize. Here the net reduction corresponded to about 8 tons CO2 ha‐1 yr‐1.