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Long-term climate change experiments are extremely valuable for studying ecosystem responses to environmental change. Examination of the vegetation and the soil should be non-destructive to guarantee long-term research. In this paper, we review field methods using isotope techniques for assessing carbon dynamics in the plant–soil–air continuum, based on recent field experience and examples from a European climate change manipulation network. Eight European semi-natural shrubland ecosystems were exposed to warming and drought manipulations. One field site was additionally exposed to elevated atmospheric CO2. We discuss the isotope methods that were used across the network to evaluate carbon fluxes and ecosystem responses, including: (1) analysis of the naturally rare isotopes of carbon (13C and 14C) and nitrogen (15N); (2) use of in situ pulse labelling with 13CO2, soil injections of 13C- and 15N-enriched substrates, or continuous labelling by free air carbon dioxide enrichment (FACE) and (3) manipulation of isotopic composition of soil substrates (14C) in laboratory-based studies. The natural 14C signature of soil respiration gave insight into a possible long-term shift in the partitioning between the decomposition of young and old soil carbon sources. Contrastingly, the stable isotopes 13C and 15N were used for shorter-term processes, as the residence time in a certain compartment of the stable isotope label signal is limited. The use of labelled carbon-compounds to study carbon mineralisation by soil micro-organisms enabled to determine the long-term effect of climate change on microbial carbon uptake kinetics and turnover. Based on the experience with the experimental work, we provide recommendations for the application of the reviewed methods to study carbon fluxes in the plant–soil–air continuum in climate change experiments. 13C-labelling techniques exert minimal physical disturbances, however, the dilution of the applied isotopic signal can be challenging. In addition, the contamination of the field site with excess 13C or 14C can be a problem for subsequent natural abundance (14C and 13C) or label studies. The use of slight changes in carbon and nitrogen natural abundance does not present problems related to potential dilution or contamination risks, but the usefulness depends on the fractionation rate of the studied processes. Comisión Europea-FTP7- ICT- 2008-1/227628 Gobierno español-CGL2016-79835-P Gobierno catalán-SGR2014-274 European Research Council (ERC)-SyG-610028 IMBALANCE Hungarian Scientific Research Fund-OTKA K112576

Soil microbial carbon-use efficiency (CUE), described as the ratio of growth over total carbon (C) uptake, i.e. the sum of growth and respiration, is a key variable in all soil organic matter (SOM) models and critical to ecosystem C cycling. However, there is still a lack of consensus on microbial CUE when estimated using different methods. Furthermore, the significance of many fundamental drivers of CUE remains largely unknown and inconclusive, especially for tropical ecosystems. For these reasons, we determined CUE and microbial indicators of soil nutrient availability in seven tropical forest soils along an altitudinal gradient (circa 900–2200 m a.s.l) occurring at Taita Hills, Kenya. We used this gradient to study the soil nutrient (N and P) availability and its relation to microbial CUE estimates. For assessing the soil nutrient availability, we determined both the soil bulk stoichiometric nutrient ratios (soil C:N, C:P and N:P), as well as SOM degradation related enzyme activities. We estimated soil microbial CUE using two methods: substrate independent 18O-water tracing and 13C-glucose tracing method. Based on these two approaches, we estimated the microbial uptake efficiency of added glucose versus native SOM, with the latter defined by 18O-water tracing method. Based on the bulk soil C:N stoichiometry, the studied soils did not reveal N limitation. However, soil bulk P limitation increased slightly with elevation. Additionally, based on extracellular enzyme activities, the SOM nutrient availability decreased with elevation. The 13C-CUE did not change with altitude indicating that glucose was efficiently taken up and used by the microbes. On the other hand, 18O-CUE, which reflects the growth efficiency of microbes growing on native SOM, clearly declined with increasing altitude and was associated with SOM nutrient availability indicators. Based on our results, microbes at higher elevations invested more energy to scavenge for nutrients and energy from complex SOM whereas at lower elevations the soil nutrients may have been more readily available.

The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously. peerReviewed

The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.

Long‐term climate change experiments are extremely valuable for studying ecosystem responses to environmental change. Examination of the vegetation and the soil should be non‐destructive to guarantee long‐term research. In this paper, we review field methods using isotope techniques for assessing carbon dynamics in the plant–soil–air continuum, based on recent field experience and examples from a European climate change manipulation network.\ud Eight European semi‐natural shrubland ecosystems were exposed to warming and drought manipulations. One field site was additionally exposed to elevated atmospheric CO2. We discuss the isotope methods that were used across the network to evaluate carbon fluxes and ecosystem responses, including: (1) analysis of the naturally rare isotopes of carbon (13C and 14C) and nitrogen (15N); (2) use of in situ pulse labelling with 13CO2, soil injections of 13C‐ and 15N‐enriched substrates, or continuous labelling by free air carbon dioxide enrichment (FACE) and (3) manipulation of isotopic composition of soil substrates (14C) in laboratory‐based studies.\ud The natural 14C signature of soil respiration gave insight into a possible long‐term shift in the partitioning between the decomposition of young and old soil carbon sources. Contrastingly, the stable isotopes 13C and 15N were used for shorter‐term processes, as the residence time in a certain compartment of the stable isotope label signal is limited. The use of labelled carbon‐compounds to study carbon mineralisation by soil micro‐organisms enabled to determine the long‐term effect of climate change on microbial carbon uptake kinetics and turnover.\ud Based on the experience with the experimental work, we provide recommendations for the application of the reviewed methods to study carbon fluxes in the plant–soil–air continuum in climate change experiments. 13C‐labelling techniques exert minimal physical disturbances, however, the dilution of the applied isotopic signal can be challenging. In addition, the contamination of the field site with excess 13C or 14C can be a problem for subsequent natural abundance (14C and 13C) or label studies. The use of slight changes in carbon and nitrogen natural abundance does not present problems related to potential dilution or contamination risks, but the usefulness depends on the fractionation rate of the studied processes.

Recent advances in laser spectrometry offer new opportunities to investigate the soil–atmosphere exchange of nitrous oxide. During two field campaigns conducted at a grassland site and a willow field, we tested the performance of a quantum cascade laser (QCL) connected to a newly developed automated chamber system against a conventional gas chromatography (GC) approach using the same chambers plus an automated gas sampling unit with septum capped vials and subsequent laboratory GC analysis. Through its high precision and time resolution, data of the QCL system were used for quantifying the commonly observed nonlinearity in concentration changes during chamber deployment, making the calculation of exchange fluxes more accurate by the application of exponential models. As expected, the curvature values in the concentration increase was higher during long (60 min) chamber closure times and under high-flux conditions (FN2O > 150 µg N m−2 h−1) than those values that were found when chambers were closed for only 10 min and/or when fluxes were in a typical range of 2 to 50 µg N m−2 h−1. Extremely low standard errors of fluxes, i.e., from ∼ 0.2 to 1.7 % of the flux value, were observed regardless of linear or exponential flux calculation when using QCL data. Thus, we recommend reducing chamber closure times to a maximum of 10 min when a fast-response analyzer is available and this type of chamber system is used to keep soil disturbance low and conditions around the chamber plot as natural as possible. Further, applying linear regression to a 3 min data window with rejecting the first 2 min after closure and a sampling time of every 5 s proved to be sufficient for robust flux determination while ensuring that standard errors of N2O fluxes were still on a relatively low level. Despite low signal-to-noise ratios, GC was still found to be a useful method to determine the mean the soil–atmosphere exchange of N2O on longer timescales during specific campaigns. Intriguingly, the consistency between GC and QCL-based campaign averages was better under low than under high N2O efflux conditions, although single flux values were highly scattered during the low efflux campaign. Furthermore, the QCL technology provides a useful tool to accurately investigate the highly debated topic of diurnal courses of N2O fluxes and its controlling factors. Our new chamber design protects the measurement spot from unintended shading and minimizes disturbance of throughfall, thereby complying with high quality requirements of long-term observation studies and research infrastructures.

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.

Replacing fossil fuel-derived energy with biomass-derived energy is commonly emphasized as a means to reduce CO2 emissions. However, our study highlights the risk of large greenhouse gas emissions when wastes from bioenergy production are recycled as fertilizer for energy crops. Crop management affects the magnitude of these emissions, which in some cases negate a considerable fraction of the global warming savings associated with biofuels.

Majs-biomasse kan omdannes til klimavenlig CO2-neutral energi ved behandling til bioetanol og biogas. Men gødskes majsen med affaldet fra biogasanlægget, dannes der betragtelige mængder af den kraftige drivhusgas, lattergas. Herved reduceres klimagevinsten mærkbart. Dyrkning af majs til energiformål skal derfor ske med ingen eller lille tilførsel af N.

Økølogisk plantedyrkning udleder mindre lattergas pr. areal end konventionel dyrkning. Målt i forhold til udbytter er der ingen forskel mellem de to systemer.

Et nyt forskningsprojekt belyser fordele og ulemper ved produktion af bioenergi i økologisk jordbrug.

Key enzymes of the urea cycle and (15)N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. (15)NH(4)(+) was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of L-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. (15)N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 microg Arg mg(-1) fresh weight h(-1). The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium.

Open Access article - can be viewed online without a subscription at: www.plantphysiol.org/cgi/doi/10.1104/pp.106.090522, Key enzymes of the urea cycle and 15N-labeling patterns of arginine (Arg) were measured to elucidate the involvement of Arg in nitrogen translocation by arbuscular mycorrhizal (AM) fungi. Mycorrhiza was established between transformed carrot (Daucus carota) roots and Glomus intraradices in two-compartment petri dishes and three ammonium levels were supplied to the compartment containing the extraradical mycelium (ERM), but no roots. Time courses of specific enzyme activity were obtained for glutamine synthetase, argininosuccinate synthetase, arginase, and urease in the ERM and AM roots. 15NH4 1 was used to follow the dynamics of nitrogen incorporation into and turnover of Arg. Both the absence of external nitrogen and the presence of L-norvaline, an inhibitor of Arg synthesis, prevented the synthesis of Arg in the ERM and resulted in decreased activity of arginase and urease in the AM root. The catabolic activity of the urea cycle in the roots therefore depends on Arg translocation from the ERM. 15N labeling of Arg in the ERM was very fast and analysis of its time course and isotopomer pattern allowed estimation of the translocation rate of Arg along the mycelium as 0.13 mg Argmg21 fresh weight h21. The results highlight the synchronization of the spatially separated reactions involved in the anabolic and catabolic arms of the urea cycle. This synchronization is a prerequisite for Arg to be a key component in nitrogen translocation in the AM mycelium., European Union (grant for a short term scientific mission to C.C., which supported her stay at Risø National Laboratory, Denmark).

The contribution of biologically fixed dinitrogen (N2) to the nitrous oxide (N2O) production in grasslands is unknown. To assess the contribution of recently fixed N2 as a source of N2O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a 15N2-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass-clover pots were sampled for N2 fixation determination, whereas the remaining half were examined for emission of 15N labelled N2O for another eight days using a static chamber method. Biological N2 fixation measured in grass-clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m-2 d-1 at 16, 26 and 36 weeks after emergence, respectively. The drop in N2 fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m-2 d-1, which represented 1.7 ± 0.3 % of the N accumulated in grass shoots during the labelling period. Total N2O emission was 91, 416 and 259 μg N2O-N m-2 d-1 at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N2 was emitted as N2O on a daily basis, which accounted for 2.1 ± 0.5 % of the total N2O-N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N2O.

– Intercropping of cereals and grain legumes is a technology to enhance biodiversity in organic agro-ecosystems, enhance and stabilize yields and make better use of plant growth resources. Here we report the preliminary results of the EU FP 5 research project “Intercrop” carried out in five European regions. The project demonstrates several significant benefits which could be exploited to a greater degree in European organic farming .

In organic dairy farming, a major N input to the plant-soil system comes from biological N2 fixation by pasture legumes, but knowledge is sparse on how much of the fixed N2 is lost from the pastures as N2O. Nitrifying and denitrifying bacteria are the main contributors to the N2O production in soils. Currently, no contribution from biological N2 fixation in legume pastures is included in the national N2O inventories, partly because of uncertainties in quantifying the N2 fixation in the pastures (Mosier et al., 1998). According to the guidelines issued by The Intergovernmental Panel on Climate Change (IPCC), inventories for N2O emissions from agricultural soils should be based on the assumption that 1.25 % of added N is emitted as N2O (IPCC, 1997). The standard N2O emission factor of 1.25 % could be considerably unrepresentative for biologically fixed N2. Firstly, only a part of the fixed N is mineralised during the lifetime of the crop. Secondly, the release of inorganic N into the soil occurs slowly. A 15N2-tracer-experiment was initiated on grass-clover grown in pots. The aim was to assess: · the contribution of recently fixed N2 as a source of N2O · the translocation of N from clover to companion grass References IPCC, 1997. Greenhouse gas inventory. Reference manual. Vol. 3. Intergovernmental Panel on Climate Change. Bracknell, UK. Mosier, A. et al. 1998. Nutrient Cycling in Agroecosystems 52, 225-248.

The effect of sole and intercropping of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) and of crop residue management on crop yield, NO3- leaching and N balance in the cropping system was tested in a 2-year lysimeter experiment on a temperate sandy loam soil. The crop rotation was pea and barley sole and intercrops followed by winter-rye and a fallow period. The Land Equivalent Ratio (LER), which is defined as the relative land area under sole crops that is required to produce the yields achieved in intercropping, were used to compare intercropping performance relative to sole cropping. Crops received no fertilizer in the experimental period. Natural 15N abundance techniques were used to determine pea N2 fixation. The pea-barley intercrop yielded 4.0 Mg grain ha-1, which was about 0.5 Mg lower than the yields of sole cropped pea but about 1.5 Mg greater than harvested in sole cropped barley. Calculation of the land equivalent ratio showed that plant growth resources were used from 17 to 31% more efficiently by the intercrop than by the sole crops. Pea increased the N derived from N2 fixation from 70% when sole cropped to 99% of the total aboveground N accumulation when intercropped. However, based upon aboveground N accumulation the pea-barley intercrop yielded about 85 kg N ha-1, which was about 65 kg lower than sole cropped pea but about three times greater than harvested in sole cropped barley. Despite different preceding crops and removal or incorporation of straw there was no significant difference between the subsequent non-fertilized winter-rye grain yields averaging 2.8 Mg ha-1 Indicating an equalization of the quality of incorporated residue by the NO3- leaching pattern. NO3- leaching throughout the experimental period was 61 to 76 kg N ha-1. Leaching dynamics indicated differences in the temporal N mineralization comparing lysimeters previously cropped with pea or with barley. The major part of this N was leached during autumn and winter. Leaching tended to be smaller in the lysimeters originally cropped with the pea-barley intercrops although not significantly different from the sole cropped pea and barley lysimeters. Soil N balances indicated depletion of N in the soil-plant system during the experimental period, independent of cropping system and residue management. N complementarity in the cropping system and the synchrony between residual N availability and crop N uptake is discussed.

In organic dairy farming, a major N input to the plant-soil system comes from biological N2 fixation by pasture legumes, but knowledge is sparse on how much of the fixed N2 is lost from the pastures as N2O. Nitrifying and denitrifying bacteria are the main contributors to the N2O production in soils. Currently, no contribution from biological N2 fixation in legume pastures is included in the national N2O inventories, partly because of uncertainties in quantifying the N2 fixation in the pastures (Mosier et al., 1998). According to the guidelines issued by The Intergovernmental Panel on Climate Change (IPCC), inventories for N2O emissions from agricultural soils should be based on the assumption that 1.25 % of added N is emitted as N2O (IPCC, 1997). The standard N2O emission factor of 1.25 % could be considerably unrepresentative for biologically fixed N2. Firstly, only a part of the fixed N is mineralised during the lifetime of the crop. Secondly, the release of inorganic N into the soil occurs slowly. A 15N2-tracer-experiment was initiated on grass-clover grown in pots. The aim was to assess: * the contribution of recently fixed N2 as a source of N2O * the translocation of N from clover to companion grass References IPCC, 1997. Greenhouse gas inventory. Reference manual. Vol. 3. Intergovernmental Panel on Climate Change. Bracknell, UK. Mosier, A. et al. 1998. Nutrient Cycling in Agroecosystems 52, 225-248.

In organic as well as conventional dairy farming, grass-clover pastures is an important component of the cropping system. This is because grass-clover is an excellent cattle fodder, and because clover has the ability of fixing atmospheric N2. When budgets for N2O emissions are made accord-ing to the IPCC guidelines it is assumed that 1.25 % of added nitrogen is emitted as N2O. This emission factor is used for all nitrogen inputs although the factor relies on experiments with fertilizer and manure, only. The emission factor for biological fixed nitrogen may be lower than 1.25 %, because nitrogen is released only slowly into the soil. However knowledge is very sparse. On the other hand, when the effect of grazing cattle is added the situation might be different. In Denmark organic cattle are supposed to be on grazing fields for at least 150 days a year. Nitrogen returned to the system in urine and dung is likely to locally exceed the needs of the plants and is therefore at risk of being lost as N2O. Thus far, however, there have only been a few detailed estimates of total N2O emissions from grassland livestock productions, and understanding of the factors controlling N2O emissions remains unsatisfactory.

Root system dynamics, productivity and N use were studied in inter- and sole crops of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) on a temperate sandy loam. A 32P tracer placed at a depth of 12.5, 37.5, 62.5 or 87.5 cm was employed to determine root system dynamics by sampling crop leaves at 0, 15, 30 and 45 cm lateral distance. 15N addition was used to estimate N2 fixation by pea, using sole cropped barley as reference crop. The Land Equivalent Ratio (LER), which is defined as the relative land area under sole crops that is required to produce the yields achieved in intercropping, were used to compare the crop growth in intercrops relative to the respective sole crops. The 32P appearance in leaves revealed that the barley root system grows faster than that of pea. P uptake by the barley root system during early growth stages was approximately 10 days ahead of that of the pea root system in root depth and lateral root distribution. More than 90% of the P uptake by the pea root system was confined to the top 12.5 cm of soil, whereas barley had about 25-30% of tracer P uptake in the 12.5 to 62.5 cm soil layer. Judging from this P uptake intercropping caused the barley root system to grow deeper and faster lateral root development of both species was observed. Barley accumulated similar amounts of aboveground N when grown as inter- and sole crop, whereas the total aboveground N acquired by pea in the intercrop was only 16% of that acquired in the pea sole crop. The percentage of total aboveground N derived from N2 fixation in sole cropped pea increased from 40% to 80% during the growth period, whereas it was almost constant at 85% in intercropped pea. The total amounts of N2 fixed were 95 and 15 kg N ha-1 in sole cropped and intercropped pea, respectively. Barley was the dominant component of the pea-barley intercrop, obtaining 90% of its sole crop yield, while pea produced only 15% of the grains of a sole crop pea. Intercropping of pea and barley improved the utilization of plant growth resources (LER > 1) as compared to sole crops. Root system distribution in time and space can partly explain interspecific competition. The 32P methodology proved to be a valuable tool for determining root dynamics in intercropping systems.

The experiments demonstrated the potential of pea-barley intercropping as a means of introducing complementary N use by increasing the proportional input of pea fixed N2, increasing the protein production and reducing weed problems in systems without herbicide use.

Boreal forest soils are globally one of the most extensive carbon storages, whereas soil respiration (carbon dioxide; CO2 efflux) forms the largest carbon flux from the ecosystem to the atmosphere. Current changes in the world climate may have unpredictable effects on belowground carbon processes, and thereby, on the carbon balance of boreal forests. To better understand the various processes in soil and to quantify the potential changes in the carbon cycle, forest-floor respiration was partitioned into five different components, and tree-root respiration was estimated, using four different methods in a mature boreal Scots pine (Pinus sylvestris L.) stand in southern Finland. Non-structural carbohydrate (NSC) concentrations in tree roots were determined, and carbon allocation to belowground by trees was estimated with the whole-tree carbon model ‘CASSIA’. In addition, tree-root respiration and heterotrophic soil respiration were separated using root exclusion in seven coniferous forests along a latitudinal gradient in Northern and Central Europe. The tree-root respiration comprised almost half of the forest-floor respiration, the heterotrophic soil respiration almost a third, and ground vegetation and respiration of mycorrhizal hyphae the remaining fifth in the boreal Scots pine stand. While the annual tree-root respiration decreased throughout the first three study years, the heterotrophic soil respiration increased when the mycorrhizal roots were excluded from the treatments. The tree-root respiration and most of the NSC concentrations were higher in the warmer years and lower in the cooler, as estimated with most of the methods. Three methods resulted in rather similar tree-root respiration estimations, while the tree-root respiration estimated with root incubation was significantly lower. The tree-root respiration was over 50% of the annual photosynthesis in the northernmost forest stand, whereas in the southernmost stand it was only up to 15%. Carbon allocation to the belowground, as modelled with CASSIA was a third of the annual photosynthesis on average and almost 5% for the symbiotic mycorrhizae. Boreaaliset metsämaat ovat yksi suurimmista hiilen varastoista maailmanlaajuisesti, kun taas maahengitys (hiilidioksidipäästöt) muodostaa suurimman hiilivirran ekosysteemistä ilmakehään. Ilmastonmuutoksella voi olla arvaamattomia vaikutuksia maanalaisiin hiiliprosesseihin ja sitä kautta boreaalisten metsien hiilitaseeseen. Maaperän erilaisten prosessien ymmärtämiseksi ja mahdollisten hiilenkierron muutosten arvioimiseksi muuttuvassa ilmastossa metsänpohjan kokonaismaahengitys jaettiin viiteen eri komponenttiin ja puun juurten hengitys arvioitiin neljällä eri menetelmällä keski-ikäisessä boreaalisessa mäntymetsikössä (Pinus sylvestris L.) Etelä-Suomessa. Varastosokerien ja tärkkelyksen pitoisuudet puiden juurissa määritettiin ja juurien varastosokereiden syötteen määrä maaperään ja symbionttisille mikrobeille (sienijuurille eli mykorritsoille) arvioitiin koko puun hiilitasemallilla ’CASSIA’. Lisäksi puun juurten hengitys ja maan heterotrofisten mikrobien maahengitys erotettiin toisistaan seitsemässä eri havumetsässä mittaamalla hengitystä tavanomaisissa ympäristöissä sekä maaperässä, jossa ei ollut eläviä juuria. Puiden juurten hengitys muodosti lähes puolet kokonaismaahengityksestä, heterotrofinen maahengitys lähes kolmanneksen, ja pintakasvillisuuden sekä sienijuurten hengitys loput viidenneksen boreaalisessa mäntymetsikössä. Vuotuinen puiden juurten hengitys laski kolmen ensimmäisen tutkimusvuoden ajan, kun taas heterotrofinen maahengitys nousi niin kutsutun ’Gadgil’-ilmiön vuoksi, kun sienijuuret eristettiin pois maasta. Puiden juurten hengitys ja suurin osa varasto-hiilihydraattien pitoisuuksista olivat korkeampia lämpiminä vuosina ja alhaisempia kylmempinä. Juuret ovat alati vuorovaikutuksessa maaperän kanssa ja niiden erottaminen toisistaan on menetelmällisesti haastavaa. Kolme eri menetelmää arvioi juurten hengityksen olevan melko saman tasoista, kun taas juurihengitys yksittäisistä kaivetuista ja leikatuista juurista oli merkittävästi alhaisempaa. Leikatut juuret kaivettiin melko läheltä maan pintaa, kun taas muilla menetelmillä arvioitu juurten hengitys oli peräisin myös syvemmistä maan kerroksista, jossa kosteutta oli todennäköisesti enemmän. Juurten hengitys oli yli 50 % vuotuisesta yhteytyksestä pohjoisimmassa metsikössä, kun taas eteläisimmässä metsikössä se oli vain 15 %. Koko puun hiilitasemallilla mallinnettu hiilisyöte maahan oli keskimäärin kolmannes vuotuisesta yhteytyksestä ja lähes 5 % symbioottisille sienijuurille.