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As a key component of the carbon cycle, soil CO2 efflux (SCE) is being increasingly studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of current relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question of to what extent these relationships remain unaltered beyond the current climatic window for which observations are available to constrain the relationships. Here, we evaluate whether current responses of SCE to fluctuations in soil temperature and soil water content can be used to predict SCE under altered rainfall patterns. Of the 58 experiments for which we gathered SCE data, 20 were discarded because either too few data were available or inconsistencies precluded their incorporation in the analyses. The 38 remaining experiments were used to test the hypothesis that a model parameterized with data from the control plots (using soil temperature and water content as predictor variables) could adequately predict SCE measured in the manipulated treatment. Only for 7 of these 38 experiments was this hypothesis rejected. Importantly, these were the experiments with the most reliable data sets, i.e., those providing high-frequency measurements of SCE. Regression tree analysis demonstrated that our hypothesis could be rejected only for experiments with measurement intervals of less than 11 days, and was not rejected for any of the 24 experiments with larger measurement intervals. This highlights the importance of high-frequency measurements when studying effects of altered precipitation on SCE, probably because infrequent measurement schemes have insufficient capacity to detect shifts in the climate dependencies of SCE. Hence, the most justified answer to the question of whether current moisture responses of SCE can be extrapolated to predict SCE under altered precipitation re

Dissolved inorganic carbon (DIC) fluxes across the vadose zone are influenced by a complex interplay of biological, chemical and physical factors. A novel soil mesocosm system was evaluated as a tool for providing information on the mechanisms behind DIC percolation to the groundwater from unplanted soil. Carbon dioxide partial pressure (pCO(2)), alkalinity, soil moisture and temperature were measured with depth and time, and DIC in the percolate was quantified using a sodium hydroxide trap. Results showed good reproducibility between two replicate mesocosms. The pCO(2) varied between 0.2 and 1.1 %, and the alkalinity was 0.1-0.6 meq L-1. The measured cumulative effluent DIC flux over the 78-day experimental period was 185-196 mg L-1 m(-2) and in the same range as estimates derived from pCO(2) and alkalinity in samples extracted from the side of the mesocosm column and the drainage flux. Our results indicate that the mesocosm system is a promising tool for studying DIC percolation fluxes and other biogeochemical transport processes in unsaturated environments.

To investigate the effect of sheep dung on soil carbon (C) sequestration, a 152 days incubation experiment was conducted with soils from two different Inner Mongolian grasslands, i.e. a Leymus chinensis dominated grassland representing the climax community (2.1% organic matter content) and a heavily degraded Artemisia frigida dominated community (1.3% organic matter content). Dung was collected from sheep either fed on L. chinensis (C3 plant with δ13C = -26.8‰; dung δ13C = -26.2‰) or Cleistogenes squarrosa (C4 plant with δ13C = -14.6‰; dung δ13C = -15.7‰). Fresh C3 and C4 sheep dung was mixed with the two grassland soils and incubated under controlled conditions for analysis of 13C-CO2 emissions. Soil samples were taken at days 17, 43, 86, 127 and 152 after sheep dung addition to detect the δ 13C signal in soil and dung components. Analysis revealed that 16.9% and 16.6% of the sheep dung C had decomposed, of which 3.5% and 2.8% was sequestrated in the soils of L. chinensis and A. frigida grasslands, respectively, while the remaining decomposed sheep dung was emitted as CO 2. The cumulative amounts of C respired from dung treated soils during 152 days were 7-8 times higher than in the un-amended controls. In both grassland soils, ca. 60% of the evolved CO2 originated from the decomposing sheep dung and 40% from the native soil C. Priming effects of soil C decomposition were observed in both soils, i.e. 1.4 g and 1.6 g additional soil C kg-1 dry soil had been emitted as CO2 for the L. chinensis and A. frigida soils, respectively. Hence, the net C losses from L. chinensis and A. frigida soils were 0.6 g and 0.9 g C kg-1 soil, which was 2.6% and 7.0% of the total C in L. chinensis and A. frigida grasslands soils, respectively. Our results suggest that grazing of degraded Inner Mongolian pastures may cause a net soil C loss due to the positive priming effect, thereby accelerating soil deterioration. © 2013 Ma et al.

The potential for N2O emissions outside the main growing season may be influenced by long-term effects of cropping system. This was investigated by collecting intact soil cores (100 cm3, 0-4 cm depth) under winter wheat in three organic cropping systems and a conventional reference within a long-term crop rotation experiment. Average annual inputs of C in crop residues and manure ranged from 1.7 to 3.3 Mg ha-1. A simulated freeze-thaw cycle resulted in a flush of CO2 during the first 48 h, which could be mainly from microbial sources. Other samples were adjusted to approximately –10, -30 or –100 hPa and amended with excess 15NO3- prior to freezing and thawing. Denitrification was the main source of N2O during a 72-h incubation at 22C, as judged from N2O and total 15N evolution. Although the input of C in the conventionally managed cropping system was significantly less than in the organic cropping systems, it showed higher N2O evolution at all three matric potentials. Estimates of relative gas diffusivity (DP/D0) in soil from the four cropping systems indicated that C input affected soil aeration. Soil from the two cropping systems with highest C input showed N2O evolution at DP/D0 in excess of 0.02, which is normally considered a threshold for development of anaerobic sites in the soil, presumably because the oxygen demand was also high. The study shows that cropping system affects both soil gas diffusivity and C availability, and that both characteristics significantly influence the N2O emission potential.

More efficient use of green manure-derived nitrogen (N) may improve crop yields and reduce environmental impacts in stockless organic arable farming. In this 3-month incubation study, we tested a new strategy where green manure leys are 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. Grass-clover compost or silage was soil-incorporated by either simulated ploughing (green manure placed at 15 cm depth) or harrowing (green manure mixed into the upper 5-cm soil horizon) in order to assess treatment effects on net release of plant-available N, nitrous oxide (N2O) fluxes, and soil respiration. Grass-clover silage provided the highest net N release with similar results for the two incorporation methods. Up to one third of the total N content in silage became plant-available during the three months. In contrast, no net N release was observed for the composted grass-clover and straw mixture. In fact, soil incorporation of compost by harrowing caused temporal immobilization of soil mineral N. Silage incorporated by ploughing gave rise to increased N2O effluxes, corresponding to 0.3 % of applied total N. Possibly N2O production via denitrification was stimulated by oxygen-limited conditions near the decomposing silage. In contrast, compost incorporated by harrowing caused downwards N2O fluxes, presumably an effect of reduced mineral N availability in this treatment. Overall, our study revealed that ensiled grass-clover was the best fertilizer product, and that N2O emissions may be reduced by incorporating green manures using harrowing instead of ploughing.

En ny strategi kan måske forbedre udnyttelsen af det kvælstof, som fikseres af kløverplanter i økologisk planteavl. Vores studie viste en større frigivelse af kvælstof fra ensileret kløvergræs end fra kompost af kløvergræs iblandet halm. Samtidig fandt vi, at indarbejdning af grøngødning ved harvning frem for pløjning reducerede udledningen af den stærke drivhusgas, lattergas.

This 3-month incubation study showed that ensiled grass-clover was a better nitrogen (N) source than a composted grass-clover and straw mix (grass-clover:straw, 4:1, w:w), owing to the high content of labile compounds compared to the more degraded compost. Our study also indicated that emissions of the strong greenhouse gas nitrous oxide (N2O) can be reduced by incorporating green manure using harrowing instead of ploughing. The silage-derived N release by the end of the incubation was equivalent to 38-42 kg N ha-1, which corresponded to one third of the N applied in silage, with no difference between ploughing and harrowing. In contrast, no net release of mineral N was detected from the composted grass-clover.

The global burden of carbon monoxide (CO) is rather uncertain. In this paper we address the potential for UV-induced CO emission by living terrestrial vegetation surfaces. Real-time measurements of CO concentrations were made with a cavity enhanced laser spectrometer connected in closed loop to either an ecosystem chamber or a plant-leaf scale chamber. Leaves of all examined plant species exhibited emission of CO in response to artificial UV-radiation as well as the UV-component of natural solar radiation. The UV-induced rate of CO emission exhibited a rather low dependence on temperature, indicating an abiotic process. The emission of CO in response to the UV-component of natural solar radiation was also evident at the ecosystem scale.

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (RS) in a Danish Calluna-Deschampsia-heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO2 (+130 ppm), raised soil temperature (+0.4 degrees C) and extended summer drought (58% precipitation exclusion) was established in 2005. The average RS, observed in the control over 3 years of measurements (1.7 mu mol CO2 m-2 sec-1), increased 38% under elevated CO2, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased RS by 14%, while elevated soil temperature did not affect RS overall. A significant interaction between elevated temperature and drought resulted in further reduction of RS when these treatments were combined. A detailed analysis of short-term RS dynamics associated with drought periods showed that RS was reduced by 50% and was strongly correlated with soil moisture during these events. Recovery of RS to pre-drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts RS from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in RS. The model predicted annual sums of RS in 2006 and 2007, in the control, were 672 and 719 g C m-2 y-1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by 21% (140150 g C m-2 y-1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.

A field experiment was carried out including alternating perennial ryegrass (Lolium perenne L.) – clover (Trifolium repens + Trifolium pretense L.) pasture mix with annual vetch (Vicia villosa L.) – winter rye (Secale cereale L.) intercrops. The annuals were established after soil incorporation of a 1st-year grass-clover in a 6-m wide strip, whereas the perennials were 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 especially limiting growth of the rye component. Relative clover proportion in the sward increased with increasing distance to the annual strip indicating more available soil mineral N in the interface between the perennial and the annual strip. Compensative growth of the grass-clover when grown in close proximity to the annual strip was only partly counterbalancing the decreased total crop productivity in the rye-vetch intercrop. Across the whole growing season (Sept. - Aug.) approximately the same amount of biomass was produced when dividing the field into strips (6x6m) as compared to growing the same area with the traditional single-field cropping strategy. There was a greater total aboveground plant N uptake in sole cropped vetch and the rye-vetch intercrop compared to the rye sole crop due to vetch N2-fixation, but with severe vetch-growth depression when intercropped. The amount of vetch-N2 fixed was reduced with about 9 g N m-2 when intercropped as compared to the sole cropping situation. Light interception by the annual crop when grown in close proximity to the grass-clover strip was reduced due to the lower aboveground biomass yield and assumed belowground competitive interactions. Less soil water content below the perennial strip indicated greater water uptake, than below the annual strips. Unfortunately, the present strip cropping system did not possess the right balance of co-existence and complementarity. How

The global burden of carbon monoxide, CO, is rather uncertain. In this paper we address the potential of UV-induced CO emission by terrestrial surfaces. Real-time measurements of [CO] were made with a cavity enhanced laser connected in closed loop to either an ecosystem chamber or a leaf scale chamber. Sand and leaves of all examined plant species exhibited emission of CO in response to artificial UV-radiation and the UV-component of natural solar radiation. The UV-induced rate of CO emission exhibited a rather low dependence on temperature, indicating an abiotic process. The emission of CO in response to the UV-component of natural solar radiation was also evident at the ecosystem scale. When scaled to the global level, the UV-induced emission of CO by the major types of terrestrial surfaces, living leaves and soil (here represented by sand), amounts up to 28 Tg yr−1. This source has till now not been accounted for by IPCC, but is equivalent to 14–56% of the 50–200 Tg yr−1 from sources currently accounted for (IPCC 2001). In addition to this are other known sources that ought to be considered. The hitherto unaccounted for terrestrial sources of CO amounts up to 207 Tg yr−1, almost two-thirds of the latest estimated global CO burden of 360 Tg yr−1 (IPCC, 2001).

Over hele verden truer klimaforandringer økosystemer, og forårsager ændringer i artssammensætninger samt tab af biodiversitet. Det forventes at Jordens økosystemer kan øge deres nettoprimærproduktion, og dermed også deres kulstofbinding, på grund af den stigende atmosfæriske CO2-koncentration. Dette kan dog hæmmes hvis der opstår progressiv nitrogenbegrænsning (PNL) i systemerne. I dette forsøg blev der blev taget jord ud fra et klimamanipulationsforsøg på en dansk hede, og denne jord blev inkuberet i potter. Forsøget havde i seks års fået klimabehandlinger: forhøjet CO2, forhøjet temperatur og sommertørke, alene og i alle kombinationer. Der blev dyrket rødsvingel og byg i potterne, og mens et hold potter blev inkuberet i de respektive behandlinger, blev det andet hold potter blev inkuberet under normale forhold. Effekten af behandlingerne på tilgængeligheden af nitrogen (N) i jorden samt planternes optag af N, blev undersøgt ved hjælp af ekstraktioner og elementaranalyser, og desuden blev en 15N-tracerteknik brugt til at spore 15N-NH4+-optag i planterødder og -skud. Forhøjet CO2 havde modsatrettede virkninger, da det både stimulerede N mineralisering, men også reducerede mængden af plantetilgængeligt N, ved at øge mængden af N der var inkorporeret i organisk materiale. Forhøjet temperatur stimulerede N mineralisering, og denne øgede mineralisering modvirkede den CO2-skabte reduktion i plantetilgængeligt N. Tørkebehandlingen modvirkede de positive effekter af forhøjet CO2 og temperatur på N mineralisering. Derved førte tørkebehandlingen til nedsat tilgængelighed af N i jorden samt mindsket planteoptag af N. Resultaterne betyder, at tørkebehandling mindsker nitrogens kredsløb i systemet på kort sigt, hvilket kan kvælstofbegrænse vegetationen progressivt og dermed føre til, at kulstofbindingen i dette økosystem bliver reduceret på længere sigt., Climate change is threatening ecosystems all over the world causing changes in species composition and loss of biodiversity. Earth’s ecosystems are expected to increase net primary productivity and carbon sequestration in response to the rising atmospheric CO2 concentration, but this may be limited by progressive nitrogen limitation (PNL) of the systems. Soil, from a Danish heathland experiment, was collected and pot incubated. The experiment had received six years of climate change treatments elevated CO2, elevated temperature, and prolonged summer droughts, solely and in all combinations. Red fescue and barley was grown in the pots, and one batch of pots was incubated in the respective treatments, while one batch was incubated at ambient levels. The effect of the treatments, on nitrogen (N) soil availability and plant uptake, was examined by use of soil extractions and elemental analyses, and a 15N tracer technique was use to trace 15N-NH4+ uptake in plant roots and shoots. Elevated CO2 had contrasting effects, as it both stimulated mineralization and reduced plant available N, by increasing the amount of N incorporated into organic material. Elevated temperature stimulated mineralization. The increased mineralization counteracted the reduction in plant available N, induced by elevated CO2. Drought counteracted the positive effects of elevated temperature and CO2 on mineralization. Thereby, drought led to decreased soil availability and plant uptake of N. The results imply that drought decreases N cycling, in the short-term investigation, which may lead to progressive N limitation of the vegetation and reduce carbon sequestration, in this ecosystem after long term manipulation.

Carbon dioxide (CO2) fluxes, composing net ecosystem exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) were measured in a temperate heathland exposed to elevated CO2 by the FACE (free-air carbon enrichment) technique, raising the atmospheric CO2 concentration from c. 380 μmol mol−1 to 510 μmol mol−1. All CO2 fluxes were measured by the static chamber methodology. Although the FACE technique enriches the atmosphere with CO2 to a fixed level, the above ground CO2 concentrations may nevertheless locally vary strongly (from about ambient to 1000 μmol mol−1). Deployment of static chambers to FACE experiments should therefore be performed with great care in order to ensure reproducible conditions with respect to chamber headspace CO2 concentration. We demonstrate that that the fluxes measured by closed chambers relate linearly to the initial headspace CO2 concentration. When changing the initial headspace CO2 concentration from 380 to 510 μmol mol−1 the net CO2 assimilation expressed by NEE increased instantaneously 1.51 times in control plots and 1.71 times in FACE plots. By contrast, ER in control plots decreased, being 0.87 times that measured at the low CO2 concentration, and the flux also decreased in FACE plots, to 0.79 times that at low concentration. Similar SR in control plots was decreased 0.94 times in control plots and 0.88 times in FACE plots. We found that a useful method to achieve stable and reproducible chamber headspace and soil CO2 concentration prior to commencement of flux measurements was to turn off the FACE system at least 10 min in advance. Within 10 min a new equilibrium was established between the soil and atmosphere, apparently due to CO2 degassing from the top soil. The observed increase in SR in response to increased CO2 persisted for up to 18 h during which measurements should be performed. Soil CO2 concentrations were increased by up to 500 μmol mol−1 by the FACE treatment, substantially more than the 130 μmol mol−1 enri

In temperate regions, climate change is predicted to increase annual mean temperature and intensify the duration and frequency of summer droughts, which together with elevated atmospheric carbon dioxide (CO2) concentrations, may affect the exchange of nitrous oxide (N2O) and methane (CH4) between terrestrial ecosystems and the atmosphere. We report results from the CLIMAITE experiment, where the effects of these three climate change parameters were investigated solely and in all combinations in a temperate heathland. Field measurements of N2O and CH4 fluxes took place 1–2 years after the climate change manipulations were initiated. The soil was generally a net sink for atmospheric CH4. Elevated temperature (T) increased the CH4 uptake by on average 10 μg C m−2 h−1, corresponding to a rise in the uptake rate of about 20%. However, during winter elevated CO2 (CO2) reduced the CH4 uptake, which outweighed the positive effect of warming when analyzed across the study period. Emissions of N2O were generally low (

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.

Field emissions of N2O during cultivation of bioenergy crops may counterbalance a considerable part of the avoided fossil CO2 emissions that are achieved by fossil fuel displacement