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Anthropogenic emissions of C[O.sub.2] have been increasing, reaching 9.7billion tonnes of carbon (C) in 2012 (ref. 15). The biosphere absorbs approximately 50% of anthropogenically emitted carbon per annum (16), so [...], The rising atmospheric concentration of carbon dioxide (C[O.sub.2]) should stimulate ecosystem productivity, but to what extent is highly uncertain, particularly when combined with changing temperature and precipitation (1). Ecosystem response to C[O.sub.2] is complicated by biogeochemical feedbacks (2) but must be understood if carbon storage and associated dampening of climate warming are to be predicted (3). Feedbacks through the hydrological cycle are particularly important (4) and the physiology is well known; elevated C[O.sub.2] reduces stomatal conductance and increases plant water use efficiency (the amount of water required to produce a unit of plant dry matter) (5). The C[O.sub.2] response should consequently be strongest when water is limiting (6); although this has been shown in some experiments (7), it is absent from many (8-11). Here we show that large annual variation in the stimulation of aboveground biomass by elevated C[O.sub.2] in a mixed [C.sub.3]/[C.sub.4] temperate grassland can be predicted accurately using seasonal rainfall totals; summer rainfall had a positive effect but autumn and spring rainfall had negative effects on the C[O.sub.2] response. Thus, the elevated C[O.sub.2] effect mainly depended upon the balance between summer and autumn/spring rainfall. This is partly because high rainfall during cool, moist seasons leads to nitrogen limitation, reducing or even preventing biomass stimulation by elevated C[O.sub.2]. Importantly, the prediction held whether plots were warmed by 2°C or left unwarmed, and was similar for [C.sub.3] plants and total biomass, allowing us to make a powerful generalization about ecosystem responses to elevated C[O.sub.2]. This new insight is particularly valuable because climate projections predict large changes in the timing of rainfall, even where annual totals remain static (12). Our findings will help resolve apparent differences in the outcomes of C[O.sub.2] experiments and improve the formulation and interpretation of models that are insensitive to differences in the seasonal effects of rainfall on the C[O.sub.2] response (7,13,14).

Ammonia oxidising bacteria (AOB) are important soil microorganisms that carry out the first step in nitrification, the oxidation of ammonia to nitrite. In this paper we investigated the impact of long-term elevated C[O.sub.2] on soil nitrification and soil AOB community composition. Soil samples were taken from Hakanoa natural C[O.sub.2] springs, Kamo, Northland, New Zealand. This site has been exposed to elevated C[O.sub.2] for several decades. Soils were collected from different points near to C[O.sub.2]-emitting vents where the C[O.sub.2] concentration at canopy height had been characterised. Nitrification activity was measured using a short-term nitrification assay, and AOB community composition was characterised using polymerase chain reaction and denaturing gradient gel electrophoresis (DGGE). A principal component analysis of the DGGE banding pattern was carded out to identify the effect of C[O.sub.2] on AOB community composition. Soil nitrification activity was markedly decreased with increasing C[O.sub.2]. The variation in DGGE banding patterns revealed differences in the composition of the soil AOB community that were related to C[O.sub.2] concentration. Principal component analysis showed that the changes in community composition and nitrifying activity were linked and that these changes were related to atmospheric C[O.sub.2] concentration. Additional keywords: elevated C[O.sub.2], nitrification, ammonia oxidising bacteria, natural C[O.sub.2] springs, DGGE., Introduction Environmental settings for biological activity related to atmosphere and climate are changing rapidly. Of these changes, the increasing concentration of C[O.sub.2] in the atmosphere is particularly significant as it [...]

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 ( eCO2), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment ( FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong ( r2 = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2-grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2, and this decrease was independent of the presence or magnitude of eCO2-induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2-induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2-induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation. [ABSTRACT FROM AUTHOR]

In pastures grazed by large herbivores, nutrients cycle both through litter and animal excreta. We compared nitrogen (N) returns from sheep grazing a temperate pasture exposed to ambient or elevated CO2 (475 μmol mol−1) in a FACE (Free Air CO2 Enrichment) experiment established in the spring of 1997. In the spring of 2000 and 2001, we measured the chemical composition of the diet, sheep faeces and of individual plant species before grazing to characterize feed intake and to compare the intake of N to the N produced in faeces. In both years under elevated CO2, leaves of the individual species exhibited lower N concentrations and higher water-soluble carbohydrate (WSC) concentrations. There was a significantly greater proportion of legume in the diet at elevated CO2 but, together with the changes in chemical composition of individual species, this resulted in diets that had similar N but higher WSC and digestibility for both ambient and elevated CO2. We found that a greater proportion of dietary N was partitioned to urine at elevated CO2, probably because of the higher proportion of legume N in the diet, with possible differences in protein quality. A potentially significant consequence of this change in partitioning is greater N loss through volatilization at higher CO2 levels. [ABSTRACT FROM AUTHOR]

Abstract: The rising concentration of CO2 in the atmosphere has the potential to alter many aspects of plant growth that in turn could result in changes in soil properties, such as storage of carbon and nitrogen. Through feed-backs between soils and plants, any changes in soil properties could potentially modify how plants respond to elevated CO2. To test for possible interactions, we measured plant growth responses of the perennial herb Plantago lanceolata L. to elevated CO2 using sites exposed long-term (>20 years) to low (372 ppm) or high (574 ppm) atmospheric CO2 in the area surrounding a natural mineral spring. The soil developed long-term at high CO2 had greater organic carbon, total nitrogen and net mineral-N mineralisation than the soil developed long-term at low CO2. Seedlings were transplanted into soils at each site and into soils swapped between the low and high CO2 sites and growth was recorded over 246 days. At the final harvest, plants grown in a soil developed at low CO2 but exposed to a high CO2 atmosphere (i.e. conventional CO2 enrichment experiment) had greater root and leaf mass and higher seed production than plants grown in soils developed at and exposed to a low CO2 atmosphere. However, a more realistic comparison of current (low CO2 soil-low CO2 atmosphere) and future (high CO2 soil-high CO2 atmosphere) environments showed no difference in leaf and root biomass but reduced seed production in the high CO2 soil-high CO2atmosphere treatment compared to the low CO2 soil-low CO2 atmosphere treatment. The results indicate that the properties of soils developed long-term at elevated CO2 may affect how plants respond to an elevated CO2 atmosphere. Caution is therefore needed in how we interpret plant responses to elevated CO2 that are based on experiments using step changes in atmospheric CO2and which are of short duration. Die steigende CO2-Konzentration in der Atmosphäre besitzt das Potential zahlreiche Aspekte des pflanzlichen Wachstums zu verändern. Demzufolge könnte es zu Veränderung von Bodeneigenschaften wie z.B. der Speicherung von Kohlenstoff und Stickstoff kommen. Über die Wechselwirkung zwischen Boden und Pflanzen könnte jede Veränderung der Bodeneigenschaften möglicherweise die Reaktionen der Pflanzen auf erhöhtes CO2 modifizieren. Um die möglichen Interaktionen zu testen, maßen wir die Pflanzenwachstumsreaktion des perennierenden Krautes Plantago lanceolata L. auf erhöhtes CO2. In einem Gebiet rund um eine natürliche Mineralquelle verwendeten wir Probeflächen, die langfristig (>20 Jahre) geringem (372 ppm) oder hohem (574 ppm) atmosphärischen CO2 ausgesetzt waren. Der Boden, der sich langfristig bei hohem CO2 entwickelte, besaß mehr organischen Kohlenstoff, Gesamtstickstoff und eine größere Nettomineralisierung mineralischen Stickstoffs als der Boden, der sich langfristig bei geringem CO2 entwickelte. Keimlinge wurden in Böden der jeweiligen Probefläche verpflanzt und in Böden, die zwischen den Probeflächen mit niedrigem und hohem CO2 ausgetauscht wurden. Das Wachstum wurde über 246 Tage gemessen. Bei der abschließenden Ernte hatten die Pflanzen, die in einem Boden wuchsen, der sich bei hohem CO2 entwickelte, und die einer hohen CO2-Atmosphäre ausgesetzt waren (d.h. konventionelles CO2-Anreicherungs-Experiment), eine höhere Wurzel- und Blattmasse sowie eine größere Samenproduktion als die Pflanzen, die in Böden wuchsen, die sich bei niedrigem CO2 entwickelten, und die einer geringen CO2-Atmosphäre ausgesetzt waren. Dennoch zeigte der realistischere Vergleich der derzeitigen (CO2-armer Boden – CO2-arme Atmosphäre) und zukünftigen (CO2-reicher Boden – CO2-reiche Atmosphäre) Umweltbedingungen keine Unterschiede in der Blatt- und Wurzelbiomasse, jedoch eine reduzierte Samenproduktion bei der Behandlung CO2-reicher Boden – CO2-reiche Atmosphäre im Vergleich zur Behandlung CO2-armer Boden – CO2-arme Atmosphäre. Die Ergebnisse lassen vermuten, dass die Eigenschaften von Böden, die sich langfristig bei erhöhtem CO2 entwickelten, Auswirkungen darauf haben, wie Pflanzen auf eine erhöhte CO2-Atmosphäre reagieren. Deshalb ist Vorsicht angebracht, wenn wir die Reaktionen von Pflanzen auf erhöhtes CO2 interpretieren, die bei Experimenten auftreten, die mit abgestuften Veränderungen des atmosphärischen CO2 arbeiten und die von kurzer Dauer sind. [Copyright &y& Elsevier]

Presents information on a study which examined the soil nematode fauna in an area with naturally elevated carbon dioxide levels. Description of the site; Materials and methods; Correlation between the abundance of nematode and environmental factors; Discussion on the study findings.

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 ( eCO2), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment ( FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong ( r2 = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2-grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2, and this decrease was independent of the presence or magnitude of eCO2-induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2-induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2-induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation. [ABSTRACT FROM AUTHOR]