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Background and aims: Belowground C supply from plant roots may accelerate the decomposition of SOM through the rhizosphere priming effect, but the detailed interaction between substrate quality and rhizosphere C supply is poorly understood. We hypothesize that decomposition of organic matter is enhanced by the combined effect of assimilate C supply to the rhizosphere and substrate amendments. Methods: Birch trees (Betula pendula) planted in experimental mesocosms; half of these trees were shaded to reduce the supply of assimilate C to roots and ECM fungi. Either 13C-enriched glucose, straw, fungal necromass or C4 biochar were subsequently added to each mesocosm. CO2 efflux derived from substrates were separated from that derived from native SOM and roots based on the isotopic composition of total respired CO2. Results: The addition of all substrates increased fluxes in both un-shaded and shaded treatments, with greatest total CO2 efflux observed in soils amended with straw. Increases in un-labelled CO2 were observed to be greater in the presence of belowground C supply than in mesocosms with shaded trees. Conclusions: Turnover of SOM is closely linked to belowground C allocation. The biochemical quality and recalcitrance of litter entering the soil C pool is of critical importance to this priming, as is the interaction with rhizosphere-associated decomposition activity. [ABSTRACT FROM AUTHOR]

The carbon (C) dynamics of a bioenergy system are key to correctly defining its viability as a sustainable alternative to conventional fossil fuel energy sources. Recent studies have quantified the greenhouse gas mitigation potential of these bioenergy crops, often concluding that C sequestration in soils plays a primary role in offsetting emissions through energy generation. Miscanthus is a particularly promising bioenergy crop and research has shown that soil C stocks can increase by more than 2 t C ha yr. In this study, we use a stable isotope (C) technique to trace the inputs and outputs from soils below a commercial Miscanthus plantation in Lincolnshire, UK, over the first 7 years of growth after conversion from a conventional arable crop. Results suggest that an unchanging total topsoil (0-30 cm) C stock is caused by Miscanthus additions displacing older soil organic matter. Further, using a comparison between bare soil plots (no new Miscanthus inputs) and undisturbed Miscanthus controls, soil respiration was seen to be unaffected through priming by fresh inputs or rhizosphere. The temperature sensitivity of old soil C was also seen to be very similar with and without the presence of live root biomass. Total soil respiration from control plots was dominated by Miscanthus-derived emissions with autotrophic respiration alone accounting for ∼50 % of CO. Although total soil C stocks did not change significantly over time, the Miscanthus-derived soil C accumulated at a rate of 860 kg C ha yr over the top 30 cm. Ultimately, the results from this study indicate that soil C stocks below Miscanthus plantations do not necessarily increase during the first 7 years. [ABSTRACT FROM AUTHOR]

Boreal forests occupy nearly one fifth of the terrestrial land surface and are recognised as globally important regulators of carbon (C) cycling and greenhouse gas emissions. Carbon sequestration processes in these forests include assimilation of CO2 into biomass and subsequently into soil organic matter, and soil microbial oxidation of methane (CH4). In this study we explored how ecosystem retrogression, which drives vegetation change, regulates the important process of soil CH4 oxidation in boreal forests. We measured soil CH4 oxidation processes on a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. Across these islands the build-up of soil organic matter was observed to increase with time since fire disturbance, with a significant correlation between greater humus depth and increased net soil CH4 oxidation rates. We suggest that this increase in net CH4 oxidation rates, in the absence of disturbance, results as deeper humus stores accumulate and provide niches for methanotrophs to thrive. By using this gradient we have discovered important regulatory controls on the stability of soil CH4 oxidation processes that could not have not been explored through shorter-term experiments. Our findings indicate that in the absence of human interventions such as fire suppression, and with increased wildfire frequency, the globally important boreal CH4 sink could be diminished. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) from agricultural soil is a significant source of greenhouse gas emissions. Biochar amendment can contribute to climate change mitigation by suppressing emissions of N2O from soil, although the mechanisms underlying this effect are poorly understood. We investigated the effect of biochar on soil N2O emissions and N cycling processes by quantifying soil N immobilisation, denitrification, nitrification and mineralisation rates using 15N pool dilution techniques and the FLUAZ numerical calculation model. We then examined whether biochar amendment affected N2O emissions and the availability and transformations of N in soils. Our results show that biochar suppressed cumulative soil N2O production by 91% in near-saturated, fertilised soils. Cumulative denitrification was reduced by 37%, which accounted for 85-95% of soil N2O emissions. We also found that physical/chemical and biological ammonium (NH4+) immobilisation increased with biochar amendment but that nitrate (NO3−) immobilisation decreased. We concluded that this immobilisation was insignificant compared to total soil inorganic N content. In contrast, soil N mineralisation significantly increased by 269% and nitrification by 34% in biochar-amended soil. These findings demonstrate that biochar amendment did not limit inorganic N availability to nitrifiers and denitrifiers, therefore limitations in soil NH4+ and NO3− supply cannot explain the suppression of N2O emissions. These results support the concept that biochar application to soil could significantly mitigate agricultural N2O emissions through altering N transformations, and underpin efforts to develop climate-friendly agricultural management techniques. [ABSTRACT FROM AUTHOR]

Background and aims: Acacia senegal, a leguminous dryland tree, is economically and ecologically important to sub-Saharan Africa. Water-use efficiency (WUE) and biological nitrogen fixation (BNF) are fundamental to plant productivity and survival. We quantify provenance differences in WUE, BNF, photosynthesis, biomass and gum arabic production from A. senegal assessing genetic improvement potential. Methods: Using stable isotope ratios, we determined WUE (δC) and BNF (δN) from provenances of mature A. senegal in field-trials (Senegal), sampling leaves at the beginning (wet) and end (dry) of the rainy season. Seedling provenance trials (UK) determined photosynthesis, and biomass and δC in relation to water table. Environmental data were characterised for all provenances at their sites of origin. Results: Provenances differed in both δC and δN. Gum yield declined with increasing WUE. Virtually no BNF was detected during the dry season and seedlings and mature trees may have different WUE strategies. Wind speed and soil characteristics at provenance origin were correlated with isotope composition and gum production. Conclusion: Provenance differences suggest that selection for desirable traits, e.g., increased gum production, may be possible. As ecological strategies relate to native locality, the environmental conditions at plantation site and provenance origin are important in assessing selection criteria. [ABSTRACT FROM AUTHOR]

A correction to the article "Carbon Inputs from Miscanthus Displace Older Soil Organic Carbon Without Inducing Priming" is presented.