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Abstract: The full accounting of greenhouse gas emissions is only possible when the three key contributors (CO2, CH4 and N2O) are continuously measured throughout the observation period. While some field techniques such as eddy covariance or automatic closed chamber measurements have attempted near-continuous observations, it is always beyond the control of the experimentalist to ensure perfect continuity. Therefore, the final time series of fluxes will inevitably have periods without reliable values (i.e., gaps) and will need to be gap-filled. While there is abundant literature on methodologies for gap-filling CO2 fluxes, there is no literature on the gap-filling methods for trace gases such as nitrous oxide. We investigate three general approaches for gap-filling nitrous oxide time series: linear extrapolation, moving average and look-up tables. A five-year time-series of eddy-covariance measurements from an intensive grassland site in South-Western Ireland was used as an example. The amount of gaps varied significantly from year to year. The single-year look-up table technique produced consistently good results even when long gaps were present in the time series. In some years, the simpler annual extrapolation technique performed equally well. It is essential that this work be extended with more complex methods, and that methods in this paper are further evaluated under different environmental conditions. We believe that these methods and analysis could also be applied to other trace gas gap-filling (e.g., CH4) which have similar intermittent patterns of emission. [Copyright &y& Elsevier]

Abstract: In Ireland fertilised grasslands are major source of nitrous oxide (N2O), a powerful greenhouse gas. We present 5 years (2004–2008) of eddy-covariance (EC) observations of N2O fluxes from an ecosystem transitioning from wet managed grassland to a broadleaf forestry. One sector of the EC footprint was converted to forestry during the observation period, while the remainder of the footprint remained under intensively managed grassland. The daily N2O fluxes were responsive to the rainfall and changes in soil moisture, whereas the annual and monthly emissions depended on the amount of nitrogen fertilisers applied. During the year of forest establishment, mechanical disturbances increased N2O emissions from the forest sector, even higher than the adjacent grassland; in the following 3 years, the intensity of the nitrous oxide flux dropped to approximately one-third of the previous “grassland” level of emission. At the same time, the grassland sector recorded approximately a 20% decrease of flux intensity, coinciding with a reduction in fertiliser application. These observations led us to conclude that while reduced fertilisation and the cessation of grazing contributed to the reduction of N input, a certain period is required for the new forest to establish, settle and mature and thereby to attain a stable rate of emission of N2O emissions. [ABSTRACT FROM AUTHOR]

Colloid transport was studied in heterogeneous sand columns under unsaturated steady-state conditions, using two sizes of acid-cleaned sand to pack the column. Heterogeneity was created by placing three continuous tubes of fine sand (3.6% of the total volume) within a column of coarse sand (mean grain diameters 0.36 and 1.2 mm, respectively). Experiments were performed under three flow rates (0.1, 0.2, and 0.4 cm/min) applied by a rain simulator at the top of the column. Constant water-content profile in the coarse sand was achieved by applying corresponding suction at the column bottom. Three sizes of latex microspheres (1, 0.2, and 0.02 μm) and soluble tracers (LiBr), diluted in a weak base (pH 7.3, ionic strength 0.0023 M) solution, were used simultaneously. Introduction of preferential pathways reduced front-arrival time about 2-fold and increased colloid recovery which, at the 0.2 cm/min flow rate, was higher than at 0.4 and 0.1 cm/min. Maximum solution flux from coarse to fine sand (due to differences in matric pressure) at 0.2 cm/min, verified by hydrodynamic modeling, could explain this phenomenon. Results suggest that in heterogeneous soil, maximum colloid recovery does not necessarily occur at maximum water content This has clear implications for colloid transport in natural soils, many of which are heterogeneous. [ABSTRACT FROM AUTHOR]

Information about several topics discussed at the second ClimMani-Integrated Network for Terrestrial Ecosystem Research on Feedbacks to the Atmosphere and Climate workshop held in Mikulov, Czech Republic in June 2013 on the importance of the terrestrial ecosystem is presened. Topics include drivers of biome shifts, trait responses to environmental change, and scaling of small plots to landscapes. The workshop featured speakers including Melinda Smith, Jordi Sardans and William Hoffman.

The response of terrestrial ecosystems to rising atmospheric CO2 concentration (Ca), particularly under nutrient-limited conditions, is a major uncertainty in Earth System models. The Eucalyptus Free-Air CO2 Enrichment (EucFACE) experiment, recently established in a nutrient- and water-limited woodland presents a unique opportunity to address this uncertainty, but can best do so if key model uncertainties have been identified in advance. We applied seven vegetation models, which have previously been comprehensively assessed against earlier forest FACE experiments, to simulate a priori possible outcomes from EucFACE. Our goals were to provide quantitative projections against which to evaluate data as they are collected, and to identify key measurements that should be made in the experiment to allow discrimination among alternative model assumptions in a postexperiment model intercomparison. Simulated responses of annual net primary productivity (NPP) to elevated Ca ranged from 0.5 to 25% across models. The simulated reduction of NPP during a low-rainfall year also varied widely, from 24 to 70%. Key processes where assumptions caused disagreement among models included nutrient limitations to growth; feedbacks to nutrient uptake; autotrophic respiration; and the impact of low soil moisture availability on plant processes. Knowledge of the causes of variation among models is now guiding data collection in the experiment, with the expectation that the experimental data can optimally inform future model improvements. [ABSTRACT FROM AUTHOR]