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.
► Single-year look-up table technique produces consistently good annual sums. ► Simple extrapolation techniques perform equally well in the absence of long gaps. ► Statistical performance of ...gap-filling techniques for N2O is worse than that for CO2.
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.
▶ Nitrous oxide emissions decrease within few years after forest establishment. ▶ Short-term spike in emission is possible due to soil disturbance. ▶ Grazing and fertilisation are the main sources of ...the emission from grasslands.
In Ireland fertilised grasslands are major source of nitrous oxide (N
2O), a powerful greenhouse gas. We present 5 years (2004–2008) of eddy-covariance (EC) observations of N
2O 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 N
2O 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 N
2O 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 N
2O emissions.
In this study, the processes behind observed new particle formation (NPF) events and subsequent organic-dominated particle growth at the Pallas Atmosphere–Ecosystem Supersite in Northern Finland are ...explored with the one-dimensional column trajectory model ADCHEM. The modeled sub-micron particle mass is up to ∼ 75 % composed of SOA formed from highly oxidized multifunctional organic molecules (HOMs) with low or extremely low volatility. In the model the newly formed particles with an initial diameter of 1.5 nm reach a diameter of 7 nm about 2 h earlier than what is typically observed at the station. This is an indication that the model tends to overestimate the initial particle growth. In contrast, the modeled particle growth to CCN size ranges ( > 50 nm in diameter) seems to be underestimated because the increase in the concentration of particles above 50 nm in diameter typically occurs several hours later compared to the observations. Due to the high fraction of HOMs in the modeled particles, the oxygen-to-carbon (O : C) atomic ratio of the SOA is nearly 1. This unusually high O : C and the discrepancy between the modeled and observed particle growth might be explained by the fact that the model does not consider any particle-phase reactions involving semi-volatile organic compounds with relatively low O : C. In the model simulations where condensation of low-volatility and extremely low-volatility HOMs explain most of the SOA formation, the phase state of the SOA (assumed either liquid or amorphous solid) has an insignificant impact on the evolution of the particle number size distributions. However, the modeled particle growth rates are sensitive to the method used to estimate the vapor pressures of the HOMs. Future studies should evaluate how heterogeneous reactions involving semi-volatility HOMs and other less-oxidized organic compounds can influence the SOA composition- and size-dependent particle growth.
Climatic extreme events are expected to occur more frequently in the future, increasing the likelihood of unprecedented climate extremes (UCEs) or record-breaking events. UCEs, such as extreme ...heatwaves and droughts, substantially affect ecosystem stability and carbon cycling by increasing plant mortality and delaying ecosystem recovery. Quantitative knowledge of such effects is limited due to the paucity of experiments focusing on extreme climatic events beyond the range of historical experience. Here, we present a road map of how dynamic vegetation demographic models (VDMs) can be used to investigate hypotheses surrounding ecosystem responses to one type of UCE: unprecedented droughts. As a result of nonlinear ecosystem responses to UCEs that are qualitatively different from responses to milder extremes, we consider both biomass loss and recovery rates over time by reporting a time-integrated carbon loss as a result of UCE, relative to the absence of drought. Additionally, we explore how unprecedented droughts in combination with increasing atmospheric CO2 and/or temperature may affect ecosystem stability and carbon cycling. We explored these questions using simulations of pre-drought and post-drought conditions at well-studied forest sites using well-tested models (ED2 and LPJ-GUESS). The severity and patterns of biomass losses differed substantially between models. For example, biomass loss could be sensitive to either drought duration or drought intensity depending on the model approach. This is due to the models having different, but also plausible, representations of processes and interactions, highlighting the complicated variability of UCE impacts that still need to be narrowed down in models. Elevated atmospheric CO2 concentrations (eCO2) alone did not completely buffer the ecosystems from carbon losses during UCEs in the majority of our simulations. Our findings highlight the consequences of differences in process formulations and uncertainties in models, most notably related to availability in plant carbohydrate storage and the diversity of plant hydraulic schemes, in projecting potential ecosystem responses to UCEs. We provide a summary of the current state and role of many model processes that give way to different underlying hypotheses of plant responses to UCEs, reflecting knowledge gaps which in future studies could be tested with targeted field experiments and an iterative modeling–experimental conceptual framework.
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.
Croplands are vital ecosystems for human well-being and provide important ecosystem services such as crop yields, retention of nitrogen and carbon storage. On large (regional to global)-scale levels, ...assessment of how these different services will vary in space and time, especially in response to cropland management, are scarce. We explore cropland management alternatives and the effect these can have on future C and N pools and fluxes using the land-use-enabled dynamic vegetation model LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator). Simulated crop production, cropland carbon storage, carbon sequestration and nitrogen leaching from croplands are evaluated and discussed. Compared to the version of LPJ-GUESS that does not include land-use dynamics, estimates of soil carbon stocks and nitrogen leaching from terrestrial to aquatic ecosystems were improved. Our model experiments allow us to investigate trade-offs between these ecosystem services that can be provided from agricultural fields. These trade-offs are evaluated for current land use and climate and further explored for future conditions within the two future climate change scenarios, RCP (Representative Concentration Pathway) 2.6 and 8.5. Our results show that the potential for carbon sequestration due to typical cropland management practices such as no-till management and cover crops proposed in previous studies is not realised, globally or over larger climatic regions. Our results highlight important considerations to be made when modelling C–N interactions in agricultural ecosystems under future environmental change and the effects these have on terrestrial biogeochemical cycles.
To better understand the effects of increased N input and dry periods on soil nitrous oxide (N₂O) emission, we examined a unique data-set of weather, soil microclimate, N input, and N₂O emissions ...(using the eddy covariance method), measured at a fertilized grassland over the period 2003-2008. We found that the N₂O emission (11.5 kg N ha⁻¹ year⁻¹), the ratio of N₂O emission to N input (3.4), and the duration of elevated N₂O flux (57 days) in 2003 were about two times greater than those of the following years. 2003 had the highest annual N input (343 kg N ha⁻¹ year⁻¹) which exceeded the agronomical requirements for Irish grasslands (up to 306 kg ha⁻¹ year⁻¹). In the summer of 2003, the site had a significantly higher soil temperature, lower WFPS and lowest rainfall of all years. Large N₂O emission events followed rainfall after a long dry period in the summer of 2003, attributed to dominant nitrification processes. Furthermore, in the non summer periods, when temperature was lower and WFPS was higher and when there were prior N applications, lower N₂O emissions occurred and were attributed to dominant denitrification processes. Throughout the study period, the N input and soil dryness related factors (duration of WFPS under 50%, summer average WFPS, and low rainfall) showed exponential relationships with N₂O emission and the ratio of N₂O emission to N input. Based on these findings, we infer that the observed anomalously high N₂O emission in 2003 may have been caused by the combined effects of excess N input above the plant uptake rate, elevated soil temperature, and N₂O flux bursts that followed the rewetting of dry soil after an unusually long dry summer period. These results suggest that high N input above plant uptake rate and extended dry periods may cause abnormal increases in N₂O emissions.
The response of terrestrial ecosystems to rising atmospheric CO sub(2) concentration (C sub(a)), particularly under nutrient-limited conditions, is a major uncertainty in Earth System models. The ...Eucalyptus Free-Air CO sub(2) 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 C sub(a) 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.
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