Biogeosciences Discussions

Journal: Biogeosciences Discussions

Abbreviation

Biogeosci. discuss.

Publisher

Copernicus

ISSN

1810-6277
1810-6285

Show all metadata

Search Results

Publications 1 - 10 of 57

The influence of lateral transport on sedimentary alkenone paleoproxy signals
Ausín, Blanca; Haghipour, Negar; Bruni, Elena; et al. (2021)
Biogeosciences Discussions
Alkenone signatures preserved in marine sedimentary records are considered one of the most robust paleothermometers available, and are often used as a proxy for paleoproductivity. However, important gaps remain on the provenance and fate of alkenones, and their impact on derived environmental signals in marine sediments. Here, we analyze the abundance, distribution, and radiocarbon (14C) age of alkenones in bulk sediments and corresponding grain-size fractions in surficial sediments from seven continental margin settings in the Pacific and Atlantic Oceans in order to evaluate the impact of organo-mineral associations and hydrodynamic sorting on sedimentary alkenone signals. We find that alkenones preferentially reside within fine-grained mineral fractions of continental margin sediments, with the preponderance of alkenones residing within the fine silt fraction (2–10 µm), and most strongly influencing alkenone 14C age, and SST signals from bulk sediments as a consequence of their proportional abundance and higher degree of OM protection relative to other fractions. Our results demonstrate that selective association of alkenones with mineral surfaces and associated hydrodynamic mineral sorting processes can alter alkenone signals encoded in marine sediments (14C age, content, and distribution) and confound corresponding proxy records (productivity and SST) in the spatial and temporal domain.
Interpreting canopy development and physiology using the EUROPhen camera network at flux sites
Wingate, Lisa; Ogée, Jérôme; Cremonese, Edoardo; et al. (2015)
Biogeosciences Discussions
Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green-up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations, we found that these phenological events could be detected adequately (RMSE < 8 and 11 days for leaf out and leaf fall, respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring `green hump' observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO2 in the future.
Response of δ13C in plant and soil respiration to a water pulse
Salmon, Yann; Buchmann, Nina; Barnard, Romain L. (2011)
Biogeosciences Discussions
Stable carbon isotopes have been used to assess the coupling between changes in environmental conditions and the response of soil or ecosystem respiration, usually by studying the time-lagged response of δ13C of respired CO2 (δ13CR) to changes in photosynthetic carbon isotope discrimination (Δi). However, the lack of a systematic response of δ13CR to environmental changes in field studies stresses the need to better understand the mechanisms to this response. We experimentally created a wide range of carbon allocation and respiration conditions in Fagus sylvatica mesocosms, by growing saplings under different temperatures and girdling combinations. After a period of drought, a water pulse was applied and the short-term responses of δ13C in soil CO2 efflux (δ13CRsoil) and δ13C in aboveground plant respiration (δ13CRabove) were measured, as well as leaf gas exchange rates and soil microbial biomass δ13C responses. Both δ13CRsoil and δ 13CRabove values of all the trees decreased immediately after the water pulse. These responses were not driven by changes in Δi, but rather by a fast release of C stored in roots and shoots. Changes in δ13CRsoil associated with the water pulse were significantly positively correlated with changes in stomatal conductance, showing a strong impact of the plant component on δ13CRsoil. However, three days after the water pulse in girdled trees, changes in δ13CRsoil were related to changes in microbial biomass δ13C, suggesting that changes in the carbon source respired by soil microorganisms also contributed to the response of δ13CRsoil. Our study shows that improving our mechanistic understanding of the responses of δ13CR to changes in environmental conditions requires the understanding of not only the plant's physiological responses, but also the responses of soil microorganisms and of plant-microbial interactions.
First on-line isotopic characterization of N2O emitted from intensively managed grassland
Wolf, Benjamin; Merbold, Lutz; Decock, Charlotte; et al. (2015)
Biogeosciences Discussions
The analysis of the four main isotopic N2O species (14N14N16O, 14N15N16O, 15N14N16O, 14N14N18O) and especially the intramolecular distribution of 15N ("site preference", SP) has been suggested as a tool to distinguish source processes and to help constrain the global N2O budget. However, current studies suffer from limited spatial and temporal resolution capabilities due to the combination of discrete flask sampling with subsequent laboratory-based mass-spectrometric analysis. Quantum cascade laser absorption spectroscopy (QCLAS) allows the selective high-precision analysis of N2O isotopic species at trace levels and is suitable for in situ measurements. Here, we present results from the first field campaign, conducted on an intensively managed grassland site in central Switzerland. N2O mole fractions and isotopic composition were determined in the atmospheric surface layer (at 2.2 m height) at a high temporal resolution with a modified state-of-the-art laser spectrometer connected to an automated N2O preconcentration unit. The analytical performance was determined from repeated measurements of a compressed air tank and resulted in measurement repeatability of 0.20, 0.12 and 0.11‰ for δ15Nα, δ15Nβ and δ18O, respectively. Simultaneous eddy-covariance N2O flux measurements were used to determine the flux-averaged isotopic signature of soil-emitted N2O. Our measurements indicate that, in general, nitrifier-denitrification and denitrification were the prevalent sources of N2O during the campaign and that variations in isotopic composition were due to alterations in the extent to which N2O was reduced to N2 rather than to other pathways, such as hydroxylamine oxidation. Management and rewetting events were characterized by low values of the intramolecular 15N site preference (SP), δ15Nbulk and δ18O, suggesting that nitrifier-denitrification and incomplete heterotrophic bacterial denitrification responded most strongly to the induced disturbances. The flux-averaged isotopic composition of N2O from intensively managed grassland was 6.9 ± 4.3, −17.4 ± 6.2 and 27.4 ± 3.6‰ for SP, δ15Nbulk and δ18O, respectively. The approach presented here is capable of providing long-term data sets also for other N2O-emitting ecosystems, which can be used to further constrain global N2O inventories
The distribution, dominance patterns and ecological niches of plankton functional types in Dynamic Green Ocean Models and satellite estimates
Vogt, Meike; Hashioka, Taketo; Payne, Mark R.; et al. (2013)
Biogeosciences Discussions
We compare the spatial and temporal representation of phytoplankton functional types (pPFTs) in four different Dynamic Green Ocean Models (DGOMs; CCSM-BEC, NEMURO, PISCES and PlankTOM5) to derived phytoplankton distributions from two independent satellite estimates, with a particular focus on diatom distributions. Global annual mean surface biomass estimates for diatoms vary between 0.23 mmol C m−3 and 0.77 mmol C m−3 in the models, and are comparable to a satellite-derived estimate (0.41 mmol C m−3). All models consistently simulate a higher zonal mean diatom biomass contribution in the high latitudes than in the low latitudes, but the relative diatom contribution varies substantially between models with largest differences in the high latitudes (20% to 100% of total biomass). We investigate phytoplankton distribution in terms of annual and monthly mean dominance patterns, i.e. the distribution of locations where a given PFT contributes more than 50% to total biomass. In all models, diatoms tend to dominate large areas of the high latitudes of both hemispheres, and the area of the surface ocean dominated by diatoms is significantly higher in the models than in the satellite estimates. We estimate the realized ecological niches filled by the dominant pPFT at each location as a function of annual mean surface nitrate concentration (NO3), sea surface temperature (SST), and mixed layer depth. A general additive model (GAM) is used to map the probability of dominance of all pPFTs in niche and geographic space. Models tend to simulate diatom dominance over a wider temperature and nutrient range, whereas satellites confine diatom dominance to a narrower niche of low-intermediate annual mean temperatures (annual mean SST < 10 °C), but allow for niches in different ranges of surface NO3 concentrations. For annual mean diatom dominance, the statistically modelled probability of dominance explains the majority of the variance in the data (65.2–66.6%). For the satellite estimates, the explained deviance is much lower (44.6% and 32.7%). The differences in the representation of diatoms among models and compared to satellite estimates highlights the need to better resolve phytoplankton succession and phenology in the models. This work is part of the marine ecosystem inter-comparison project (MAREMIP).
Gap-filling strategies for annual VOC flux data sets
Bamberger, Ines; Hörtnagl, Lukas; Walser, Mario; et al. (2013)
Biogeosciences Discussions
Up to now the limited knowledge about the ex-change of volatile organic compounds (VOCs) between thebiosphere and the atmosphere is one of the factors whichhinders more accurate climate predictions. Complete long-term flux data sets of several VOCs to quantify the annualexchange and validate recent VOC models are basically notavailable. In combination with long-term VOC flux mea-surements the application of gap-filling routines is inevitablein order to replace missing data and make an importantstep towards a better understanding of the VOC ecosystem–atmosphere exchange on longer timescales.We performed VOC flux measurements above a mountainmeadow in Austria during two complete growing seasons(from snowmelt in spring to snow reestablishment in late au-tumn) and used this data set to test the performance of fourdifferent gap-filling routines, mean diurnal variation (MDV),mean gliding window (MGW), look-up tables (LUT) and lin-ear interpolation (LIP), in terms of their ability to replacemissing flux data in order to obtain reliable VOC sums. Ac-cording to our findings the MDV routine was outstandingwith regard to the minimization of the gap-filling error forboth years and all quantified VOCs. The other gap-fillingroutines, which performed gap-filling on 24 h average val-ues, introduced considerably larger uncertainties. The errorwhich was introduced by the application of the different fill-ing routines increased linearly with the number of data gaps.Although average VOC fluxes measured during the winterperiod (complete snow coverage) were close to zero, thesewere highly variable and the filling of the winter period re-sulted in considerably higher uncertainties compared to theapplication of gap-filling during the measurement period.The annual patterns of the overall cumulative fluxes forthe quantified VOCs showed a completely different behaviour in 2009, which was an exceptional year due tothe occurrence of a severe hailstorm, compared to 2011.Methanol was the compound which, at 381.5 mg C m−2and449.9 mg C m−2, contributed most to the cumulative VOCcarbon emissions in 2009 and 2011, respectively. In contrastto methanol emissions, however, considerable amounts ofmonoterpenes (−327.3 mg C m−2)were deposited onto themountain meadow during 2009 caused by a hailstorm. Otherquantified VOCs had considerably lower influences on the annual patterns.
Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones
von Buttlar, Jannis; Zscheischler, Jakob; Rammig, Anja; et al. (2017)
Biogeosciences Discussions
Temporal and spatial variations of CO2, CH4 and N2O fluxes at three differently managed grasslands
Imer, Dennis; Merbold, Lutz; Eugster, Werner; et al. (2013)
Biogeosciences Discussions
Global ocean storage of anthropogenic carbon
Khatiwala, Samar P.; Tanhua, Toste; Mikaloff Fletcher, Sara E.; et al. (2012)
Biogeosciences Discussions
he global ocean is a significant sink for anthropogenic carbon (Cant), absorbing roughly a third of human CO2 emitted over the industrial period. Robust estimates of the magnitude and variability of the storage and distribution of Cant in the ocean are therefore important for understanding the human impact on climate. In this synthesis we review observational and model-based estimates of the storage and transport of Cant in the ocean. We pay particular attention to the uncertainties and potential biases inherent in different inference schemes. On a global scale, three data-based estimates of the distribution and inventory of Cant are now available. While the inventories are found to agree within their uncertainty, there are considerable differences in the spatial distribution. We also present a review of the progress made in the application of inverse and data assimilation techniques which combine ocean interior estimates of Cant with numerical ocean circulation models. Such methods are especially useful for estimating the air–sea flux and interior transport of Cant, quantities that are otherwise difficult to observe directly. However, the results are found to be highly dependent on modeled circulation, with the spread due to different ocean models at least as large as that from the different observational methods used to estimate Cant. Our review also highlights the importance of repeat measurements of hydrographic and biogeochemical parameters to estimate the storage of Cant on decadal timescales in the presence of the variability in circulation that is neglected by other approaches. Data-based Cant estimates provide important constraints on forward ocean models, which exhibit both broad similarities and regional errors relative to the observational fields. A compilation of inventories of Cant gives us a "best" estimate of the global ocean inventory of anthropogenic carbon in 2010 of 155 ± 31 PgC (±20% uncertainty). This estimate includes a broad range of values, suggesting that a combination of approaches is necessary in order to achieve a robust quantification of the ocean sink of anthropogenic CO2.
Cereal-legume mixtures increase net CO2 uptake in a forage system of the Eastern Pyrenees
Ibañez, Mercedes; Altimir, Núria; Ribas, Àngela; et al. (2020)
Biogeosciences Discussions
Forage systems are the major land use, and provide essential resources for animal feeding. Assessing the influence of forage species on net ecosystem CO2 exchange (NEE) is key to develop management strategies that can help to mitigate climate change, while optimizing productivity of these systems. However, little is known about the effect of forage species on CO2 exchange fluxes and net biome production (NBP), considering: species ecophysiological responses; growth and fallow periods separately; and the management associated with the particular sown species. Our study assesses the influence of cereal monocultures vs. cereal legume mixtures on (1) ecosystem scale CO2 fluxes, for the whole crop season and separately for the two periods of growth and fallow; (2) potential sensitivities of CO2 exchange related to short-term variations in light, temperature and soil water content; and (3) NBP during the growth period; this being the first long term (seven years) ecosystem scale CO2 fluxes dataset of an intensively managed forage system in the Pyrenees region. Our results provide strong evidence that cereal-legume mixtures lead to higher net CO2 uptake than cereal monocultures, as a result of higher gross CO2 uptake, while respiratory fluxes did not significantly increase. Also, management associated with cereal legume mixtures favoured vegetation voluntary regrowth during the fallow period, which was decisive for the cumulative net CO2 uptake of the entire crop season. All cereal legume mixtures and some cereal monocultures had a negative NBP (net gain of C) during the growth period, indicating C input to the system, besides the yield. Overall, cereal legume mixtures enhanced net CO2 sink capacity of the forage system, while ensuring productivity and forage quality.

Publications 1 - 10 of 57

Journal: Biogeosciences Discussions

Abbreviation

Publisher

Journal Volumes

ISSN

Description

Filters

Search Results