Lukas Hörtnagl

Last Name

Hörtnagl

First Name

Lukas

ORCID

0000-0002-5569-0761

Organisational unit

03648 - Buchmann, Nina / Buchmann, Nina

Show all metadata

Search Results

Publications 1 - 10 of 63

Management matters: testing a mitigation strategy for nitrous oxide emissions using legumes on intensively managed grassland
Fuchs, Kathrin; Hörtnagl, Lukas; Buchmann, Nina; et al. (2018)
Biogeosciences
Replacing fertiliser nitrogen with biologically fixed nitrogen (BFN) through legumes has been suggested as a strategy for nitrous oxide (N2O) mitigation from intensively managed grasslands. While current literature provides evidence for an N2O emission reduction effect due to reduced fertiliser input, little is known about the effect of increased legume proportions potentially offsetting these reductions, i.e. by increased N2O emissions from plant residues and root exudates. In order to assess the overall effect of this mitigation strategy on permanent grassland, we performed an in situ experiment and quantified net N2O fluxes and biomass yields in two differently managed grass–clover mixtures. We measured N2O fluxes in an unfertilised parcel with high clover proportions vs. an organically fertilised control parcel with low clover proportions using the eddy covariance (EC) technique over 2 years. Furthermore, we related the measured N2O fluxes to management and environmental drivers. To assess the effect of the mitigation strategy, we measured biomass yields and quantified biologically fixed nitrogen using the 15N natural abundance method. The amount of BFN was similar in both parcels in 2015 (control: 55±5kgNha−1yr−1; clover parcel: 72±5kgNha−1yr−1) due to similar clover proportions (control: 15% and clover parcel: 21%), whereas in 2016 BFN was substantially higher in the clover parcel compared to the much lower control (control: 14±2kgNha−1yr−1 with 4% clover in DM; clover parcel: 130±8kgNha−1yr−1 and 44% clover). The mitigation management effectively reduced N2O emissions by 54% and 39% in 2015 and 2016, respectively, corresponding to 1.0 and 1.6tha−1yr−1CO2 equivalents. These reductions in N2O emissions can be attributed to the absence of fertilisation on the clover parcel. Differences in clover proportions during periods with no recent management showed no measurable effect on N2O emissions, indicating that the decomposition of plant residues and rhizodeposition did not compensate for the effect of fertiliser reduction on N2O emissions. Annual biomass yields were similar under mitigation management, resulting in a reduction of N2O emission intensities from 0.42gN2O-Nkg−1DM (control) to 0.28gN2O-Nkg−1DM (clover parcel) over the 2-year observation period. We conclude that N2O emissions from fertilised grasslands can be effectively reduced without losses in yield by increasing the clover proportion and reducing fertilisation.
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.
Relationship of leaf elongation rate of young wheat leaves, gross primary productivity and environmental variables in the field with hourly and daily temporal resolution
Merz, Quirina Noëmi; Walter, Achim; Maier, Regine; et al. (2022)
Agricultural and Forest Meteorology
Plant growth is controlled by an interplay of internal and external factors. The production of biomass via photosynthesis is dependent on the plant response to environmental variables such as temperature, vapour pressure deficit and light intensity. Short-term responses of plant growth to these variables at fine temporal scales of hours are not well investigated, especially under field conditions. The present study explores the relationship between leaf elongation rate (LER) of young wheat leaves in the field in very high temporal resolution (minutes). Turbulent fluxes of CO2 were measured with the eddy covariance technique and used to derive GPP, and environmental variables such as air and soil temperature, short wave radiation and vapour pressure deficit were simultaneously measured. The analysis revealed the importance of different variables on different temporal scales (hourly, daily). On an hourly scale, GPP and shortwave radiation explain most of the variance of LER, however on a daily scale, air temperature is the main driver. A cross-correlation analysis confirmed that the strongest immediate relationship can be found between LER and GPP and incoming shortwave radiation; variables that are determining photosynthesis. In principal, LER also shows the same diurnal patterns as air temperature and soil temperature, however air and soil temperature lag behind LER. Multivariate growth models show that combinations with GPP or incoming shortwave radiation and air temperature perform best. These results indicate that short term growth processes in young wheat leaves in the field are mainly controlled by incoming shortwave radiation, while the magnitude of growth is controlled by temperature.
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.
Long‐term changes in forest response to extreme atmospheric dryness
Shekhar, Ankit; Hörtnagl, Lukas; Buchmann, Nina; et al. (2023)
Global Change Biology
Atmospheric dryness, as indicated by vapor pressure deficit (VPD), has a strong influence on forest greenhouse gas exchange with the atmosphere. In this study, we used long-term (10–30 years) net ecosystem productivity (NEP) measurements from 60 forest sites across the world (1003 site-years) to quantify long-term changes in forest NEP resistance and NEP recovery in response to extreme atmospheric dryness. We tested two hypotheses: first, across sites differences in NEP resistance and NEP recovery of forests will depend on both the biophysical characteristics (i.e., leaf area index [LAI] and forest type) of the forest as well as on the local meteorological conditions of the site (i.e., mean VPD of the site), and second, forests experiencing an increasing trend in frequency and intensity of extreme dryness will show an increasing trend in NEP resistance and NEP recovery over time due to emergence of long-term ecological stress memory. We used a data-driven statistical learning approach to quantify NEP resistance and NEP recovery over multiple years. Our results showed that forest types, LAI, and median local VPD conditions explained over 50% of variance in both NEP resistance and NEP recovery, with drier sites showing higher NEP resistance and NEP recovery compared to sites with less atmospheric dryness. The impact of extreme atmospheric dryness events on NEP lasted for up to 3 days following most severe extreme events in most forests, indicated by an NEP recovery of less than 100%. We rejected our second hypothesis as we found no consistent relationship between trends of extreme VPD with trends in NEP resistance and NEP recovery across different forest sites, thus an increase in atmospheric dryness as it is predicted might not increase the resistance or recovery of forests in terms of NEP.
Large nitrous oxide emissions from arable soils after crop harvests prior to sowing
Maier, Regine; Hörtnagl, Lukas; Buchmann, Nina (2025)
Nutrient Cycling in Agroecosystems
Global agriculture is the largest anthropogenic source for nitrous oxide (N2O) emissions. During crop rotations, periods with arable soils without crops, thereafter called “bare soils” are often impossible to avoid after the crop is harvested, prior to sowing of the next crop. However, such periods are underrepresented in studies focussing on N2O emissions. Here, we present continuous, high-temporal-resolution N2O fluxes during bare soil periods after four major crops, using the eddy-covariance technique at two sites in Switzerland. Overall, periods with bare soil were net sources for N2O as well as for carbon dioxide (CO2) and methane (CH4). Daily average sums of N2O emissions varied between 10 ± 2 and 38 ± 5 g N2O-N ha−1 d−1 after the respective rapeseed, winter wheat, pea, and maize harvests. While CO2 emissions contributed 86–96% to the total GHG budgets, N2O fluxes accounted for 2% after pea, but for 10–12% after rapeseed, winter wheat, and maize. In contrast, CH4 fluxes were negligible (< 2%). N2O fluxes during bare soil periods increased for all cropland sites with increasing water-filled pore space, particularly at high soil temperatures. Thus, our study emphasizes the significance of avoiding bare soil periods to mitigate N2O emissions from croplands.
Canopy photosynthesis of six major arable crops is enhanced under diffuse light due to canopy architecture
Emmel, Carmen; D'Odorico, Petra; Revill, Andrew; et al. (2020)
Global Change Biology
Diffuse radiation generally increases photosynthetic rates if total radiation is kept constant. Different hypotheses have been proposed to explain this enhancement of photosynthesis, but conclusive results over a wide range of diffuse conditions or about the effect of canopy architecture are lacking. Here, we show the response of canopy photosynthesis to different fractions of diffuse light conditions for five major arable crops (pea, potato, wheat, barley, rapeseed) and cover crops characterized by different canopy architecture. We used 13 years of flux and microclimate measurements over a field with a typical 4 year crop rotation scheme in Switzerland. We investigated the effect of diffuse light on photosynthesis over a gradient of diffuse light fractions ranging from 100% diffuse (overcast sky) to 11% diffuse light (clear‐sky conditions). Gross primary productivity (GPP) increased with diffuse fraction and thus was greater under diffuse than direct light conditions if the absolute photon flux density per unit surface area was kept constant. Mean leaf tilt angle (MTA) and canopy height were found to be the best predictors of the diffuse versus direct radiation effect on photosynthesis. Climatic factors, such as the drought index and growing degree days (GDD), had a significant influence on initial quantum yield under direct but not diffuse light conditions, which depended primarily on MTA. The maximum photosynthetic rate at 2,000 µmol m−2 s−1 photosynthetically active radiation under direct conditions strongly depended on GDD, MTA, leaf area index (LAI) and the interaction between MTA and LAI, while under diffuse conditions, this parameter depended mostly on MTA and only to a minor extent on canopy height and their interaction. The strongest photosynthesis enhancement under diffuse light was found for wheat, barley and rapeseed, whereas the lowest was for pea. Thus, we suggest that measuring canopy architecture and diffuse radiation will greatly improve GPP estimates of global cropping systems.
Assimilating phenology datasets automatically across ICOS ecosystem stations
Hufkens, Koen; Filippa, Gianluca; Cremonese, Edoardo; et al. (2018)
International Agrophysics
The presence or absence of leaves within plant canopies exert a strong influence on the carbon, water and energy balance of ecosystems. Identifying key changes in the timing of leaf elongation and senescence during the year can help to understand the sensitivity of different plant functional types to changes in temperature. When recorded over many years these data can provide information on the response of ecosystems to long-term changes in climate. The installation of digital cameras that take images at regular intervals of plant canopies across the Integrated Carbon Observation System ecosystem stations will provide a re- liable and important record of variations in canopy state, colour and the timing of key phenological events. Here, we detail the procedure for the implementation of cameras on Integrated Carbon Observation System flux towers and how these images will help us understand the impact of leaf phenology and ecosystem function, distinguish changes in canopy structure from leaf physiology and at larger scales will assist in the validation of (future) remote sensing products. These data will help us improve the representation of phenological responses to climatic variability across Integrated Carbon Observation System stations and the terrestrial biosphere through the improvement of model algorithms and the provision of validation datasets.
Towards long-term standardised carbon and greenhouse gas observations formonitoring Europe's terrestrial ecosystems
Franz, Daniel; Buchmann, Nina; D'Odorico, Petra; et al. (2018)
International Agrophysics
Compound soil and atmospheric drought (CSAD) events and CO₂ fluxes of a mixed deciduous forest: the occurrence, impact, and temporal contribution of main drivers
Scapucci, Liliana; Shekhar, Ankit; Aranda-Barranco, Sergio; et al. (2024)
Biogeosciences
With global warming, forests are increasingly exposed to “compound soil and atmospheric drought” (CSAD) events, characterized by low soil water content (SWC) and high vapour pressure deficit (VPD). Such CSAD events trigger responses in both ecosystem and forest-floor CO₂ fluxes, which we know little about. In this study, we used multi-year daily and daytime above-canopy (18 years; 2005–2022) and daily forest-floor (5 years; 2018–2022) eddy covariance CO₂ fluxes from a Swiss forest site by the name of CH-Lae (a mixed deciduous montane forest). The objectives were (1) to characterize CSAD events at CH-Lae, (2) to quantify the impact of CSAD events on ecosystem and forest-floor CO₂ fluxes, and (3) to identify the major drivers and their temporal contributions to changing ecosystem and forest-floor CO₂ fluxes during CSAD events and CSAD growing seasons. Our results showed that the growing seasons of 2015, 2018, and 2022 were the three driest at CH-Lae since 2005 (referred to as the CSAD years), exhibiting similar intensity and duration of the CSAD events but considerably different pre-drought conditions. The CSAD events reduced daily mean net ecosystem productivity (NEP) in all 3 CSAD years by about 38 % compared to the long-term mean, with the highest reduction observed during 2022 (41 %). This reduction in daily mean NEP was largely due to decreased gross primary productivity (GPP; > 16 % below the long-term mean) rather than increased ecosystem respiration (Reco) during CSAD events. Furthermore, forest-floor respiration (Rff) decreased during the CSAD events in 2018 and 2022 (with no measurements in 2015), with a larger reduction in 2022 (41 %) than in 2018 (16 %), relative to the long-term mean (2019–2021). Using data-driven machine learning methods, we identified the major drivers of NEP and Rff during CSAD events. While daytime mean NEP (NEP_DT) during the 2015 and 2018 CSAD events was limited by VPD and SWC, respectively, NEP_DT during the 2022 CSAD event was strongly limited by both SWC and VPD. Air temperature had negative effects, while net radiation showed positive effects on NEP_DT during all CSAD events. Daily mean Rff during the 2018 CSAD event was driven by soil temperature and SWC but was severely limited by SWC during the 2022 CSAD event. We found that a multi-layer analysis of CO₂ fluxes in forests is necessary to better understand forest responses to CSAD events, particularly if the first signs of NEP acclimation to CSAD events – evident in our forest – are also found elsewhere. We conclude that CSAD events have multiple drivers with different temporal contributions, making predictions about site-specific CSAD events and long-term forest responses to such conditions more challenging.

Publications 1 - 10 of 63

Lukas Hörtnagl

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results