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Detecting and quantifying CH4 emissions from scarab larvae using stable carbon isotopes

Periodic Reporting for period 1 - CH4ScarabDetect (Detecting and quantifying CH4 emissions from scarab larvae using stable carbon isotopes)

Reporting period: 2016-05-01 to 2018-04-30

The greenhouse gas (GHG) methane (CH4) is an excellent candidate for short-term climate change mitigation strategies. It has an atmospheric lifetime of only nine years and slight changes in even small CH4 sources and sinks can ultimately determine whether the Earth, as a whole ecosystem, is a net source or sink of atmospheric CH4. However, this mitigation potential can only be accessed if all sinks and sources contributing to the global CH4 budget are known. One of the still poorly understood aspects of the global CH4 budget is the effect of land use on the CH4 budget of soils which can be either sinks or sources of atmospheric CH4 depending on the ratio of gross CH4 production and gross CH4 oxidation. The central drawback in our understanding of the mechanisms controlling the net soil CH4 flux is our insufficient knowledge on these gross fluxes and the involved organisms. Especially one group of organisms has thus far been almost completely neglected in soil CH4 cycle research – methanogens living in the digestive tracts of soil invertebrates. The second large drawback in terrestrial CH4 cycle research is the lack of non-invasive field methods to track soil-dwelling insects as well as to separate gross CH4 production and gross CH4 oxidation. The project “CH4ScarabDetect: Detecting and quantifying CH4 emissions from scarab larvae using stable carbon isotopes” addressed these two major drawbacks by a) measuring CH4 emissions from scarab larvae in situ to derive the first quantitative estimate of their importance for net CH4 soil fluxes, and b) combining this with the development of a non-invasive field monitoring method to detect scarab larvae using stable carbon isotope signatures of atmospheric CH4 and bioacoustics. The project focused on the larvae of the common cockchafer (Melolontha melolontha) and the forest cockchafer (M. hippocastani) which are important European agricultural and forest pests, respectively.
The project was divided into two subprojects. In the first subproject, survey measurements were conducted at seven cockchafer infested sites in Central and Southern Germany during the vegetation period in 2017 (April – October) in close cooperation with relevant stakeholders in agriculture and forestry. Each site was visited once and measurements conducted at two to four randomly chosen plots (50 cm x 50 cm) per site. Measurements comprised soil audio recordings, excavation and characterization of larvae, and incubation of larvae to quantify CH4 emissions. In total, CH4 emissions of 91 cockchafer larvae were measured directly in the field. Over the entire dataset, average CH4 emissions were 1.3 µg CH4 h-1 larva-1, but there was a large variability within and between the different sampling sites. The highest CH4 emissions with up to 7.1 µg CH4 h-1 larva-1 were observed in a meadow in May 2017 where M. melolontha larvae had reached their final larval stage (3rd instar) with weights ranging between 2.0 and 2.7 g. For M. hippocastani, CH4 emissions were mostly below 0.5 µg CH4 h-1 larva-1. However, Melolontha spp. are pest insects and can reach abundance levels of more than 50 individuals m-2, thus upscaled fluxes can easily reach values of more than 10 µg CH4 h-1 m-2. Overall, emission strength and variability tended to increase with larval body weight and decrease with increasing soil excavation depth.
In the second subproject (October 2016 – October 2017), the effect of different M. melolontha larvae abundances on net soil CH4 fluxes were quantified under a wide range of environmental conditions in a controlled field experiment consisting of 27 soil mesocosms. These mesocosms were planted with either turf, carrots or a combination of both, and infested with cockchafers. Larval abundances ranged between 0 and 16 larvae m-2. On average every two weeks throughout the vegetation period, net soil CH4 fluxes were quantified with closed static chambers. These chambers were combined with a 13CH4 isotope pool dilution technique to non-invasively estimate gross CH4 production and CH4 oxidation in the soil. Since the soil in this experiment was well-aerated, any gross CH4 production was very likely attributable to the cockchafer larvae. Acoustic soil monitoring was conducted alongside the chamber measurements in cooperation with the University of York (England). Stable carbon isotope analysis of the gas samples was performed at the University College Dublin (Irleland) with a state-of-the-art high precision cavity ring-down spectrometer (Picarro G2201-i) equipped with a Small Sample Isotope Module (SSIM) for processing small discrete gas sample volumes. Four general trends were observed in this experiment: a) the soils acted as net sinks for atmospheric CH4, b) net CH4 uptake rates increased throughout the vegetation period, c) net CH4 uptake rates were higher in mesocosms covered completely with permanent vegetation, and d) net CH4 uptake rates tended to be higher in the mesocosms with the highest infestation level within each vegetation type, pointing towards a stimulation of gross CH4 oxidation rates in the presence of CH4 emission hotspots in the soil. With respect to the method development, two new data analysis routines for non-invasive soil monitoring were developed. One for the simultaneous analysis of 13C-CH4 and CH4 concentrations in small, discrete gas samples by cavity ring-down spectroscopy, and one for the estimation of cockchafer infestation levels based on active larval communication (=stridulation).
The project provided the first in situ CH4 emission data for soil-dwelling Scarabaeidae larvae which were significantly higher than values previously reported by laboratory-based studies. It was also demonstrated for the first time in the field that soil-dwelling Scarabaeidae larvae have the potential to significantly affect net soil CH4 fluxes, underlining the need to include Scarabaeidae larvae in terrestrial CH4 cycle modelling in the future. The combination of the closed static chamber technique with a 13CH4 isotope pool dilution technique for the non-invasive monitoring of gross CH4 fluxes in soils is a promising tool for furthering our understanding of the processes controlling soil net CH4 fluxes, but Is not widely applied yet. The new data analysis routine for the simultaneous analysis of 13C-CH4 and CH4 concentrations in small, discrete gas samples will simplify the application of this technique in future field monitoring and facilitate its wider application. The new acoustic data analysis routine opens up new possibilities for non-invasive and species-specific monitoring of soil-dwelling Scarabaeidae larvae. Acoustic soil monitoring has the potential to greatly increase our ecological knowledge of these animals which is not only beneficial for understanding their role in the terrestrial CH4 cycle, but also for the development of new environmentally friendly pest control strategies. This is especially important in times of climate change since cockchafer infestations are expected to spread again in Europe with increasing soil temperatures. The stridulations recorded during CH4ScarabDetect were the first verified recordings for M. melolontha and M. hippocastani. However, their ecological meaning remains unclear.
Overview measurement sites and techniques