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Carbon fluxes from decaying beech litter : insights from a ¹³C-tracer experiment and a new method to analyse the stable isotopes in soil CO₂ effluxes


Kammer, A. Carbon fluxes from decaying beech litter : insights from a ¹³C-tracer experiment and a new method to analyse the stable isotopes in soil CO₂ effluxes. 2011, University of Zurich, Faculty of Science.

Abstract

Forest soils contain large amounts of organic C, and thus are important potential sinks or sources of atmospheric CO₂. The C accumulation in forest soils is greatly driven by the rates at which C from aboveground plant litter is either mineralised to CO₂ or transported to the mineral soil via soil fauna and dissolved organic C (DOC). However, very few field studies, commonly using an isotopic tracer, have quantitatively assessed the different pathways of litter-derived C. Information is especially sparse for the decomposition of fine-woody litter, even though this litter type accounts for about 30% of the annual litter fall in temperate forests and, as it might mineralise more slowly than non-woody litter, could contribute substantially to the C pool in forest soils.
In my PhD research, I performed a tracer experiment in two forest soils (Rendzina and Cambisol) in the Swiss Jura mountains using ¹³C-depleted beech litter (leaves and twigs). The main goal was to quantify the different pathways of litter-derived C for one year by tracking the labelled C into the CO₂ and DOC fluxes as well as into different fractions of the topsoil. In addition, I applied for the first time a new generation of spectrometer (QCLS: quantum cascade laser-based spectrometer) in a closed soil-chamber system to analyse the isotopic ratios of soil-respired CO₂ (δ¹³Cresp, δ¹⁸Oresp) at unique temporal resolutions.
The ¹³C-tracer experiment revealed that the fate of the litter C depended on the 'quality' of the litter. While the woody twig litter decomposed largely in situ on the soil surface, large amounts of the leaf litter were transported to the mineral soil via soil fauna (~ 35% of the initial litter C). Moreover, the DOC leached from the twigs amounted only to half of that from the leaves throughout the year. However, the mineralisation rates differed little between the two litter types. After one year, the leaves had lost 29–34% of their initial C through CO₂ and the twigs 22–27%. This small difference between woody and non-woody litter is against the assumption of most soil C models. In combination, my findings suggest that twig litter is clearly less important for the C storage in these forest soils than leaf litter.
Although in deciduous forests leaf litter falls on the forest floor mostly at the beginning of the winter, we still know little about the decay and transport of this labile C pool during the cold season. My results show that the leaching of DOC from the ¹³C-labelled litter occurred mainly in winter (~ 80%). By contrast, the C mineralisation during the five winter months contributed to 'only' 20–25% of the annual C losses from the litter through CO₂. This indicates that C mineralization and leaching of DOC from fresh litter are not basically linked. My tracer experiment also revealed that litter-derived CO₂ is a highly variable component of the winter soil respiration greatly driven by the air temperature. While on warm winter days (T-air > 5°C) litter mineralisation accounted for up to 60% of the soil CO₂ effluxes, this CO₂ source was almost negligible on cold winter days (T-air < 1°C). On an annual scale, decaying leaf litter contributed to 10–12% of the soil CO₂ effluxes and twig litter 4–6%.
The DOC fluxes measured at three different depths (0, 5, 10 cm) were rather small as compared to other forest ecosystems. The leaching of DOC from the litter, for instance, contributed 6–10 times less to the total C loss from the litter than the C mineralisation. Nevertheless, in the long term, litter-derived DOC could be important for the storage of soil C in this forest ecosystem. I have several evidences that the litter DOC was strongly adsorbed on mineral surfaces, and thus was likely an important source of the 'new' C recovered in the heavy soil fraction (>1.6 g cm–3) at 0–2 cm depth (2–4% of the initial litter C). This fraction is known to have much longer residence times in the soil than the light fraction. Moreover, I have evidence that a substantial part of the litter-derived DOC (20–40%) was rapidly mineralised by soil microbes. Together, physico-chemical interactions and biodegradation retained more than 90% of the DOC leached from the litter layer within the top centimetres of the mineral soil.
Laser spectroscopy is an emerging technique to analyse the isotopic composition of soil CO2 effluxes in situ and at high temporal resolution. In collaboration with the EMPA, I employed the most recent spectrometer (QCLS) during a short field campaign. With a one-second interval, the QCLS measured the stable isotopes of CO₂ that accumulated in the headspace of a closed soil-chamber system. The unique resolution of the QCLS measurements allowed us to analyse non-linearities in the isotopic composition of CO₂ effluxes. This information was used to improve the estimation of δ¹³Cresp and δ¹⁸Oresp.
To determine δ¹³Cresp, for instance, we used only the first 10 out of 20 minutes of CO₂ accumulation because there was a systematic shift in δ¹³Cresp of 1.9‰ in the second part of the chamber measurements. This bias was probably the result of a ‘kinetic fractionation’ during CO₂ diffusion as it has recently been suggested by simulation studies, but so fare, has not been proven in field experiments. We estimated δ¹³Cresp from both high resolution Keeling plots and directly from the ratio of the ¹²CO₂- and ¹³CO₂-flux rates. If the isotopic fluxes were derived from simple linear regression, the flux ratios were equal to the Keeling plot intercepts. The calculation of the fluxes with quadratic curve fits, however, resulted in, on average, 0.8‰ lower values for δ¹³Cresp. For the estimation of δ¹⁸Oresp, we used a new approach based on quadratic curve fits of the ¹⁸O-Keeling plots. This was necessary as the δ¹⁸Oresp increased immediately after the chamber system was closed probably due to the invasion of chamber CO₂ into the first few centimetres of soil, where the ¹⁸O of the CO₂ equilibrated partly with the soil water.
In summary, this thesis provides deep insights into the dynamic of both the decomposition of beech litter and the isotopic composition of soil CO₂ effluxes. It suggests rethinking the importance of fine-woody litter for the C storage in forest soils, and thus could contribute to improved soil C models. The evaluation of a QCLS in combination with a closed soil- chamber system clears the way for future studies that will use this new and promising method to determine the stable isotopes in soil-respired CO₂.

Forest soils contain large amounts of organic C, and thus are important potential sinks or sources of atmospheric CO₂. The C accumulation in forest soils is greatly driven by the rates at which C from aboveground plant litter is either mineralised to CO₂ or transported to the mineral soil via soil fauna and dissolved organic C (DOC). However, very few field studies, commonly using an isotopic tracer, have quantitatively assessed the different pathways of litter-derived C. Information is especially sparse for the decomposition of fine-woody litter, even though this litter type accounts for about 30% of the annual litter fall in temperate forests and, as it might mineralise more slowly than non-woody litter, could contribute substantially to the C pool in forest soils.
In my PhD research, I performed a tracer experiment in two forest soils (Rendzina and Cambisol) in the Swiss Jura mountains using ¹³C-depleted beech litter (leaves and twigs). The main goal was to quantify the different pathways of litter-derived C for one year by tracking the labelled C into the CO₂ and DOC fluxes as well as into different fractions of the topsoil. In addition, I applied for the first time a new generation of spectrometer (QCLS: quantum cascade laser-based spectrometer) in a closed soil-chamber system to analyse the isotopic ratios of soil-respired CO₂ (δ¹³Cresp, δ¹⁸Oresp) at unique temporal resolutions.
The ¹³C-tracer experiment revealed that the fate of the litter C depended on the 'quality' of the litter. While the woody twig litter decomposed largely in situ on the soil surface, large amounts of the leaf litter were transported to the mineral soil via soil fauna (~ 35% of the initial litter C). Moreover, the DOC leached from the twigs amounted only to half of that from the leaves throughout the year. However, the mineralisation rates differed little between the two litter types. After one year, the leaves had lost 29–34% of their initial C through CO₂ and the twigs 22–27%. This small difference between woody and non-woody litter is against the assumption of most soil C models. In combination, my findings suggest that twig litter is clearly less important for the C storage in these forest soils than leaf litter.
Although in deciduous forests leaf litter falls on the forest floor mostly at the beginning of the winter, we still know little about the decay and transport of this labile C pool during the cold season. My results show that the leaching of DOC from the ¹³C-labelled litter occurred mainly in winter (~ 80%). By contrast, the C mineralisation during the five winter months contributed to 'only' 20–25% of the annual C losses from the litter through CO₂. This indicates that C mineralization and leaching of DOC from fresh litter are not basically linked. My tracer experiment also revealed that litter-derived CO₂ is a highly variable component of the winter soil respiration greatly driven by the air temperature. While on warm winter days (T-air > 5°C) litter mineralisation accounted for up to 60% of the soil CO₂ effluxes, this CO₂ source was almost negligible on cold winter days (T-air < 1°C). On an annual scale, decaying leaf litter contributed to 10–12% of the soil CO₂ effluxes and twig litter 4–6%.
The DOC fluxes measured at three different depths (0, 5, 10 cm) were rather small as compared to other forest ecosystems. The leaching of DOC from the litter, for instance, contributed 6–10 times less to the total C loss from the litter than the C mineralisation. Nevertheless, in the long term, litter-derived DOC could be important for the storage of soil C in this forest ecosystem. I have several evidences that the litter DOC was strongly adsorbed on mineral surfaces, and thus was likely an important source of the 'new' C recovered in the heavy soil fraction (>1.6 g cm–3) at 0–2 cm depth (2–4% of the initial litter C). This fraction is known to have much longer residence times in the soil than the light fraction. Moreover, I have evidence that a substantial part of the litter-derived DOC (20–40%) was rapidly mineralised by soil microbes. Together, physico-chemical interactions and biodegradation retained more than 90% of the DOC leached from the litter layer within the top centimetres of the mineral soil.
Laser spectroscopy is an emerging technique to analyse the isotopic composition of soil CO2 effluxes in situ and at high temporal resolution. In collaboration with the EMPA, I employed the most recent spectrometer (QCLS) during a short field campaign. With a one-second interval, the QCLS measured the stable isotopes of CO₂ that accumulated in the headspace of a closed soil-chamber system. The unique resolution of the QCLS measurements allowed us to analyse non-linearities in the isotopic composition of CO₂ effluxes. This information was used to improve the estimation of δ¹³Cresp and δ¹⁸Oresp.
To determine δ¹³Cresp, for instance, we used only the first 10 out of 20 minutes of CO₂ accumulation because there was a systematic shift in δ¹³Cresp of 1.9‰ in the second part of the chamber measurements. This bias was probably the result of a ‘kinetic fractionation’ during CO₂ diffusion as it has recently been suggested by simulation studies, but so fare, has not been proven in field experiments. We estimated δ¹³Cresp from both high resolution Keeling plots and directly from the ratio of the ¹²CO₂- and ¹³CO₂-flux rates. If the isotopic fluxes were derived from simple linear regression, the flux ratios were equal to the Keeling plot intercepts. The calculation of the fluxes with quadratic curve fits, however, resulted in, on average, 0.8‰ lower values for δ¹³Cresp. For the estimation of δ¹⁸Oresp, we used a new approach based on quadratic curve fits of the ¹⁸O-Keeling plots. This was necessary as the δ¹⁸Oresp increased immediately after the chamber system was closed probably due to the invasion of chamber CO₂ into the first few centimetres of soil, where the ¹⁸O of the CO₂ equilibrated partly with the soil water.
In summary, this thesis provides deep insights into the dynamic of both the decomposition of beech litter and the isotopic composition of soil CO₂ effluxes. It suggests rethinking the importance of fine-woody litter for the C storage in forest soils, and thus could contribute to improved soil C models. The evaluation of a QCLS in combination with a closed soil- chamber system clears the way for future studies that will use this new and promising method to determine the stable isotopes in soil-respired CO₂.

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Item Type:Dissertation
Referees:Schmidt M W I, Hagedorn F
Communities & Collections:07 Faculty of Science > Institute of Geography
Dewey Decimal Classification:910 Geography & travel
Language:English
Date:2011
Deposited On:19 Mar 2012 14:29
Last Modified:05 Apr 2016 15:39
Number of Pages:122
Official URL:http://opac.nebis.ch/ediss/20121297.pdf
Related URLs:http://opac.nebis.ch/F/?local_base=NEBIS&CON_LNG=GER&func=find-b&find_code=SYS&request=006847382
Permanent URL: https://doi.org/10.5167/uzh-59796

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