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The temporal and spatial variability of soil respiration in boreal forests : a case study of Norunda forest, Central Sweden

Vogel, Elisabeth LU (2013) In Student thesis series INES NGEM01 20122
Dept of Physical Geography and Ecosystem Science
Abstract
Previous studies have shown that, contrary to expectations, the boreal forest at the Norunda research site in central Sweden is acting as a carbon source. Therefore, the aim of this study was to investigate the spatial and temporal variability of soil carbon dioxide (CO2) efflux (FSoil) in order to provide insights into its role for the net carbon balance of this forest. The investigation is divided into three parts: (1) a general description of the soils, (2) the analysis of the spatial variability and (3) the analysis of the temporal variability of FSoil.
Soil samples were collected at three soil profiles and analysed for physical and chemical properties. Measurements of FSoil and auxiliary, environmental data were carried out at 90... (More)
Previous studies have shown that, contrary to expectations, the boreal forest at the Norunda research site in central Sweden is acting as a carbon source. Therefore, the aim of this study was to investigate the spatial and temporal variability of soil carbon dioxide (CO2) efflux (FSoil) in order to provide insights into its role for the net carbon balance of this forest. The investigation is divided into three parts: (1) a general description of the soils, (2) the analysis of the spatial variability and (3) the analysis of the temporal variability of FSoil.
Soil samples were collected at three soil profiles and analysed for physical and chemical properties. Measurements of FSoil and auxiliary, environmental data were carried out at 90 plots in the forest. The impact of environmental factors on the spatial variability of FSoil were investigated using a multiple, linear regression. Based on time series of FSoil and net ecosystem exchange (NEE), which are continuously measured at the study site, monthly and annual means of CO2 fluxes were derived and the contribution of soil respiration to ecosystem respiration was calculated. Furthermore, the correlations between FSoil, soil temperature (TSoil) and gross primary production (GPP) were investigated using the wavelet analysis.
Soil pH as well as total C and N stocks were in typical ranges for boreal forest soils. The analysis of the spatial variability of FSoil yielded a coefficient of variation of 46 %. Only soil temperature and moisture had a significant effect on FSoil, they explained about 27 % of its variance. Stone content of the soil, distance to and species of the closest three trees were not found to be significant. Annual means of FSoil were found to be very high and are about 4-5 times as large as mean, annual NEE. Furthermore, it was found that, on average, soil respiration contributes 70%-100% of ecosystem respiration. While 70% is in agreement with previous studies, a value of 100% appears to be unrealistic and potential sources of errors should be further investigated. The wavelet analysis revealed significant correlations of FSoil, TSoil and GPP at the daily and yearly cycle, as expected. Additional correlations with periods of several days to weeks were found and are likely related to weather phenomena. Soil moisture appeared to affect the dynamics between FSoil and GPP. An analysis of phase differences revealed a time lag of 1-5 days between GPP and FSoil, confirming findings of previous studies.
The results of this study highlight the importance of FSoil for the carbon balance of Norunda forest. However, in order to reliably estimate the contribution of soil respiration to ecosystem respiration, further investigations are required to rule out potential errors, as the results indicate that the continuously measured FSoil values are comparatively high. Furthermore, the variability of FSoil at the spatial scale could only be partly explained. Future studies on drivers of FSoil, which include more potential predictor variables and apply a non-linear, multiple regression, might yield further valuable insights. (Less)
Abstract
Popular science
Boreal forests play an important role within the global climatic system, since they generally act as a carbon sink. However, contrary to expectations, measurements have shown that the boreal forest at the Norunda research site in Central Sweden is acting as a carbon source, i.e. emitting more of the greenhouse gas carbon dioxide (CO2) as taking up. Soil CO2 efflux plays an important role within the carbon cycle of a forest, therefore, the aim of this study was to evaluate its relative importance for the carbon balance of the forest in Norunda and to identify underlying mechanisms resulting in large CO2 emissions.
The study was divided into three parts: Firstly, the soils in Norunda were described based on excavated soil... (More)
Popular science
Boreal forests play an important role within the global climatic system, since they generally act as a carbon sink. However, contrary to expectations, measurements have shown that the boreal forest at the Norunda research site in Central Sweden is acting as a carbon source, i.e. emitting more of the greenhouse gas carbon dioxide (CO2) as taking up. Soil CO2 efflux plays an important role within the carbon cycle of a forest, therefore, the aim of this study was to evaluate its relative importance for the carbon balance of the forest in Norunda and to identify underlying mechanisms resulting in large CO2 emissions.
The study was divided into three parts: Firstly, the soils in Norunda were described based on excavated soil profiles in three different, representative locations in the forest. Soil samples were collected from different soil layers and analysed for physical and chemical properties. Secondly, the variation of soil CO2 efflux along with the variation of different environmental factors within the forest were analysed. A portable soil respiration chamber was used to measure soil CO2 efflux in 90 plots; environmental data, such as soil temperature and soil moisture, were noted for each plot. The relationships between those environmental factors and soil CO2 efflux were analysed using a multiple, linear regression. Lastly, long-term time series of soil CO2 efflux as well as the net exchange of CO2 between the atmosphere and the forest, referred to as net ecosystem exchange (NEE), were used to analyse how soil CO2 efflux varies over time and in relation to overall ecosystem exchange. Both variables were aggregated to monthly and yearly averages and the contribution of soil CO2 efflux to total ecosystem exchange was calculated. Furthermore, the wavelet analysis, a technique that allows to analyse correlations between two signals at a common periodicity (e.g. daily or yearly), was applied to the data to investigate the relationships between soil CO2 efflux, soil temperature and gross primary production (GPP, a proxy for photosynthetic activity in the forest).
The general soil description revealed that physical and chemical properties (soil pH, C and N stocks) of the soils were in typical ranges for boreal forest soils, therefore, no information on the sources of high CO2 fluxes could be derived from the soil sampling. The spatial variability of soil CO2 efflux within the forest was relatively high, with a coefficient of variation of 46%. Only soil temperature and moisture could be identified as influencing variables for soil CO2 efflux. The other variables (stone content, distance to and species of the closest 3 trees) did not have a significant effect. The long-term chamber data showed that annual means of soil CO2 efflux are very high, about 4-5 times as large as mean, annual NEE. Furthermore, it was found that, on average, soil respiration contributes 70%-100% of ecosystem respiration. While 70% is in agreement with previous studies, a value of 100% appears unrealistic, as it would mean that no above-ground respiration occurs, and potential sources of errors should be further investigated. The wavelet analysis revealed significant correlations of soil CO2 exchange, soil temperature and GPP at the daily and yearly cycle. This is an expected result, since all three variables individually have an annual cycle. Additional correlations with periods of several days to weeks were found and are likely related to weather phenomena. Soil moisture seemed to have an influence on the dynamics between soil CO2 efflux and GPP. An analysis of phase differences showed a time lag of 1-5 days between GPP and soil CO2 efflux.
The results of this study highlight the importance of soil CO2 efflux for the carbon balance of Norunda forest. However, in order to reliably estimate the contribution of soil respiration to ecosystem respiration, further investigations are necessary to rule out potential errors, as the results indicate that the continuously measured soil CO2 efflux values are relatively high. Furthermore, the variability of soil CO2 efflux at the spatial scale could only be partly explained. Future studies on the drivers of soil CO2 efflux, which analyse more environmental variables that might explain differences in soil CO2 efflux within the forest, might provide further valuable insights. (Less)
Please use this url to cite or link to this publication:
author
Vogel, Elisabeth LU
supervisor
organization
course
NGEM01 20122
year
type
H2 - Master's Degree (Two Years)
subject
keywords
boreal forests, soil respiration, carbon dioxide exchange, automatic chamber measurements, Eddy covariance, Wavelet analysis, Norunda, physical geography and ecosystem analysis
publication/series
Student thesis series INES
report number
284
language
English
additional info
External supervisor: Prof. Martin Kaupenjohann, Technical University, Berlin
id
4076662
date added to LUP
2013-10-08 09:09:02
date last changed
2013-10-08 09:09:02
@misc{4076662,
  abstract     = {Popular science
Boreal forests play an important role within the global climatic system, since they generally act as a carbon sink. However, contrary to expectations, measurements have shown that the boreal forest at the Norunda research site in Central Sweden is acting as a carbon source, i.e. emitting more of the greenhouse gas carbon dioxide (CO2) as taking up. Soil CO2 efflux plays an important role within the carbon cycle of a forest, therefore, the aim of this study was to evaluate its relative importance for the carbon balance of the forest in Norunda and to identify underlying mechanisms resulting in large CO2 emissions.
The study was divided into three parts: Firstly, the soils in Norunda were described based on excavated soil profiles in three different, representative locations in the forest. Soil samples were collected from different soil layers and analysed for physical and chemical properties. Secondly, the variation of soil CO2 efflux along with the variation of different environmental factors within the forest were analysed. A portable soil respiration chamber was used to measure soil CO2 efflux in 90 plots; environmental data, such as soil temperature and soil moisture, were noted for each plot. The relationships between those environmental factors and soil CO2 efflux were analysed using a multiple, linear regression. Lastly, long-term time series of soil CO2 efflux as well as the net exchange of CO2 between the atmosphere and the forest, referred to as net ecosystem exchange (NEE), were used to analyse how soil CO2 efflux varies over time and in relation to overall ecosystem exchange. Both variables were aggregated to monthly and yearly averages and the contribution of soil CO2 efflux to total ecosystem exchange was calculated. Furthermore, the wavelet analysis, a technique that allows to analyse correlations between two signals at a common periodicity (e.g. daily or yearly), was applied to the data to investigate the relationships between soil CO2 efflux, soil temperature and gross primary production (GPP, a proxy for photosynthetic activity in the forest).
The general soil description revealed that physical and chemical properties (soil pH, C and N stocks) of the soils were in typical ranges for boreal forest soils, therefore, no information on the sources of high CO2 fluxes could be derived from the soil sampling. The spatial variability of soil CO2 efflux within the forest was relatively high, with a coefficient of variation of 46%. Only soil temperature and moisture could be identified as influencing variables for soil CO2 efflux. The other variables (stone content, distance to and species of the closest 3 trees) did not have a significant effect. The long-term chamber data showed that annual means of soil CO2 efflux are very high, about 4-5 times as large as mean, annual NEE. Furthermore, it was found that, on average, soil respiration contributes 70%-100% of ecosystem respiration. While 70% is in agreement with previous studies, a value of 100% appears unrealistic, as it would mean that no above-ground respiration occurs, and potential sources of errors should be further investigated. The wavelet analysis revealed significant correlations of soil CO2 exchange, soil temperature and GPP at the daily and yearly cycle. This is an expected result, since all three variables individually have an annual cycle. Additional correlations with periods of several days to weeks were found and are likely related to weather phenomena. Soil moisture seemed to have an influence on the dynamics between soil CO2 efflux and GPP. An analysis of phase differences showed a time lag of 1-5 days between GPP and soil CO2 efflux.
The results of this study highlight the importance of soil CO2 efflux for the carbon balance of Norunda forest. However, in order to reliably estimate the contribution of soil respiration to ecosystem respiration, further investigations are necessary to rule out potential errors, as the results indicate that the continuously measured soil CO2 efflux values are relatively high. Furthermore, the variability of soil CO2 efflux at the spatial scale could only be partly explained. Future studies on the drivers of soil CO2 efflux, which analyse more environmental variables that might explain differences in soil CO2 efflux within the forest, might provide further valuable insights.},
  author       = {Vogel, Elisabeth},
  keyword      = {boreal forests,soil respiration,carbon dioxide exchange,automatic chamber measurements,Eddy covariance,Wavelet analysis,Norunda,physical geography and ecosystem analysis},
  language     = {eng},
  note         = {Student Paper},
  series       = {Student thesis series INES},
  title        = {The temporal and spatial variability of soil respiration in boreal forests : a case study of Norunda forest, Central Sweden},
  year         = {2013},
}