THE IMPACT OF SOIL NUTRIENT STATUS ON CARBON DYNAMICS IN FORESTED
PEATLANDS – COMBINING CONTINUOUS FLUX MEASUREMENTS AND ISOTOPIC
METHODS
LINKOSALMI, M.1, LOHILA, A.1, BIASI, C.2, PUMPANEN, J.3, MINKKINEN, K.3, OJANEN, P.3,
LAURILA, T.1
1
Finnish Meteorological Institute, Climate and Global Change Research, P.O. Box 503, FI-00101
Helsinki, Finland
2
University of Kuopio, Department of Environmental Science, Bioteknia 2, P.O. Box 1627, FI-70211
Kuopio, Finland
3
Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland
Keywords: 13C, Soil respiration, Drained peatland
INTRODUCTION
Soils in general contain a vast amount of carbon and peatlands are a major carbon store in the world
(Gorham, 1991). Peatlands cover 30 % of Finland’s land area and more than a half has been drained,
mainly for forestry purposes (Vasander, 1996). Land use changes, for example drainage of wetlands, cause
soil carbon (C) losses into the atmosphere. The carbon flow through drained and forested peatlands is still
poorly understood although peatlands have a major role in the global carbon cycle (Gorham, 1991). This is
due to the methodological difficulties in measuring gas exchange of forests. Eddy covariance method is
used to produce continuous area-integrated data on net ecosystem CO2 exchange. It is highly assumable
that soil nutrient status affects soil respiration rate and the rate of C allocation into the root system and
further into the soil (Martikainen, 1996). The aim of this study is to examine the carbon balance in two
drained forested peatlands, a nutrient rich and a nutrient poor site. The main questions are, how does the
photosynthetic carbon transfer through the plant–soil system and how does it return back to the
atmosphere through ecosystem respiration. We will investigate how this differs in these two forested
peatland sites, i.e., how does the soil nutrient status affect the carbon dynamics.
EXPERIMENTAL SETUP
In belowground C cycling of plants, carbon is used for root respiration, growth and storage. It is difficult
to observe and quantify the belowground C cycling by using geological or flux based methods only. The
stable isotope method (13C) is a novel and potential tool to get important information on carbon allocation
and carbon exchange between soil and atmosphere (Biasi et al., 2008, Kuzyakov, 2006).
The study is planned to take place mainly in laboratory conditions. Soil samples will be collected from
two forestry drained peatlands, Kalevansuo and Lettosuo. Kalevansuo is a nutrient poor site in southern
Finland and Lettosuo is a nutrient rich site about 20 km west from Kalevansuo. Kalevansuo is a dwarfshrub pine bog with a tree stand consisting mainly of Scots pine (Pinus sylvestris). Lettosuo has the same
climatic conditions and the same species, but the trees are taller, the stand is denser and the forest floor is
sparser. The annual net ecosystem CO2 exchange is measured at both sites with the eddy covariance
method, and results of the experiments with stable isotopes will be used to interpret the assumed
differences in the carbon balances between these different sites.
Our plan is to study 1) the priming effect and 2) C-allocation in tree seedlings. The priming effect study
consists of two experiments: a) a simulation of root exudation by adding sugar ( 13C –glucose) and b)
priming effect by using microcosmic and natural tracer (in old soil). In the root exudation experiment 13C labelled glucose is added to soil samples. The soil samples will be taken from the nutrient poor (NP) and
the nutrient rich (NR) site. The study is planned to be executed with 2 different treatments, pure soil from
nutrient poor and nutrient rich sites and nutrient poor and nutrient rich soil with 13C-glucose.
NR
NP
NR + 13Cglucose
NP + 13Cglucose
Figure 1. The conceptual drawing of the root exudation simulation
The respiration of the soil samples is measured over 3 weeks. It is assumed that the root exudates increase
the respiration rate and that there is no priming effect in the nutrient poor soil samples; the microbes have
already used the reservoir. When the respiration peaks in the glucose-treated soils, the proportion of SOMand glucose derived respiration will be determined, calculations will be done with an isotopic mixing
model. Picarro G1111-i Analyzer for Isotopic CO2 (Picarro, Inc., CA), based on cavity ring down
spectroscopy, is planned to be used in this experiment, to enable the continuous on-line measurement of
the 13C/12C ratio.
In the experiment b) the priming effect is studied by using microcosmic and natural tracer in old soil. In
this experiment no labelling will be used, but the experiment is based in the natural abundance of 14CO2.
The soil samples will be taken deeper from the peat to get older soil. In old soil the 14C is more depleted,
the signal of plant derived 14C thus being stronger. In this experiment we plan to have also 2 treatments;
old nutrient rich and nutrient poor samples with plants and same soils without plants. The plants bring in a
modern 14C-signal. The duration of this experiment will be about 6 months. Soil respiration will be
measured constantly from the samples. After 6 months, 14CO2 flux is measured from both treatments. We
will be able to estimate the soil-derived and root-derived respiration by using a mixing model. Here we
are interested in the soil component and how do the plants affect the soil decomposition.
Old NR
Old NP
No plants
NR
No plants
NP
Figure 2. The conceptual drawing of the priming effect experiment 1.b
The other approach is to study the C-allocation in tree seedlings as dependent on the soil nutrient status. In
this maximum 1 year experiment we will use the uppermost soil from the nutrient poor and nutrient rich
sites. After seeding, the saplings need to grow for ca. 6 months before the measurements. When the
saplings are big enough, they will be covered with a transparent chamber, and 13C- labelled CO2 is added
as a pulse. The saplings are allowed to fix labelled CO 2 for about 6h to ensure that the label is assimilated
by the plants and brought to the roots. The 13CO2 from the soil respiration is planned to be measured with
the Picarro 13CO2 analyzer. The aim is to find out how much C is lost from the root and soil system, and
ultimately how much is mineralized. Sampling will be done before the labelling (day -1) and after the
labelling 6 times (day 0, day 3, day 7, 30, 60, 360). The experiment will be executed also with nonlabelled samples. At each sampling, also the 13C content of the leaves, stems, roots and dissolved organic
carbon will be determined with the isotope ratio mass spectrometer (IRMS).
NR
NP
Figure 3. The conceptual drawing of the C-allocation experiment
In addition to laboratory incubation studies, we are planning to measure the dynamics in 13C/12C ratio in
the soil CO2 efflux with the Picarro 13CO2 analyzer directly at the field sites, as well as from gas samples
brought to the lab.
EXPECTED RESULTS
The expected results are going to help us to better understand the transfer of photosynthetically
assimilated carbon through the plant–soil system, and how does the carbon return back to the atmosphere
through the ecosystem respiration. We will also investigate how the soil nutrient status affects the carbon
dynamics. We will apply a novel stable isotope method (13C) to improve the knowledge about the carbon
cycling by using Picarro G1111-i Analyzer for Isotopic CO2 (Picarro, Inc., CA). We are going to combine
micrometeorological CO2 flux measurements and isotope-based laboratory and field experiments. We
have the opportunity to use state-of-the-art methods to understand the carbon balance of peat soils.
REFERENCES
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