Approximately 15 percent of Sweden's land area is covered by peat ( 30 cm deep). In drained peatlands, decomposition of the peat produces greenhouse gases such as CO2 and N2O. Electric conductivity, measured with the instrument EM38, can be used to assess spatial variation in soil properties. To easier decide the quantity of the CO2-emissions from a peatland the possible relation between electric conductivity and CO2-emissions, water content, peat depth and soil temperature is investigated. The test site is located at Bälinge mossar, approximately 20 kilometers north of Uppsala. On the peat field 40 test points are marked where the measurements take place. The results of the investigation show that the only correlation that exists is the one between electric conductivity and water content.
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In their natural state peatlands are accumulating organic matter, which means that
they bind large quantities of carbon, about 5 and 50 g C / m 2
/ year. The drainage
and cultivation of peat soils increase the aeration of the soil, which in turn results in
faster brake down of the organic material than if the land is left untouched. The
degradation of the organic material release greenhouse gases such as CO2, N2O and
CH 4
most) and greatly affect the agricultural sector's contribution of
greenhouse gases. Greenhouse gas measurements from cultivated land are often
done on few points per field, or by integrating over a larger area with eddy
covariance methods
CO2 emissions from cultivated peat soils exhibit a large spatial variation, which gives uncertain estimates of greenhouse gas emissions.
Hypotheses:
CO2 emission varies with soil properties The spatial variability of soil properties in a cultivated peat soil can be identified by measuring the electrical conductivity or gamma radiation By correlating the CO2 emission with different soil characteristics, a better estimate of gas emissions from cultivated peat soils can be made. 2
The field site was a fen peat soil with a peat depth of about 1 meter. Soil from the
field has previously been used in greenhouse gas measurements in lysimeters in
EUROPEAT, a European project which ran between 2002 and 2006
The measurements were carried out October 21, 2008 and 14 May 2009 in order to compare the variation at different seasons and moisture contents.
Conductivity measurements with the EM38 give approximately a value every 2.5 meters in the direction of travel and the distance between the rows was about five meters. In the field a total of 3448 points was measured. Fig. 2 shows all data points from the EM38 run. The reason to run a few zig-zag turns is to be able to ensure that the EM38 value is stable during the entire run.
To obtain values for the entire field the values were interpolated with kriging
In the autumn survey the soil dielectric properties were also measured (a form of moisture measurement) with a Profile Probe from Delta-T.
We made two transects across the fields and in Fig. 4 the Profile Probe value (mv) and EM38 value from the deeper run is shown. Although both EM38 value and Profile Probe value is a measure of water content, the relationship was not entirely clear, with relatively low R2 values. The deeper EM38 measurements (0-100 cm) gave the best agreement with Profile Probe measurements (Fig. 5).
In the spring (14/05/2009) we conducted a more detailed study in which an EM38 survey was supplemented with gamma-ray measurements with a mole (Fig. 6). The correlation table (Table 1) shows that the EM38 values show a similar variation both between depth and between autumn and spring but there is no correlation between the different gamma ray measurements. At the same time as the EM38 measurements was done we also carried out a field survey of 40 points in which CO2 emissions, water content (gravimetrically), moisture (ProfileProbe), C, N, pH, peat depth (19 points) was measured.
40K
We only found a correlation between C and N and between water levels. Fig. 7
shows the CO2 emission from the 40 sampling points. There is considerable variation
in gas emission between the points, from 521 mg CO2 m-2 h-1 to 6638 mg CO2 m-2
h1. We found no correlation between gas emission and any of the other parameters we
measured. The field was divided into eight conductivity zones (EM38) to obtain a
better correlation between conductivity and water content measured with the Profile
Probe. When excluding two outliers the relationship between EM38-value and
Profile Probe-value of 20 cm depth was much better (R2 = 0.89), but there was still
no relationship between the EM38-value and the CO2 emission or peat depth.
Table 1. Correlation between the deep EM38 mea