Due to frequent volcanic activity and erosion of dryland soils, peatlands in Iceland receive regular additions of mineral aeolian deposits (tephra and eroded material). Therefore, their soils may develop not only histic, but also andic characteristics. Here we present data sets of a study that elucidates interactions between carbon characteristics and andic soil properties in Histosols of three Icelandic peatlands. The data sets contain information about the soil carbon structure derived by ¹³C NMR spectroscopy, andic soil properties based on selective dissolution of Al, Fe and Si, decomposition proxies C/N, δ¹³C and δ¹⁵N, information on total carbon and nitrogen content, dry bulk density, soil organic matter content and pH measured in deionised water and NaF solution.

1. Description of methods used for collection/generation of data: 
1.1 Soil Sampling Approach
The research area is in Austur Húnavatnssýsla in northwest Iceland. Soil samples were collected from three peatlands along a north-south transect from the coast (Torfdalsmýri - TDM), via the lowlands (Tindar - TIN) to the highland fringe (Hrafnabjörg - HRAFN). At each peatland, one soil profile was excavated. Each profile contains well preserved tephra deposits from the Hekla 3 and Hekla 4 eruption. Pooled bulk soil samples were collected at 10 cm intervals down to a depth of 20 cm below the Hekla 4 tephra layer. Above and below the tephra layers, the sampling interval was reduced to 2 x 5 cm. 

1.2 Laboratory analyses

1.2.1 Solid state 13C NMR spectroscopy 
Solid state ¹³C NMR spectroscopy was used to determine the chemical composition or structure of soil organic carbon (see e.g. Kögel-Knabner, 1997). 

1.2.2 Dry Bulk Density, Soil Organic Matter, %C and %N, stable isotope ratios δ¹³C and δ¹⁵N 
The dry bulk density of the soils (DBD) was determined by weighing after the drying of a known volume of soil. Soil organic matter content (SOM) and %Ash was measured by loss on ignition at 550 °C (e.g. Heiri et al., 2001). Determination of total carbon (%C) and nitrogen (%N) by dry combustion and of the stable isotope ratios δ¹³C and δ¹⁵N was conducted by the Cornell Isotope Laboratory, USA.  

1.2.3 Phosphate retention and pH in DiH2O and NaF
Phosphate retention was measured by spectrophotometrical absorbance at 466 nm after color reaction with nitric vanadomolybdate acid reagent (Blakemore et al., 1987). The pH in 1N NaF solution (pHNaF) was determined following Blakemore et al. (1987) and Soil Survey Staff (2014). Soil acidity (pH_H2O) was measured in deionised water (DiH₂O), using a soil:water ratio of 1:10 (Blakemore et al., 1987; Rayment & Lyons, 2011). 
 
1.2.4 Selective dissolution of Al, Fe and Si
Extraction of Al, Fe and Si with ammonium oxalate (0.2 M, pH 3.0) followed Soil Survey Staff (2014; method 4G2). The sum of Al_o + ½Fe_o was calculated, which is a diagnostic criterion of andic soil properties (≥ 2%; IUSS Working Group WRB, 2015). Ferrihydrite was estimated as %ferrihydrite = %Fe_o x 1.7 (Childs, 1985). Extraction of Al and Fe with sodium pyrophosphate followed Soil Survey Staff (2014; method 4G3). Allophane or allophane-like constituents were estimated by the equation proposed by 
Mizota and van Reeuwijk (1989), based on Parfitt and Wilson (1985). 

A more detailed description of the sampling approach and laboratory analyses is provided in the following manuscript:

Möckel, S. C., Erlendsson, E., & Gísladóttir, G. (2021). Andic Soil Properties and Tephra Layers Hamper C Turnover in Icelandic Peatlands. 
Submitted to the Journal of JGR - Biogeosciences. https://doi.10.1002/essoar.10506826.1.

References

Blakemore, L. C., Searle, P. L., & Daly, B. K. (1987). Methods for Chemical Analysis of Soils. Lower Hutt, NZ: NZ Soil Bureau, Department of Scientific and Industrial Research. 

Childs, C. W. (1985). Towards understanding soil mineralogy II. Notes on ferrihydrite. N.Z. Soil Bureau Laboratory Report CM7. Lower Hutt, New Zealand: N.Z. Soil Bureau. 

Heiri, O., Lotter, A. F., & Lemcke, G. (2001). Loss on ignition as a method for estimating organic and carbonate content in sediments: reproducibility and comparability of results. Journal of Paleolimnology, 25(1), 101-110. https://doi.org/10.1023/A:1008119611481

IUSS Working Group WRB. (2015). World Reference Base for Soil Resources 2014, update 2015. International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports No. 106. FAO, Rome.

Kögel-Knabner, I. (1997). 13C and 15N NMR spectroscopy as a tool in soil organic matter studies. Geoderma, 80(3), 243-270. https://doi.org/10.1016/S0016-7061(97)00055-4

Mizota, C., & van Reeuwijk, L. P. (1989). Clay Mineralogy and Chemistry of Soils Formed in Volcanic Material in Diverse Climatic Regions. Soil Monograph 2. Wageningen: ISRIC. 

Parfitt, R. L., & Wilson, A. D. (1985). Estimation of allophane and halloysite in three sequences of volcanic soils, New Zealand. Volcanic Soils Catena Supplement, 7, 1-8. 

Rayment, G. E., & Lyons, D. J. (2011). Soil Chemical Methods - Australasia. Australia: CSIRO Publishing.
 
Soil Survey Staff. (2014). Soil Survey laboratory Methods manual: soil Survey investigations Report no. 42, version 5.0. U.S. Department of Agriculture, Natural Resources Conservation Service.

DATA-SPECIFIC INFORMATION FOR: Moeckel et al 2021_andic soil properties_decomposition proxies.xlsx

1. Number of variables: 20

2. Number of observations: 45 soil samples

3. Variable List: 
The table shows raw data of:
- Selective extraction of aluminum (Al), silicon (Si) and iron (Fe) with ammonium oxalate (Al_o, Fe_o, Si_o) reported in g kg^-1. 
- Selective extraction of Al and Fe with sodium pyrophosphate (Al_p, Fe_p) reported in g kg^-1.
- Al_o+0.5*Fe^_o, allophane and ferrihydrite reported in %.
- Al_p/Al_o and Fe_p/Fe_o ratios.
- Phosphate retention (P-ret.) reported in %.
- Total carbon (C) and nitrogen (N), reported in %.
- C/N ratio.
- Stable isotope ratios δ¹⁵N and δ¹³C, reported in ‰.
- Dry bulk density (DBD) reported in g cm^-3.
- Soil organic matter content (SOM) reported in %.
- pH measured in deionised water (pH_H2O) and in 1N NaF solution (pH_NaF)

DATA-SPECIFIC INFORMATION FOR: Moeckel et al 2021_13C NMR.xlsx

1. Number of variables: 2

2. Number of observations: 45 soil samples

3. Variable description: 
The data tables show results of ¹³C NMR spectroscopy of 45 soil samples, i.e. the chemical shift (ppm) and the according relative intensity (%).