Soil Science

Supplemental Digital Content

The stabilisation of organic carbon via chemical interactions with Fe and Al oxides in gley soils

J. M. Cloya*1, C. A. Wilsonb and M. C. Grahama

a School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh, UK b Biological and Environmental Sciences, School of Natural Sciences, University of Stirling, Stirling, UK

*Corresponding author. Tel.: +44(0)131 535 4155. E-mail address: [email protected]

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Supplementary Material Table 1. Comparison of oxalate-extractable concentrations of Al and Fe using different oxalate concentrations for selected Harwood forest soil a -1 samples . Table shows oxalate-extractable concentrations of Al (AlO) and Fe (FeO) (mg g ) obtained using 0.02 M and 0.2 M oxalate concentration methods. Gleysol and Stagnosol measurements are the mean of single soil samples from five replicate profiles and Cambisol measurements are values determined for single soil samples from one profile (figures in brackets show one standard deviation).

Soil type AlO AlO FeO FeO (sampling depth (mg g-1) (mg g-1) (mg g-1) (mg g-1) (cm)) 0.02M methodb 0.2M method 0.02M methodb 0.2M method Gleysol

10-20 0.49 (0.15) 0.97 (0.40) 1.3 (0.3) 4.8 (1.6)

20-30 1.2 (0.6) 1.4 (0.2) 1.3 (0.3) 3.8 (1.2)

Stagnosol

10-20 0.44 (0.01) 1.1 (0.2) 1.3 (0.2) 5.7 (1.5)

20-30 0.44 (0.13) 0.93 (0.2) 1.2 (0.2) 5.2 (1.4)

Gleyic Cambisol

10-20 0.31 0.79 0.96 7.6

20-30 0.29 1.1 0.75 5.9 aTest extractions using the modified 0.02 M and conventional 0.2 M ammonium oxalate methods were carried out for selected single soil sub-samples from 10-20 cm and 20- 30 cm depths and results indicated that the Al and Fe extraction by the 0.02 M oxalate method averaged 48 ± 19 % and 23 ± 9 %, respectively, of those extracted by the conventional method. Paired t-tests for Al (t = 7.18, df = 20, P<0.05) and Fe (t = 11.76, df = 20, P<0.05) confirmed that extractable concentrations for the two methods were significantly different (although the chosen 0.02 M oxalate extraction method was necessary to reduce oxalate contamination of the extracted OM in order to study the chemical nature of the OM associated with weakly crystalline Fe/Al oxides). For this reason, 0.2 M oxalate extraction Fe and Al values, rather than 0.02 M oxalate extraction values, were used to calculate the amounts of weakly and strongly crystalline Fe and Al oxides present in these soils. bFor direct comparison, values quoted here are different from those in Table 3 in the main manuscript because they represent values measured in the samples selected for test extractions.

2 80 80

2 2 R = 0.0018

60 R = 0.0327 60

)

) -1 -1 Gleysol 10-20 cm Gleysol 10-20 cm Gleysol 20-30 cm Gleysol 20-30 cm Stagnosol 10-20 cm Stagnosol 10-20 cm 40 40 Stagnosol 20-30 cm Stagnosol 20-30 cm

Gleyic cambisol 10-20 cm Gleyic cambisol 10-20 cm Total Total SOC (mg g Total Total SOC (mg g Gleyic cambisol 20-30 cm Gleyic cambisol 20-30 cm 20 20

0 0 0 2 4 6 8 10 0 0.5 1 1.5 2

FeO (mg g-1) AlO (mg g-1)

80 80

60 R2 = 0.0114 60

R2 = 0.0283

)

) -1 -1 Gleysol 10-20 cm Gleysol 10-20 cm Gleysol 20-30 cm Gleysol 20-30 cm Stagnosol 10-20 cm Stagnosol 10-20 cm 40 40 Stagnosol 20-30 cm Stagnosol 20-30 cm

Gleyic cambisol 10-20 cm Gleyic cambisol 10-20 cm Total Total SOC (mg g Total Total SOC (mg g Gleyic cambisol 20-30 cm Gleyic cambisol 20-30 cm 20 20

0 0 0 10 20 30 40 0 2 4 6 8 10

FeDCB-O (mg g-1) (Fe + Al)O (mg g-1)

Supplementary Material Figure 1 Relationship between total SOC and the amounts of 0.2 M oxalate extractable Fe (FeO) and Al (AlO), 0.2 M oxalate extractable Fe and Al

(Fe + AlO), and total 0.02 M oxalate and DCB extractable Fe minus 0.2 M oxalate extractable Fe (FeDCB-O) for Harwood forest Gleysols, Stagnosols and gleyic Cambisols plotted using values determined for each profile at depths of 10-20 and 20-30 cm.

3 140 140

120 120 R2 = 0.0363 Gleysol 0-10 cm R2 = 0.0385 Gleysol 0-10 cm 100 Gleysol100 10-20 cm

) Gleysol 10-20 cm -1 Gleysol) 20-30 cm -1 Gleysol 20-30 cm 80 Stagnosol80 0-10 cm Stagnosol 0-10 cm Stagnosol 10-20 cm Stagnosol 10-20 cm 60 Stagnosol 20-30 cm 60 Stagnosol 20-30 cm

Gleyic cambisol 0-10 cm content (mg g content

Total Total SOC (mg g Gleyic cambisol 0-10 cm Gleyic cambisol 10-20 cm 40 40 Gleyic cambisol 10-20 cm Gleyic cambisol 20-30 cm Gleyic cambisol 20-30 cm

20 20 Calculated oxalate and DCB extractable SOC extractable and DCB oxalate Calculated 0 0 0 10 20 30 40 50 0 10 20 30 40 50 Fe (mg g-1) -1 DCB FeDCB (mg g )

140 140

120 120

2 2 Gleysol 0-10 cm R = 0.0255 Gleysol 0-10 cm 100 R = 0.0048 100 Gleysol 10-20 cm Gleysol 10-20 cm Gleysol 20-30 cm Gleysol 20-30 cm 80 80 Stagnosol 0-10 cm Stagnosol 0-10 cm Stagnosol 10-20 cm Stagnosol 10-20 cm 60 60 Stagnosol 20-30 cm Stagnosol 20-30 cm

Gleyic (mg g-1) content cambisol 0-10 cm Gleyic cambisol 0-10 cm Total Total SOC (mg g-1) 40 Gleyic cambisol40 10-20 cm Gleyic cambisol 10-20 cm Gleyic cambisol 20-30 cm Gleyic cambisol 20-30 cm

20 20 Calculated oxalate and DCB extractable SOC extractable and DCB oxalate Calculated 0 0 0 1 2 3 4 0 1 2 3 4

AlDCB (mg g-1) AlDCB (mg g-1)

a) b) Supplementary Material Figure 2 Relationship between a) total SOC and b) calculated 0.02 M oxalate and DCB extractable SOC with total 0.02 M oxalate and DCB extractable Fe (FeDCB) and Al (AlDCB) for Harwood forest Gleysols, Stagnosols and gleyic Cambisols plotted using values determined for each profile and depth.

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