The impact of sampling techniques on soil pore water carbon measurements of an Icelandic Histic Andosol

The carbon in soil pore water from a Histic Andosol from Western Iceland was studied at three different scales; in the field, in undisturbed outdoor mesocosms and in laboratory repacked microcosms. Pore water was extracted using suction cup lysimeters and hollow-fibre tube sampler devices (Rhizon samplers). There were significant differences in all measured variables, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and pH values between the scales of the experiment. Gaseous constituents of soil solution and pH were more susceptible to changes in scale and the type of sampling devices used. Dissolved inorganic carbon concentrations did not differ significantly between field and mesocosm solutions but where up to 14 times lower in microcosms compared to mesocosms solutions. Rhizon samplers yielded solutions with up to 4.7 times higher DIC concentrations than porous cup lysimeters. Mesocosm surface horizon DOC concentrations were 20 and 2 times higher than in field and microcosms respectively. There was difference in DOC concentration between sampling methods (up to 8 times higher in suction cups than rhizon samplers) above 50 cm depth. Soil solution pH values did not differ between field and mesocosms and mesocosms and microcosms respectively down to 80 cm depth. Direct comparison between field and microcosms was not possible due to the nature of sampling devices. Soil solutions sampled with Rhizon samplers yielded lower pH values (up to 1.3 pH units) than those sampled with suction cups. Twenty percent of annually bound organic carbon at the soils surface under field conditions was lost by leaching of DOC and through decomposition to DIC in disturbed non-vegetated microcosms. This percentage increased to 38% in undisturbed vegetated mesocosms highlighting the importance of surface vegetation in importing carbon to soils. Increased influx of nutrients will increase growth and photosynthesis but decrease carbon sequestration in near surface horizons. Although field studies considering long-term anthropogenic changes in pedogenesis require considerable experimental duration, more rapid experiments can be conducted with confidence in micro- and mesocosms as in this research.