Dissolved organic carbon in soil solutions: a comparison of collection methods

A field study was undertaken to compare the DOC concentrations in soil solutions obtained with three different sampling methods over a range of soil types. The sampling devices were a tension-free collector, a tension Prenart collector and a tension Rhizon collector. Samples were collected fortnightly for a year at seven sites in northern England, each collection being replicated three times. The soil solution DOC ranged from 1.3 gm-3 in an acid ranker to 34.7 gm-3 in a peat. The DOC concentrations obtained with the three methods were reasonably well correlated (r2 of 0.6 to 0.8) but with an indication of bias, as the best fit line differed from the 1:1 line. The tension-free collector gave generally higher DOC concentrations except at very low concentrations (in the acid ranker soil). The DOC concentrations measured with the tension-free collectors were significantly (p < 0.05) higher those obtained with Prenart and Rhizon collectors at four and six sites, out of seven, respectively. Subsequent laboratory tests on tension-free collected samples showed no DOC loss on filtration through 0.1 and 0.22-m membranes, whereas a significant loss of DOC occurred when tension-free collected samples were subsequently passed through Prenart and Rhizon collectors, indicating a probable sampling artefact with the tension devices. The difficulties of acquiring representative soil solution samples are discussed, together with the advantages and disadvantages of tension and tension-free methods.