Geovariability and biodegradability of the water-extractable organic material in an agricultural soil

Abstract

An analysis of the quantity, biodegradability. three-dimensional distribution and u.v. absorption at 254 nm of the water-extractable organic carbon (WEOC) from an agricultural loess soil was made. In addition, the total organic carbon (TOC) in the samples was quantified. The following results were obtained. The average concentration (9.3μg g −1 DW) of the WEOC was fairly constant to a depth of 50cm. which was approximately the boundary between the B and C horizons. Below 50cm it again remained constant at significantly lower amounts (6.5 μg g −1 DW) down to 100cm. The WEOC consisted of two fractions: one that was not biodegradable with an average concentration of 3.9μg g −1 DW and one that was 85% degradable under the conditions used. This was independent of depth. The WEOC in the Ap horizon absorbed more u.v. radiation than the WEOC from the deeper horizons. This was especially the case for the refractory material. An analysis of the data with semivariograms indicated that the WEOC concentrations in the upper 50 cm were random while the ones below this depth were isotropic, being dependent on the direction of tillage. This may have been the result of preferential flow, resulting from root channels. Another possibility is organic solute enrichment below the crop rows due to the water movement into the plants. TOC decreased sharply below 25 cm depth and tended to decline further with depth. The amount of the WEOC relative to TOC increased with depth. There was no tight statistical covariation between WEOC and TOC.