Molecular Weight of Dissolved Organic Carbon, Nitrogen, and Phenolics in Grassland Soils

Accurate characterization of dissolved organic matter (DOM) in soil is vital for understanding its functional significance. In this study, we used ultrafiltration to determine the molecular weight (MW) distribution of dissolved organic C (DOC), N (DON), and phenolics in two contrasting agricultural grassland soils. This MW fractionation (>100, 10–100, 1–10, and <1 kDa) was undertaken with soil solutions, distilled water extracts, and 2 mol L −1 KCl extracts. We showed that water or KCl extracts removed different amounts and forms of DOM than those in soil solution extracted by the centrifugation technique. Therefore, the ecological significance of such batch extraction methods and associated data requires careful consideration. Overall, soil solutions from the two grassland soils possessed different MW signatures, although the majority of DON and DOC was recovered in the higher MW fractions (>1 kDa) in both soils. This high‐MW fraction contained a significant amount of organic N, however, and was no more enriched in phenolic‐containing substances than the low‐MW fraction. Our results are consistent with the view that soil solutions contain a diverse size range of organic compounds but that the high‐MW fraction does not contain a disproportionate amount of phenolics or N.