In search of the elusive “active” fraction of soil organic matter: Three size-density fractionation methods for tracing the fate of homogeneously 14C-labelled plant materials

Abstract

To improve the understanding of nutrient cycling in soil there is a need for development of methods to quantify biologically-meaningful fractions of soil organic matter which turn over in the short or medium-term. Homogeneously 14C-labelled shoots from ryegrass grown at ambient (350 μl l −1) and elevated (700 μl l −1) CO 2 concentrations were added to a loamy sand and incubated for up to 200 days. Three size-density methods were tested in order to elucidate the breakdown of the plant material. One approach involved density separation in Ludox TM40 (a colloidal silica suspension) but only included soil materials > 150 μm. The other two approaches in which sodium polytungstate was used as density agent included all solid and soluble soil material. One of these involved a size separation (at 100 μm) prior to density separation, while the other was performed on whole soil. Density fractionation in a centrifuge (10,000 g ) without initial size-separation substantially reduced the recovery of freshly-added plant material in the light fraction. We assume that this was partly due to the loss of air entrapped in intact tissue during centrifugation, and partly due to interactions between small heavy particles and the large light plant material. Fractionation by size and density thus seems a more powerful approach for separating soil organic matter fractions than fractionation based on density alone. Separation of finer textured materials (< 100 μm) by density resulted in fractions with similar specific activity, indicating that they did not differ greatly in their turnover rates. The changes with time in the specific activity of the fine fractions indicated that they acted as sinks for microbial products, and only contributed slightly to the mineralization of the freshly-added C. The soluble carbon was consistently the most 14C-enriched fraction and contained a substantial amount of 14C throughout the incubation. The large, light fractions consisted of identifiable plant residues and were enriched in 14C during the 200 day incubation. Subdivision of the large fraction by density resulted in fractions with considerably different initial enrichment, presumably due to greater airfilled porosity in less decomposed or frayed materials. Losses of “native” soil carbon were small, compared with the analytical uncertainties, and thus the identification of active “native” soil fractions was hampered. Differences in the decomposition patterns between ryegrass grown at ambient and elevated CO 2 concentrations, measured by CO 2 respiration after 10 days, were observed with the large (> 150 μm) light Ludox fractions. At the end of the experiment no differences between plant material grown at ambient and elevated CO 2 concentrations were detected in earlier CO 2 evolution or in the different soil organic matter fractions. Mineralization of C from previously leached plant materials was considerably enhanced by exposure to Ludox and retarded by exposure to sodium polytungstate.