Comparison of nitrogen mineralisation patterns from root residues of Trifolium subterraneum and Medicago sativa

Crops grown in the first or second year after Medicago sativa L. (lucerne or alfalfa) in southern Australia have sometimes yielded less than crops grown after Trifolium subterraneum L. (subterranean clover), despite the greater annual accumulation of legume nitrogen (N) and enhanced N-2 fixation in lucerne-based pastures. To understand why, we studied the N mineralisation patterns of root residues collected from 16- or 20-week-old plants using incubation assays, in two separate experiments with contrasting soil types (a red loam and a grey clay loam). Fine roots of both species added to soil produced more mineral N than the control soil with no root residues added. In contrast, less mineral N accumulated in the presence of coarse roots than in the control soil. These patterns were not explained by differences in physical size or surface-area, but appeared to be more related to the carbon to nitrogen ratio (C:N) of fine and coarse roots. Fine roots of both species had a C:N of about 11, while the C:N of coarse roots ranged from 28 to 37. Subterranean clover had a high proportion of fine roots giving a weighted average C:N of 19 for the whole root system, and could be expected to result in a rapid net mineralisation of N. The root systems of lucerne on the other hand, consisted mainly of coarse roots giving an average C:N of 26, and would be likely to induce a transient net immobilisation of N and a delay in net mineralisation. The same general patterns of N mineralisation/immobilisation were observed in the second experiment. Tissue chemical analyses suggested that even where the total C:N of subterranean clover and lucerne residues were similar, the amounts of C and N in the soluble fraction of the residues and the C:N of that soluble fraction could largely account for the observed differences in N mineralisation/immobilisation.