Effects of high and low fertility plant species on dead root decomposition and nitrogen mineralisation

The influence of growing grass species Holcus lanatus and Festuca ovina on the decomposition of dead roots of H. lanatus, F. rubra and F. ovina and on the nitrogen (N) mineralisation from these residues was studied in a greenhouse experiment. H. lanatus, F. rubra and F. ovina are typical of soils with high, intermediate and low fertility, respectively. Dead roots of high fertility species were expected to decompose faster compared to those of low fertility species and living roots of high fertility species were expected to accelerate the rate of decomposition of dead roots more than low fertility species. To test this hypothesis, decomposition of 15N-labelled roots of these three species was measured after a 6-week incubation period in soil planted with either H. lanatus or F. ovina plants. After this period, the remaining dead root mass, living plant biomass, and the N and 15N distribution among plant, soil and dead roots was measured. The decomposition rate of dead roots from the three plant species was not significantly different. However, H. lanatus dead roots caused a lower N uptake by the growing plants (18.3 mg N) compared to F. ovina (21.5 mg N) and F. rubra (21.9 mg N) dead roots, as a result of a higher N immobilisation by H. lanatus dead roots. The presence of growing plants stimulated dead root decomposition and N mineralisation, the effect of Holcus plants being larger than that of Festuca. H. lanatus plants took up more N and 15N (26.3 mg N and 0.30 mg 15N) than F. ovina plants (14.1 mg N and 0.17 mg 15N). Our results provide evidence that living plant roots stimulate the nitrogen release from plant residues and thereby facilitate their own growth. This effect was stronger for high fertility species than for the low fertility species, because the high fertility species produced more root biomass. On the other hand, dead roots of high fertility species immobilised more N, due to their lower N concentration, resulting in a lower N availability to the growing plants.