Rhizodeposition flux of competitive versus conservative graminoid: contribution of exudates and root lysates as affected by N loading

Abstract

Carbon distribution between root production and rhizodeposition represents the plant strategy for growth and nutrient capture. It can shift in response to changed availability of limiting nutrients, with important consequences for ecosystem functioning. We studied the influence of nitrogen (N) availability on the belowground C fluxes of two wetland graminoids, the competitive Glyceria maxima and the conservative Carex acuta. Plants grown in pots under two levels of N availability were pulse-labeled with 13CO2 and the 13C distribution in the plant–soil systems was followed for 15 days. Together with 13C allocation measurements, root production and death were estimated to constrain the belowground C fluxes, including rhizodeposition. Higher N supply enhanced root biomass and, subsequently, the total rhizodeposition. Both species shifted partitioning of belowground C towards higher mass-specific root production and turnover, with lower investments into root exudation. Therefore, the rhizodeposition was enriched in root-derived lysates over soluble exudates. Increased total rhizodeposition and its changed quality enhanced the concentration of soluble organic C. The N fertilization induced changes in belowground C fluxes were species-specific. Contrary to Glyceria, Carex enhanced mass-specific root growth rate, which implied a markedly larger root-derived C flux to soil. In general, soil N loading enhanced total C rhizodeposition and, simultaneously, the proportion of predominantly more complex root lysates over soluble root exudates, with consequences for soil organic matter dynamics. Our results also underline the importance of species-specific responses to N loading in predicting total rhizodeposition flux and changes in its quality.