A larger investment into exudation by competitive versus conservative plants is connected to more coupled plant–microbe N cycling

Abstract

Plant–microbe interactions actively control nitrogen (N) cycling in the ecosystem. We hypothesize that the investment into exudation and the coupling of plant–microbe N cycling will be larger in competitive plants compared to the more conservative species. Root exudation of competitive (Glyceria maxima) and conservative (Carex acuta) plants was estimated by 13C-CO2 labeling. Seasonal changes in plant, microbial, and soil soluble N pools as well as potential net microbial N transformations were determined to interconnect the C and N cycling within grassland ecosystems dominated by these species. We showed that competitive Glyceria, as compared to conservative Carex, appears to affect soil N cycling through a more direct temporal and spatial influence on soil microbes due to a larger investment into root exudation. This makes the system highly dynamic, with faster soil N cycling and pronounced seasonal N redistribution between plants and microbes. The conservative Carex, irrespective of its larger root system, invested less C to exudation. In this case, the plant–microbe relationships appear to be less-coupled in time and space with the plant N supply likely relying mainly on the relatively slow microbial mineralization of organic matter than on rhizosphere priming effect. We showed that differences in soil N cycling associated with competitive versus conservative plants are closely connected with their different investments into root exudation, which govern the coupling of plant–microbe interactions in time and space.