High Additions of Nitrogen Affect Plant Species-Specific Differences in the Composition of Main Microbial Groups and the Uptake of Rhizodeposited Carbon in a Grassland Soil

The effects of four different rates of high nitrogen (N) addition (220, 300, 450, and 750 kg N ha−1) on carbon (C) rhizodeposition, the composition of main soil microbial groups, and microbial processing of rhizodeposited C were investigated for two common grassland species, Lolium perenne L. (perennial ryegrass) and Plantago lanceolata L. (ribwort plantain). We measured net ecosystem carbon dioxide (CO2) exchange and used a 13CO2 pulse-labelling technique to trace recent products of photosynthesis through the plant-soil system. Overall, net C uptake was 10% higher for P. lanceolata than for L. perenne. This was associated with a 62% higher concentration of rhizodeposited C in the soil under P. lanceolata than under L. perenne. Concentrations of rhizodeposited C further increased by 30% per 100 kg N ha−1 added. For both plant species, increasing N addition was associated with compositional differences in soil microbial groups towards a more bacteria-dominated system and increased microbial uptake of rhizodeposited 13C. However, the N-induced changes in rhizodeposited 13C uptake by different microbial groups were much more pronounced for L. perenne than those for P. lanceolata. This suggests that microbial processing of rhizodeposited 13C was more susceptible to the response of L. perenne to high N addition compared to P. lanceolata. The findings highlight the importance of the responses of plant species with contrasting traits to high N inputs and the associated distinct effects on soil C cycling processes through altering the composition of the main soil microbial groups and microbial uptake of rhizodeposited C.