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

Abstract

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.