Post-fire nitrogen uptake and allocation by two resprouting herbaceous species with contrasting belowground traits

Abstract

Abstract Immediately after fire, soil nitrogen availability increases due to ash deposition and the stimulation of mineralization by increased temperature. The coexistence of plant resprouters with contrasting root traits that grow soon after fire suggests different strategies operating for the re-acquisition of space, capturing resources and resisting additional stresses. Brachypodium retusum and Dactylis glomerata are two perennial grass species that co-occur in fire-prone Mediterranean basin ecosystems. B. retusum (Br) is a slow-growing grass with a developed rhizome, whereas D. glomerata (Dg) is a fast-growing grass that resprouts from rootstock buds. We hypothesized that the fast-growing grass would take advantage of the flush in available N after fire, whereas the aboveground growth of the slow-growing grass would rely on rhizome N reserves. The objective of this study was to ascertain the role of the root system characteristics and plant growth traits in relation to soil N uptake and plant N assimilation soon after fire followed by grazing. Therefore, in a pot experiment with plants and soil transplanted from the field, we applied 15N-NH4+ to the surface of the soil, immediately after cutting the aboveground plant biomass to simulate post-fire disturbance. Grazing defoliation was simulated using four successive cuttings. Both species resprouted quickly after the fire-simulation cutting, with the recovery of approximately 15% of the initial aboveground biomass only 5 days afterwards; 20 days later, the biomass reached 50% for Dg and 28% for Br. This regrowth was parallel to a reduction in belowground biomass. Later, the aboveground biomass remained approximately constant after four cuts over 75 days, maintaining the same total N content (close to 2.0\u2009g\u2009N m−2 for Dg and 1.0–1.5\u2009g\u2009N m−2 for Br). Both species showed high 15N atom % in their leaves after 20 days of 15N application, which slowly decreased over the successive cuts. After 55 days and two cuts, only Br showed a decline in the total plant 15N content. Plant growth and soil N uptake were higher in the non-rhizomatous species (Dg) showing greater root development and higher belowground/aboveground ratio. The rhizome of Br supported early resprouting but this species did not take much advantage of the initial N mineralization after fire. In Dg pots, the 15N-NO3- that leached out was lower than in Br pots. Dg showed higher nitrate reductase activity, in comparison to Br. In agreement with our hypothesis, the results suggest that the fast-growing strategy of Dg takes advantage of increased N availability after fire, whereas the slow growing Br relies on N rhizome reserves, which were slowly replenishing from available soil N. Both species showed high regeneration capacity after fire and high resilience to intensive post-fire grazing pressure.