Festuca campestris alters root morphology and growth in response to simulated grazing and nitrogen form

P> Large herbivores are known to spatially concentrate and alter the form of nitrogen (N) in grassland systems, which can modify aboveground plant chemical composition and productivity, and result in shifts in grazing pressure in these areas. Root responses to grazing under high N inputs may facilitate these changes, but their responses are less well understood. We examined biomass, root morphology and demography, and nitrogen pool responses of Festuca campestris Rydb. seedlings grown in pots in an environmental chamber and subject to heavy fertilization with isotopically-labelled N in a NO3-N or NH4-N fertilizer treatment, and to no clipping or clipping to 4 cm twice during the experiment. Festuca campestris exhibited re-growth patterns after defoliation that reflected preferential allocation to aboveground shoots, with percent recovery of shoot biomass twice that of roots. Clipping did not increase the risk of root mortality. Instead, reduced root biomass in clipped plants resulted from slowed elongation and decreased production of new roots. Although roots with smaller diameters and at lower density had a greater risk of root mortality, clipping did not alter the mean root diameter or level of root branching. Under high N fertilization, NH4-N increased the amount of aboveground biomass, ameliorated the effects of clipping on root production and further reduced the risk of root mortality in clipped plants. N-form had no effect on root morphology. While total plant N uptake and the percent utilization of fertilizer were lower in clipped plants, total plant nitrogen accumulated per root biomass (g g-1) at harvest was higher in clipped plants. Our findings indicate that where defoliation is coupled with high nitrogen return, Festuca campestris recuperates by increasing nitrogen uptake, shifting production to favour recovery of aboveground biomass, and mitigating carbon losses by decreasing investment in new, actively growing roots. Because application of a more mobile form of nitrogen (NO3-N) did not alter root morphology, root physiological responses may be most important for re-growth when nitrogen is not limiting. Root reductions associated with grazing may have a greater impact in locations where grazer-mediated nitrogen return is spatially decoupled from defoliation.