Preferences for different nitrogen forms in three dominant plants in a semi-arid grassland under different grazing intensities

Plant species generally absorb different nitrogen (N) forms from soil in ways that minimize N niche overlap, but whether and how grazing affects a plant's preferred form of N remains unclear. Using the in-situ stable N isotope-labeling technique, we explored the preference of plants in taking up N of different chemical forms along an experimental gradient of grazing intensity in a semi-arid grassland in Inner Mongolia. 15N-labelled nitrate (NO3−), ammonium (NH4+) and glycine solutions were injected into the soil under nil (G0, as a control), light (G1) and heavy grazing (G2) grassland plots in June and August. Uptake of these three N forms was determined for three dominant plants, Leymus chinensis, Stipa grandis and Cleistogenes squarrosa. L. chinensis had the highest N acquisition capacity. Regardless of species, N absorption was higher in August than in June. While all three plant species absorbed glycine-N, they preferentially absorbed N in its inorganic forms (NO3−-N + NH4+-N), with NO3−-N having the highest uptake. Irrespective of grazing intensity, during peak growth (August) all three plant species preferred to absorb NO3−-N (the dominant N form) than NH4+-N or glycine-N. In contrast, in June plants differed in their preferred form of N uptake depending on the grazing treatment. Under heavy grazing (G2), all plants preferentially absorbed NO3−-N. Under light grazing (G1), all plants absorbing all three N chemical forms with no significant difference. Under no grazing (G0), L. chinensis preferentially absorbed NO3−-N but otherwise there was no significant effect of grazing intensity on the preferred form of N. Our results indicate that the three species of plants that we observed altered their N-uptake preference in response to grazing intensity differently in the early growth and vigorous growth stages. Plant N niche differentiation in time, space and chemical N forms needs to be explored systematically in future studies to aid understand of grazing effects on N processes in grassland ecosystem.