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.

植物物种通常以最小化氮生态位重叠的方式从土壤中吸收不同的氮 (N) 形式，但放牧是否以及如何影响植物偏好的氮形式仍不清楚。利用原位稳定N同位素标记技术，我们在内蒙古半干旱草原沿放牧强度的实验梯度探索了植物吸收不同化学形式N的偏好。⁶月^和_{_} ^_ _ _{_} _八月。确定了三种优势植物Leymus chinensis对这三种 N 形式的吸收，大针茅和Cleistogenes squarrosa。L. chinensis的 N 获取能力最高。无论物种如何，8 月份的 N 吸收都高于 6 月份。虽然所有三种植物都吸收了甘氨酸-N，但它们优先吸收无机形式的 N（NO ₃ ^- -N + NH ₄ ⁺ -N），其中 NO ₃ ^- -N 的吸收量最高。无论放牧强度如何，在生长高峰期（8 月），所有三种植物物种都更喜欢吸收 NO ₃ ^- -N（主要的 N 形式）而不是 NH ₄ ⁺-N 或甘氨酸-N。相比之下，6 月份植物对 N 吸收的偏好形式因放牧处理而异。在重度放牧（G2）下，所有植物都优先吸收NO ₃ ^- -N。在轻放牧 (G1) 下，所有植物都吸收了所有三种 N 化学形式，没有显着差异。在不放牧（G0）的情况下，L. chinensis优先吸收 NO ₃ ^--N，但放牧强度对 N 的首选形式没有显着影响。我们的结果表明，我们观察到的三种植物在早期生长和旺盛生长中不同地改变了它们对放牧强度的 N 吸收偏好阶段。在未来的研究中，需要系统地探索植物氮在时间、空间和化学氮形式上的生态位分化，以帮助了解放牧对草原生态系统氮过程的影响。