Nitrogen stable isotope (¹⁵N) natural abundance is widely used to study nitrogen cycling. In grazed ecosystems, urine patches are hot-spots of nitrogen inputs, losses, and changes in δ¹⁵N. Understanding δ¹⁵N dynamics in urine-affected vegetation is therefore crucial for accurate inferences from ¹⁵N natural abundance in grasslands. We hypothesized that leaf δ¹⁵N following urine deposition varies with time and plant functional group. Specifically, we expected (i) short-term decreases in δ¹⁵N due to foliar absorption of ¹⁵N-depleted volatilized ammonia, (ii) followed by increases in δ¹⁵N due to uptake of ¹⁵N-enriched soil inorganic nitrogen, and (iii) that the magnitude of these changes is less in legumes than in grasses. The latter should be expected because ammonia absorption depends on leaf nitrogen concentration, which is higher in legumes than grasses, and because biological nitrogen fixation will modify the influence of urine-derived nitrogen on δ¹⁵N in legumes. We applied cattle urine to a mixture of Lolium perenne and Trifolium repens in a pot experiment. Nitrogen concentration and δ¹⁵N were determined for successive leaf cohorts and bulk biomass either 17 (early) or 32 (late) days after urine application. Early after urine application, leaves of L. perenne were ¹⁵N-depleted compared to control plants (δ¹⁵N 0.1 vs. 5.8‰, respectively), but leaves of T. repens were not (-1.1 vs. -1.1‰, respectively). Later, both species increased their δ¹⁵N, but T. repens (4.5‰) less so than L. perenne (5.9‰). Vegetation sampled within and outside urine patches in the field further supported these results. Our findings confirm that foliar ammonia uptake can substantially decrease grass foliar δ¹⁵N, and that in both grass and legume the direction of the δ¹⁵N response to urine changes over time. Temporal dynamics of plant δ¹⁵N at urine patches therefore need to be explicitly addressed when ¹⁵N natural abundance is used to study nitrogen cycling in grazed grasslands.

氮稳定同位素 ( ¹⁵ N) 自然丰度广泛用于研究氮循环。^{在放牧生态系统中，尿液斑块是氮输入、损失和 δ 15} N变化的热点。因此，了解受尿液影响的植被中 δ ¹⁵ N 动态对于准确推断草原¹⁵ N 自然丰度至关重要。我们假设尿液沉积后的叶子 δ ^{15 N 随时间和植物功能群的变化而变化。}具体来说，我们预计 (i)由于叶面吸收¹⁵ N 贫化的挥发性氨， δ ¹⁵ N会短期下降， (ii) 随后由于吸收^{15 N 富集的土壤无机氮，δ 15} N 会增加，以及(iii) 豆科植物中这些变化的幅度小于禾本科植物。后者是可以预期的，因为氨吸收取决于叶片氮浓度，豆科植物中的氮浓度高于禾本科植物，而且生物固氮作用会改变尿源性氮对豆科植物 δ 15 N 的影响^。我们在盆栽实验中将牛尿加入黑麦草和白三叶草的混合物中。在施用尿液后 17 天（早期）或 32 天（晚期）测定连续叶群和大量生物量的氮浓度和 δ ^{15 N。}尿液施用后早期，L的叶子。与对照植物相比，多年生植物的N 消耗量为^{15 （δ 15} N 分别为 0.1 vs. 5.8‰），但T的叶子。匍匐植物不是（分别为-1.1与-1.1‰）。后来，两个物种都增加了δ ¹⁵ N，但T。repens (4.5‰) 小于L。多年生植物（5.9‰）。在田间尿斑内外采集的植被样本进一步支持了这些结果。我们的研究结果证实，叶面氨吸收可以显着降低草叶 δ ^{15 N，并且在草和豆科植物中，δ 15} N 对尿液的反应方向会随着时间的推移而变化。^{因此，当使用15} N 自然丰度来研究放牧草地的氮循环时，需要明确解决尿斑处植物 δ ¹⁵ N的时间动态。