Urine effects on grass and legume nitrogen isotopic composition: Pronounced short-term dynamics of delta N-15

Nitrogen stable isotope (N-15) natural abundance is widely used to study nitrogen cycling. In grazed ecosystems, urine patches are hot-spots of nitrogen inputs, losses, and changes in delta N-15. Understanding delta N-15 dynamics in urine-affected vegetation is therefore crucial for accurate inferences from N-15 natural abundance in grasslands. We hypothesized that leaf delta N-15 following urine deposition varies with time and plant functional group. Specifically, we expected (i) short-term decreases in delta N-15 due to foliar absorption of N-15-depleted volatilized ammonia, (ii) followed by increases in delta N-15 due to uptake of N-15-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 delta N-15 in legumes. We applied cattle urine to a mixture of Lolium perenne and Trifolium repens in a pot experiment. Nitrogen concentration and delta N-15 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 N-15-depleted compared to control plants (delta N-15 0.1 vs. 5.8 parts per thousand, respectively), but leaves of T. repens were not (-1.1 vs. -1.1 parts per thousand, respectively). Later, both species increased their delta N-15, but T. repens (4.5 parts per thousand) less so than L. perenne (5.9 parts per thousand). 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 delta N-15, and that in both grass and legume the direction of the delta N-15 response to urine changes over time. Temporal dynamics of plant delta N-15 at urine patches therefore need to be explicitly addressed when N-15 natural abundance is used to study nitrogen cycling in grazed grasslands.