Forests losing large quantities of nitrogen have elevated 15N:14N ratios.

Urea (U) and ammonium nitrate (AN) had been applied to a Scots pine (Pinus sylvestris L.) forest in northern Sweden for 18 consecutive years at four doses resulting in total N applications ranging from 0 to 1980 kg ha-1. The 15N abundance (δ 15N) of the grass Deschampsia flexuosa (L.) Trin. increased linearly (from -0.7 to 11.0‰) with application rate in the case of U. The response to AN was in the same direction but smaller. While others have shown that the initial response of nitrogen-limited systems to additions of N is a change of 15N abundance towards that of added N, this study shows that further and excessive additions leads to a retention of 15N. Monitoring 15N abundance over time in dose-response trials of this type thus opens new possibilities to estimate "critical loads" of N and the point of "nitrogen saturation".