Changes in nitrogen content and isotopic composition in subarctic Empetrum nigrum seeds in the period 1976-2010

High latitude ecosystems are believed to be sensitive to increases in the availability of nitrogen (N) which may be the result of anthropogenic emissions or may be due to climate warming which accelerates the release of N from organic matter in the soil. These changes may be detected in the changes in N pools and their isotopic composition. We studied N content (N%) and isotopic composition (delta N-15 parts per thousand) along an elevational gradient in a subarctic environment in northern Finland, using Empetrum nigrum (crowberry) seeds collected annually over the past 35 years. We studied whether N% or delta N-15 parts per thousand shows trends that can be attributed to anthropogenic nitrogen, and investigated the effect of climate variables on the N content of seeds. Among the years, variations in N% and delta N-15 parts per thousand, were mainly dependent on previous summer or growing season temperatures and were in opposite directions for N% and delta N-15 parts per thousand. Among the sites, N% and delta N-15 parts per thousand were highest at a low elevation site, and lowest at the highest elevation site. Trends in delta N-15 parts per thousand over time were contrasting among the sites: at the low elevation site trend was increasing, whereas at the highest elevation site the trend was decreasing. The trend in N% over time was increasing at the low elevation site. Our results suggest that N cycling in these systems has changed in the past 35 years, but there were differences among sites.