Seasonal variation in 15N natural abundance in subarctic plants of different life-forms

Plants can be expected to utilize different sources of nitrogen with different proportions of N-15 at different times of the year. We expected this to be reflected in a seasonal variation in the natural abundance of plant N-15, and that this pattern would vary among life-forms or species. To test this hypothesis, we studied the delta(15)N of eight different life-forms, selecting two representatives from each of four categories (woody deciduous, woody evergreen, graminoid, and cryptogam life-forms) at two locations having different levels of precipitation, over a six-month period. Sampling was conducted in mid-winter, during snowmelt in May, after leaf emergence, in mid-August, and in September. The sampled species showed a highly significant seasonal pattern in the natural abundance of N-15. Within each species and site, the delta(15)N showed a difference on average of 3.6% (range from 2.1 to 5.3%) between minimum and maximum over the sampling period. In most cases delta(15)N was highest in mid-winter and lowest at start: of the growing season. Most species studied showed some common trends: (i) a decline in delta(15)N from mid-winter to pre-snowmelt (May); (ii) an increase from snowmelt to mid-June (mainly in plants sampled at one site); and (iii) a late-season decline in delta(15)N (August to September). Life-forms differed from each other in term of their pattern of seasonal variation (harvest x life-form interaction) and between sites (sire x life-form interaction). Thus, the outcome of comparisons of natural delta(15)N within and among species or sires depends on the time of year of sampling.