Nitrogen Uptake During One Year in Subarctic Plant Functional Groups and in Microbes After Long-Term Warming and Fertilization

For the first time in an arctic long-term warming and fertilization experiment, the short-term (days) and longer-term (month and year) nitrogen (N) uptake and allocation in plants, microbes, and soil pools were studied, with N-15-labeling of an organic nitrogen form, glycine. The long-term warming and fertilization had no marked effect on soil inorganic N content, but both dissolved organic N (DON) and plant biomass did increase after fertilization. Soil microbes initially immobilized most of the added N-15, but in the following months, they lost two-thirds, while label concentration in plants increased. After a year, however, the N-15 recovered in microbes was still 10-fold higher than that in the plant biomass, showing the high importance of soil microbes in nutrient retention in arctic ecosystems, irrespective of the impact of long-term warming or fertilization. The effects of the treatments on the uptake of label by deciduous shrubs and evergreens paralleled that of their N pool sizes, suggesting that their N uptake potential was unaffected by long-term warming and fertilizer addition. Mosses and herbs had high uptake potential but in fertilized plots they took up less N-15, that is, they were N saturated. The fraction of N-15 in microbes tended to decrease after fertilization, but this was an effect of higher N pool dilution after 1 month and a year, and not due to lower initial uptake. Although the concentration of soil inorganic N did not change after fertilization, both increased DON and the results of the N-15 label addition showed that the N availability in the ecosystem had increased. By contrast, warming had little effect on soil N pools and microbial N-15 uptake, and, hence, had no detectable effects on N-15 accumulation.