Black spruce assimilates nitrate in boreal winter

Abstract

Winter has long been considered a dormant season in boreal forests regarding plant physiological activity such as nutrient acquisition. However, biogeochemical data clearly show that soil can remain unfrozen with substantial rates of nutrient transformation for several weeks following autumn snowfall. Here we examined nitrate (NO₃–-N) assimilation by black spruce (Picea mariana (Mill.) Britton, Sterns and Poggenb.) during summer and winter in Interior Alaska to test our hypothesis that this boreal species is able to assimilate NO₃–-N, even at the very low temperatures typical of early winter. Nitrate reductase activity (NRA) was measured in current year needles and fine roots of black spruce as an indicator of NO₃–-N assimilation in the summer and winter at two boreal forest sites. Nitrate concentration in the needles and roots were also measured to determine whether NO₃–-N was available in plant tissue for the enzyme. Nitrate reductase activity and NO₃–-N were detected in needles and roots in the winter as well as the summer. The results of a generalized linear mixed model showed that season had minimal effects on NRA and NO₃–-N concentration in this species. Additionally, the effect of incubation temperature for the NRA assays was tested at 30 °C and −3 °C for samples collected in the winter. Substantial enzyme activity was detected in winter-collected samples, even in incubations conducted at −3 °C. These results indicate that this dominant tree species in the boreal forests of Interior Alaska, black spruce, has the capacity to assimilate NO₃–-N below freezing temperatures, suggesting that the physiological activity required for nitrogen (N) resource acquisition may extend beyond the typical growing season. Our findings coupled to biogeochemical evidence for high microbial activity under the snow also indicate that winter N acquisition should be taken into account when estimating the annual N budgets of boreal forest ecosystems.