Deepened winter snow significantly influences the availability and forms of nitrogen taken up by plants in High Arctic tundra

Abstract

Climate change may alter nutrient cycling in Arctic soils and plants. Deeper snow during winter, as well as summer warming, could increase soil temperatures and thereby the availability of otherwise limiting nutrients such as nitrogen (N). We used fences to manipulate snow depths in Svalbard for 9 consecutive years, resulting in three snow regimes: 1) <i>Ambient</i> with a maximum snow depth of 35 cm, 2) <i>Medium</i> with a maximum of 100 cm and 3) <i>Deep</i> with a maximum of 150 cm. We increased temperatures during one growing season using Open Top Chambers (OTCs), and sampled soil and vascular plant leaves throughout summer 2015. Labile soil N, especially inorganic N, during the growing season was significantly greater in <i>Deep</i> than <i>Ambient</i> suggesting N supply in excess of plant and microbial demand. However, we found no effect of <i>Medium</i> snow depth or short-term summer temperature increase on soil N, presumably due to minor impacts on soil temperature and moisture. The temporal patterns of labile soil N were similar in all snow regimes with high concentrations of organic N immediately after snowmelt, thereafter dropping towards peak growing season. Concentrations of all N forms increased at the end of summer. Vascular plants had high N at the start of growing season, decreasing as summer progressed, and leaf N concentrations were highest in <i>Deep</i>, corresponding to the higher soil N availability. Short-term summer warming was associated with lower leaf N concentrations, presumably due to growth dilution. Deeper snow enhanced labile soil organic and inorganic N pools and plant N uptake. Leaf ¹⁵N natural abundance levels (δ¹⁵N) in <i>Deep</i> indicated a higher degree of utilization of inorganic than organic N, which was especially pronounced in mycorrhizal plants.