Gross and net nitrogen transformation rates and availability in late summer in tall shrub and birch hummock ecosystems of the Canadian low Arctic

Climate change is occurring most rapidly in the Arctic where warming has been twice as fast as the rest of the globe over the last few decades. Arctic soils contain a vast store of carbon and warmer arctic soils may mediate current atmospheric CO2 concentrations and global warming trends. Warmer soils could increase nutrient availability to plants, leading to increased primary production and sequestration of CO2. Presumably because of these effects of warming on shrub ecosystems, shrubs have been expanding across the arctic over the last 50 years, Arctic shrub expansion may track or cause changes in nutrient cycling and availability that favour growth of larger, denser shrubs. This study aimed at measuring gross and net nitrogen cycling rates, major soil nitrogen and carbon pool sizes, and elucidating controls on nutrient cycling and availability between a mesic birch (Betula nana) hummock tundra ecosystem and an ecosystem of dense, tall, birch (B. nana) shrubs. Nitrogen cycling and availability was enhanced at the tall shrub ecosystem compared to the birch hummock ecosystem. Net nitrogen immobilization by microbes was approximately threefold greater at the tall shrub ecosystem. This was in part because of larger microbial biomass nitrogen and carbon (interpreted as a larger microbial community) at the tall shrub ecosystem. Nitrogen inputs via litter were significantly larger at the tall shrub ecosystem and were hypothesized to be the major contributor to the higher dissolved organic and inorganic nitrogen pools in the soil at the tall shrub ecosystem. The results from this study suggest a positive feedback mechanism between litter nitrogen inputs and the enhancement of nitrogen cycling and availability as a driver of shrub expansion across the Arctic.

Summary 1. Nitrogen cycling was enhanced at the tall shrub ecosystem. The immobilization rate was approximately three times higher at the tall shrub ecosystem than at the birch hummock ecosystem. There was a net nitrogen immobilization by microbes in the soil at both the birch hummock and the tall shrub ecosystem late in the growing season. 2. Microbial biomass carbon (MBC) and nitrogen (MBN) were greater at the tall shrub ecosystem. The greater MBC and MBN at the tall shrub ecosystem may explain why net immobilization was greater at the tall shrub ecosystem because the greater microbial biomass at the tall shrub ecosystem can be expected to immobilize more nitrogen. 3. MBC:MBN was lower at the tall shrub ecosystem, suggesting a more bacteria-dominated microbial community. Combined with the enhanced immobilization rate, the microbial community at the tall shrub ecosystem may have the capacity to immobilize more nitrogen than the community at the birch hummock ecosystem. 4. Nitrogen availability was enhanced at the tall shrub ecosystem. Dissolved inorganic (ammonium) and dissolved organic nitrogen (DON) soil pools were larger at the tall shrub ecosystem. 5. Litter input quantity and quality (measured as litter C:N) was greater at the tall shrub ecosystem. Nitrogen inputs via litter most likely contribute significantly to the enhanced nitrogen cycling and availability at the tall shrub ecosystem compared to the birch hummock ecosystem.