Site: Bonanza Creek LTER
Mid-successional balsam poplar trees rely on several mechanisms of nitrogen acquisition, including nitrate uptake from hyporheic water, ammonium from soil N mineralization, and direct uptake of free amino acids in the soil.
K. Kielland

Biogeochemical investigations have a long history at BNZ. These studies have demonstrated how slowly soil organic matter turn over in boreal forest soils, because of low biological activity coupled to a very short growing season. Recently, however, we have learned that nitrogen cycling in boreal forest soils continues past freeze-up and that about 40% of the annual nitrogen flux occurs during winter. Whereas decomposition and nitrogen mineralization may appear slow compared to more temperate ecosystems, there are other avenues of soil N supply which support the demand of the vegetation across primary succession on the Tanana River floodplain. For example, the production and turnover of dissolved organic nitrogen in boreal forests appear to be as rapid as that of inorganic nitrogen, and the uptake of amino acids by plants and microbes appears equally important as the uptake of dissolved inorganic nitrogen (McFarland et al. 2010). Soils on the Tanana River floodplain exhibit both qualitative and quantitative shifts in biogeochemical processes across succession. Early successional soils which have low organic matter content are characterized by low in situ rates of nitrogen mineralization and proteolysis (Kielland et al. 2007). Nitrogen mineralization increases with increasing soil organic matter content, reaches a peak in mid-succession, and declines thereafter possibly due to an accumulation of recalcitrant soil organic matter derived from the dominant coniferous tree species. By contrast, soil proteolytic activity and the turnover of free amino acids increases steadily across succession despite marked reductions in soil temperatures (Kielland et al. 2007). This diversity of biogeochemical processes is reflected in qualitative changes of the nitrogen economy of successional vegetation.

Thus, early successional, riparian species such as willows rely to a large extent on nitrogen in the form of nitrate supplied by sub-surface water flow. Mid-successional species absorb nitrate (from river water), and ammonium and amino acids derived from soil. Late successional conifers appear to take up nitrogen in the form of amino acids and ammonium derived from soil organic matter turnover (Näsholm et al. 2009). The substantial range of biogeochemical processes controlling plant nitrogen supply, and the large variation in the forms of nitrogen taken up by boreal forest species suggests that these forests may be more resilient to disturbance, such as climate change, than has hitherto been posited.

Graph for
The production of free amino acids via proteolysis increases across succession from the warm, alkaline soils of deciduous shrub stands to the acidic, permafrost-dominated stands of black spruce.
Kielland et al. 2007.