Incorporating mass flow strongly promotes N flux rates in boreal forest soils

Large differences in productivity and species composition are characteristic for the boreal forest and nitrogen (N) availability has been deemed the proximate cause of this variation. We used a modified microdialysis technique to assess N availability through monitoring in situ inorganic and organic soil N fluxes in the presence and absence of mass flow in two forest ecosystems of contrasting fertility, a nutrient rich Norway spruce forest and a nutrient poor Scots pine forest. This was enabled by using solutions of different osmotic potentials as perfusates. In the absence of mass flow, amino acids dominated soil N fluxes of both ecosystems representing 62 and 82% of total flux in the nutrient rich and the nutrient poor ecosystem respectively. In the presence of mass flow, N flux increased by nine times in the nutrient rich and four times in the nutrient poor soil and nitrate comprised a greater share of total N flux. Our results suggest that mass flow may be a strong driver for plant N acquisition in boreal forests through delivering higher amounts of amino acids and NOT to plant roots and mycorrhizas. These results points to a strong interaction between water and N availabilities, the former enhancing the supply of the latter through enabling high rates of transpiration. (C) 2017 The Authors. Published by Elsevier Ltd.