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

•Measurement of mass flow of soil N fluxes was done in undisturbed boreal forest soil.•Amino acids dominated soil N diffusive fluxes.•In presence of mass flow N flux increased by up to nine times, mainly due to nitrate.•Mass flow, and hence transpiration, may be a strong driver of plant N acquisition.•These results points to a strong interaction between water and N availabilities.

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 NO3− 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.