Summary: Understanding how plant water uptake interacts with acquisition of soil nitrogen (N) and other nutrients is fundamental for predicting plant responses to a changing environment, but it is an area where models disagree. We present a novel isotopic labelling approach which reveals spatial patterns of water and N uptake, and their interaction, by trees. The stable isotopes¹⁵N and²H were applied to a small area of the forest floor in stands with high and low soil N availability. Uptake by surrounding trees was measured. The sensitivity of N acquisition to water uptake was quantified by statistical modelling. Trees in the high‐N stand acquired twice as much¹⁵N as in the low‐N stand and around half of their N uptake was dependent on water uptake (²H enrichment). By contrast, in the low‐N stand there was no positive effect of water uptake on N uptake. We conclude that tree N acquisition was only marginally dependent on water flux toward the root surface under low‐N conditions whereas under high‐N conditions, the water‐associated N uptake was substantial. The results suggest a fundamental shift in N acquisition strategy under high‐N conditions.

Summary: Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses. We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance. N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20×). Only fine‐root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m⁻² yr⁻¹. Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon‐use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m⁻² yr⁻¹; P < 0.01) that eddy covariance did not (19 (62) g C m⁻² yr⁻¹; ns). The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.

The new European Union Forest Strategy for 2030 aims to plant an additional 3 billion trees on non-forest land to mitigate climate change. However, the choice of tree species for afforestation to achieve the maximum climate benefit is unclear. We compared the climate benefit of six different species in terms of carbon (C) sequestration in biomass and the harvested wood substitution in products to avoid carbon dioxide (CO₂) emissions from fossil-based materials over the 100-year period by afforesting about ¼ of the available area in northern Europe. The highest climate benefit was observed for larch, both at a stand scale (1626 Mg CO₂ eqv. ha⁻¹) and at the landscape level for the studied scenario (579 million Mg CO₂ eqv.). Larch was followed by Norway spruce, poplar, hybrid aspen and birch, showing a climate benefit about 40–50% lower than that for larch. The climate benefit of willow was about 70% lower than larch. Willow showed 6–14-fold lower C stocks at the landscape level after 100 years than other tree species. The major climate benefit over the 100-year period comes from wood substitution and avoided emissions, but C stock buildup at the landscape level also removes significant amounts of CO₂ already present in the atmosphere. The choice of tree species is important to maximize climate change mitigation.

Boreal trees are capable of taking up organic nitrogen (N) as effectively as inorganic N. Depending on the abundance of soil N forms, plants may adjust physiological and morphological traits to optimize N uptake. However, the link between these traits and N uptake in response to soil N sources is poorly understood. We examined Pinus sylvestris L. seedlings’ biomass growth and allocation, transpiration and N uptake in response to additions of organic N (the amino acid arginine) or inorganic N (ammonium nitrate). We also monitored in situ soil N fluxes in the pots following an addition of N, using a microdialysis system. Supplying organic N resulted in a stable soil N flux, whereas the inorganic N resulted in a sharp increase of nitrate flux followed by a rapid decline, demonstrating a fluctuating N supply and a risk for loss of nitrate from the growth medium. Seedlings supplied with organic N achieved a greater biomass with a higher N content, thus reaching a higher N recovery compared with those supplied inorganic N. In spite of a higher N concentration in organic N seedlings, root-to-shoot ratio and transpiration per unit leaf area were similar to those of inorganic N seedlings. We conclude that enhanced seedlings’ nutrition and growth under the organic N source may be attributed to a stable supply of N, owing to a strong retention rate in the soil medium.

Active hydroponic substrates that stimulate on demand the plant growth have not been demonstrated so far. Here, we developed the eSoil, a low-power bioelectronic growth scaffold that can provide electrical stimulation to the plants’ root system and growth environment in hydroponics settings. eSoil’s active material is an organic mixed ionic electronic conductor while its main structural component is cellulose, the most abundant biopolymer. We demonstrate that barley seedlings that are widely used for fodder grow within the eSoil with the root system integrated within its porous matrix. Simply by polarizing the eSoil, seedling growth is accelerated resulting in increase of dry weight on average by 50% after 15 d of growth. The effect is evident both on root and shoot development and occurs during the growth period after the stimulation. The stimulated plants reduce and assimilate NO ₃ ⁻ more efficiently than controls, a finding that may have implications on minimizing fertilizer use. However, more studies are required to provide a mechanistic understanding of the physical and biological processes involved. eSoil opens the pathway for the development of active hydroponic scaffolds that may increase crop yield in a sustainable manner.