Since the first vascular plants appeared on land at least 430 million years ago, plant-soil feedback has started through the root-soil interface. Plant species have inherently specific and diverse root traits, but root functional and morphological plasticity is important to respond to soil changes or diversity in terms of nutrient forms and availabilities, especially in ecosystems with low plant species diversity. This paper synthesized how tree plasticity facilitates soil nutrient acquisition from the tropics to the Arctic. The fine roots of dipterocarp (Shore laevis) and rhizosphere microbes increase malate release in acidic soils for phosphorus solubilization, aluminum detoxification, and lignin degradation. The development of finer roots is a well-known strategy for the acquisition of limited nutrients, but the allocation of roots foraging “nutrient hotspots” in deeper soil is an alternative strategy. Scots pine increases the allocation of finer roots into the subsoil to solubilize P bonded to Al/Fe oxides in fine-textured podzol, but not in the coarse-textured podzol with deeper nutrient hotspots. The black spruce trees increase the biomass allocation to the belowground to acquire soil nitrogen, especially when black spruce roots absorb urea in the shallow soil on permafrost. Even in northern ecosystems with limited species diversity, a combination of functional plasticity and vertical plasticity of root system architecture facilitates soil phosphorus or nitrogen limitation.

Biomass productivity is limited by soil nitrogen in shallow active layer on permafrost. Trees and mycorrhizal roots are known to absorb amino acids to bypass slow nitrogen mineralization in nitrogen-limited soils. However, amino acid uptake strategy of tree roots cannot fully explain their advantages in competition for soil nitrogen with other plants and microbes. We demonstrated that black spruce roots have potentials to absorb intact urea in shallow soil on permafrost. Urea uptake is limited to soils with shallow permafrost, where urea accumulates due to limited microbial mineralization activity. This contrasts with soils with deep permafrost table, where roots absorb amino acids and inorganic nitrogen. Allocation of fine roots to colder subsoil above permafrost provides advantages for trees monopolizing urea-nitrogen. Despite lower energy efficiency of urea utilization compared to inorganic nitrogen and amino acids, urea uptake is one of nitrogen acquisition strategies for spruce growing on nitrogen-limited subarctic soil.

Although the amount of acidic deposition has recently decreased in Japan, acid deposition has deteriorated some forest ecosystems during the past few decades. We investigated the yearly variations in stream water chemistry for more than 20 years in two areas (Yusuhara and Taisho) in the Shmanto River Basin, southern Japan, where the effects of acidic deposition are considered to be modest. Stream water samples were collected monthly from three forest watersheds selected at each site. The annual means of the stream chemistry were predicted by multiple regression analysis. The sunlight hours were positively related with the potassium, magnesium, calcium, nitrate, sulfate, and bicarbonate concentrations in stream water. The results suggest that long sunlight hours boost the photosynthetic activities, thus promoting soil respiration and decomposition of soil organic matter; moreover, a higher carbonic acid concentration in the soil solution promotes cation weathering and carbonic acid dissociation to bicarbonate. The ammonium, nitrate, and sulfate concentrations in the bulk precipitation have decreased at Yusuhara and the sodium, magnesium, calcium, chloride, nitrate, and sulfate concentrations in the stream water have decreased in both areas. The nitrate and sulfate concentrations apparently responded to the decreasing input of acidic deposition. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previous inputs of acidic deposition enhanced the rate of rock weathering.