Nitrogen additions modify the nursing effect of Scots pine on Sitka spruce via plant-soil feedback

Zhou, Y. and Tao, T. and Cox, F. and Johnson, D. (2026) Nitrogen additions modify the nursing effect of Scots pine on Sitka spruce via plant-soil feedback. Forest Ecosystems, 16: 100485.

Abstract

Nitrogen availability is thought to be a key factor that modifies the strength and direction of plant-soil feedback (PSF), but how nitrogen availability and PSF interact in forests remains unresolved. For example, PSF is one of the drivers underpinning the well-known nursing effect in the Atlantic forests of the UK and Ireland. In these systems, the nursing effect occurs when Scots pine (Pinus sylvestris L.) facilitates the establishment of economically valuable Sitka spruce (Picea sitchensis (Bong.) Carr). However, this effect has primarily been observed in nitrogen-limited environments. To isolate the PSF-driven component of the nursing effect, we used a two-phase PSF experiment and tested whether nitrogen additions diminish biomass differences among Sitka spruce grown in soils conditioned solely by Scots pine and Sitka spruce, and a soil that was a mix of both conditioning treatments. Our findings show that increasing nitrogen may promote Sitka spruce growth via microbial-mediated pathways (such as enhanced ectomycorrhizal (ECM) colonization and indirectly altered inorganic nitrogen availability), while inhibiting growth via inorganic nitrogen-mediated pathways (such as nutrient limitation). Thus, with increasing nitrogen additions, Sitka spruce biomass in conspecific soil increased to become similar to that in heterospecific soils, thereby weakening the PSF-driven nursing effect. Additionally, nitrogen additions shifted Sitka spruce in conspecific soil toward a faster root nutrient-acquisition strategy, potentially enhancing Sitka spruce nitrogen uptake and reducing biomass differences among soil types. Finally, Scots pine exerted a disproportionate influence in the mixed soil, as plant and soil responses more closely resembled those in Scots pine-conditioned soil than in Sitka spruce-conditioned soil. Our study highlights the interaction between soil microbial properties and nitrogen availability in shaping plant-soil feedback during early stages of tree establishment. These findings suggest matching forest planting to soil nitrogen availability, using nursing mixtures in low-nitrogen soils and Sitka spruce monocultures in high-nitrogen soils.