Soil carbon, nitrogen and phosphorus stoichiometry (C : N : P) in relation to conifer species productivity and nutrition across British Columbia perhumid rainforests

Abstract

<p><strong>Abstract.</strong> Temperate rainforest soils of the Pacific Northwest are often carbon (C) rich and encompass a wide range in fertility reflecting varying nitrogen (N) and phosphorus (P) availability. Soil resource stoichiometry (C : N : P) may provide an effective measure of site nutrient status and help refine species-dependent patterns in forest productivity across edaphic gradients. We described the nature of soil organic matter for mineral soil and forest floor substrates across very wet (perhumid) rainforest sites of southwestern Vancouver Island (Canada), and employed soil element ratios as covariates in a long-term planting density trial to test their utility in defining basal area growth response of four conifer species. There were strong positive correlations in mineral soil C, N and organic P (Po) concentrations, and close alignment in C : N and C : Po both among and between substrates. Stand basal area after five decades was best reflected by soil C : N but included a significant species-soil interaction. The conifers with ectomycorrhizal fungi had diverging growth responses displaying either competitive (<i>Picea sitchensis</i>) or stress-tolerant (<i>Tsuga heterophylla</i>, <i>Pseudotsuga menziesii</i>) attributes, in contrast to a more generalist response by an arbuscular mycorrhizal tree (<i>Thuja plicata</i>). Despite the consistent patterns in organic matter quality we found no evidence via foliar nutrition for increased P availability with declining element ratios as we did for N. The often high C : Po ratios (as much as 3000) of these soils may reflect a stronger immobilization sink for P than N, which, along with ongoing sorption of PO<sub>4</sub><sup>−</sup>, could limit the utility of C : Po or N : Po to adequately reflect P supply. The dynamics and availability of soil P to trees, particularly as Po, deserves greater attention as many perhumid rainforests were co-limited by N and P, or, in some stands, possibly P alone.</p>

Contrasting conifer species productivity in relation to soil carbon, nitrogen and phosphorus stoichiometry of British Columbia perhumid rainforests

“…Data availability. All data related to the study are archived with Dryad at https://doi.org/10.5061/dryad.0gb5mkkwr (Kranabetter et al, 2019b).…”