Post‐disturbance conifer tree‐ring δ¹⁵N reflects openness of the nitrogen cycle across temperate coastal rainforests

Abstract

Post‐disturbance losses in nitrogen (N) may diminish forest productivity, and soils with inherently ‘open’ N cycles are considered the most vulnerable to leaching losses of NO3‐. Monitoring ongoing N depletion from soil profiles is challenging, but tree‐ring δ15N of regenerating stands may offer an effective method for assessing site‐specific, long‐term soil N dynamics. Evidence to date is mixed, however, and includes increasing, unchanging or decreasing tree‐ring δ15N in young stands following stand‐level disturbances, possibly because of contrasting soil N availability among study sites. In addition, a consensus on post‐disturbance N trajectories is hampered by the inconsistent patterns in tree‐ring δ15N found between tree species of differing mycorrhizal association. We compared tree‐ring δ15N of two conifer species (Picea sitchensis with ectomycorrhizal fungi and Thuja plicata with arbuscular mycorrhiza) from a replicated silviculture trial across temperate rainforests of Vancouver Island (Canada). A natural gradient in soil N status across the six sites, driven largely by topography and parent materials, was demonstrated by in situ increases in N mineralization and nitrification rates with declining C:N ratios for both organic horizons and mineral soils. Five decades after timber harvest, the overall trend in tree‐ring δ15N was positive, indicating a loss of nitrate from the system, but among individual plots the slope of δ15N ranged from nearly 0 to 0.13. We found the gains in tree‐ring δ15N with time were consistent between mycorrhizal types and escalated (up to 6‰) with increasing N mineralization, although less so on flat terrain with seasonal water tables. The most recent sapwood was also enriched in 15N on soils with higher N mineralization rates, perhaps slightly more so for T. plicata than P. sitchensis. Synthesis. The alignment of tree‐ring δ15N with soil N cycles may be especially strong in regenerating forests because of ontogeny effects, including the expansion of rooting depth and increases in N resorption efficiency with stand age. Sharp increases in tree‐ring δ15N underscore the vulnerability of low C:N soils with open N cycles to post‐disturbance N losses, and highlight how multiple, frequent harvesting cycles may risk substantial N depletion from these productive rainforest ecosystems.

Sporocarp nutrition of ectomycorrhizal fungi indicates an important role for endemic species in a high productivity temperate rainforest

“…High rates of N mineralization are typical for fertile (low C : N) soils of perhumid rainforests (Kranabetter et al ., 2021), and consequently, the enhanced concentration of sporocarp N observed in the endemic group can likely be attributed to effective exploitation of NH 4 + . This may be achieved by differential expression and regulation of membrane proteins (Casieri et al ., 2013), a reduction in futile cycling and NH 4 + efflux from roots (Hawkins & Robbins, 2022), and unique storage and transfer mechanisms, such as intracellular vesicle packaging and diffusion, that permit high N status without toxicity (Chalot et al ., 2006).…”

“…High rates of N mineralization are typical for fertile (low C : N) soils of perhumid rainforests (Kranabetter et al, 2021), and consequently, the enhanced concentration of sporocarp N observed in the endemic group can likely be attributed to effective exploitation of NH 4 + . This may be achieved by differential expression and regulation of membrane proteins (Casieri et al, 2013), a reduction in futile cycling and NH 4…”

“…, and, to varying degrees, NO 3 À (Perakis & Sinkhorn, 2011;Kranabetter et al, 2021). With extreme rainfall, soils of the perhumid rainforest are also typically acidic and frequently wet, with widespread phosphorus (P) deficiencies due to intense podzolization that can supersede N as the primary limitation on ecosystem productivity (Kranabetter et al, 2020;Nelson et al, 2021).…”

Leaf enzyme plays a more important role in leaf nitrogen resorption efficiency than soil properties along an elevation gradient

“…Plants with higher NRE generally produce lower quality litter, which could slow down litter decomposition, N mineralization rates and the following soil N cycling processes (Aerts, 1997; Vitousek, 1982), resulting in less N losses from the ecosystem. Ecosystems with low‐fertility soils have been found to have more closed N cycle, which reinforces the competition between plants and microbes for N (Kranabetter et al, 2020). On the contrary, ecosystems with high‐fertility soils may have more open N cycle with relatively high nitrification and denitrification rates and relatively large fluxes of N leaching and N gaseous losses (Högberg et al, 2017; Schimel & Bennett, 2004).…”

“…The magnitude of N resorption fluxes not only influences plant nutrients (Zhang et al, 2020), but also affects N losses from the ecosystem (Kranabetter et al, 2020), and then the eco-environmental problems caused by excessive N leaching or emission of reactive N gases. The openness of N cycling is defined as the relative importance of internal N cycling compared to the fluxes of N inputs and outputs (Austin & Vitousek, 1998).…”