Components explain, but do eddy fluxes constrain? Carbon budget of a nitrogen‐fertilized boreal Scots pine forest

Abstract

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 enduring mystery of differences between eddy covariance and biometric measurements for ecosystem respiration and net carbon storage in forests

“…Reference stand of Scots pine ( Pinus sylvestris L.) trees from the Marshall et al . (2023; 2166–2179), a study recently published in New Phytologist , was c . 100 yr of age located at Rosinedalsheden c .…”

“…Reference (R) and Fertilized (F) are the NEP EC vs biometric comparison from the study by Marshall et al . (2023; 2166–2179) published in this issue of New Phytologist .…”

“…This article is a Commentary on Marshall et al (2023), 239: 2166-2179. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.…”

“…(b) Data fromCampioli et al (2016) for NEP only plotted with a best-fit linear model (colored line), and 95% confidence intervals (colored, shaded area) and 1 : 1 (dashed black line) showing that EC estimates of NEP are higher than biometric estimates. Reference (R) and Fertilized (F) are the NEP EC vs biometric comparison from the study byMarshall et al (2023; 2166-2179 published in this issue of New Phytologist.…”

“…EC estimates of ecosystem respiration (R ECO ) are typically lower than biometric-based estimates, while annual net C storage values (net ecosystem production, NEP) are typically higher (Speckman et al, 2015;Campioli et al, 2016). In an article published in this issue of New Phytologist, Marshall et al (2023;2166-2179 compared C flux and storage estimates from EC and biometric measurements at two simple forest ecosystem sites in Sweden, one fertilized with nitrogen (N) and one not. The simple forests with a single overstory species, low soil organic C, and sparse understory (Fig.…”