Informing Nature-based Climate Solutions for the United States with the best-available science.

Nature-based Climate Solutions (NbCS) are managed alterations to ecosystems designed to increase carbon sequestration or reduce greenhouse gas emissions. While they have growing public and private support, the realizable benefits and unintended consequences of NbCS are not well understood. At regional scales where policy decisions are often made, NbCS benefits are estimated from soil and tree survey data that can miss important carbon sources and sinks within an ecosystem, and do not reveal the biophysical impacts of NbCS for local water and energy cycles. The only direct observations of ecosystem-scale carbon fluxes, for example, by eddy covariance flux towers, have not yet been systematically assessed for what they can tell us about NbCS potentials, and state-of-the-art remote sensing products and land-surface models are not yet being widely used to inform NbCS policymaking or implementation. As a result, there is a critical mismatch between the point- and tree-scale data most often used to assess NbCS benefits and impacts, the ecosystem and landscape scales where NbCS projects are implemented, and the regional to continental scales most relevant to policymaking. Here, we propose a research agenda to confront these gaps using data and tools that have long been used to understand the mechanisms driving ecosystem carbon and energy cycling, but have not yet been widely applied to NbCS. We outline steps for creating robust NbCS assessments at both local to regional scales that are informed by ecosystem-scale observations, and which consider concurrent biophysical impacts, future climate feedbacks, and the need for equitable and inclusive NbCS implementation strategies. We contend that these research goals can largely be accomplished by shifting the scales at which pre-existing tools are applied and blended together, although we also highlight some opportunities for more radical shifts in approach.

Integration of C1 and C2 Metabolism in Trees.

C1 metabolism in plants is known to be involved in photorespiration, nitrogen and amino acid metabolism, as well as methylation and biosynthesis of metabolites and biopolymers. Although the flux of carbon through the C1 pathway is thought to be large, its intermediates are difficult to measure and relatively little is known about this potentially ubiquitous pathway. In this study, we evaluated the C1 pathway and its integration with the central metabolism using aqueous solutions of 13C-labeled C1 and C2 intermediates delivered to branches of the tropical species Inga edulis via the transpiration stream. Delivery of [13C]methanol and [13C]formaldehyde rapidly stimulated leaf emissions of [13C]methanol, [13C]formaldehyde, [13C]formic acid, and 13CO2, confirming the existence of the C1 pathway and rapid interconversion between methanol and formaldehyde. However, while [13C]formate solutions stimulated emissions of 13CO2, emissions of [13C]methanol or [13C]formaldehyde were not detected, suggesting that once oxidation to formate occurs it is rapidly oxidized to CO2 within chloroplasts. 13C-labeling of isoprene, a known photosynthetic product, was linearly related to 13CO2 across C1 and C2 ([13C2]acetate and [2-13C]glycine) substrates, consistent with reassimilation of C1, respiratory, and photorespiratory CO2. Moreover, [13C]methanol and [13C]formaldehyde induced a quantitative labeling of both carbon atoms of acetic acid emissions, possibly through the rapid turnover of the chloroplastic acetyl-CoA pool via glycolate oxidation. The results support a role of the C1 pathway to provide an alternative carbon source for glycine methylation in photorespiration, enhance CO2 concentrations within chloroplasts, and produce key C2 intermediates (e.g., acetyl-CoA) central to anabolic and catabolic metabolism.