COS 186-5 - Dynamic root:shoot modeling explains changes in salt marsh carbon storage and nitrogen cycling observed in elevated CO2 and temperature experiment

Abstract: Plant-soil feedbacks complicate predictions of the combined impact of multiple global change facets on long term C storage in terrestrial ecosystems. A primary input to soil C and N stocks is fine root growth, while plants dynamically adjust fine root growth according to soil N availability. In this work, we show how the classic "functional balance" model depicting root vs. shoot allocation explains field data from a 6-year (2017-present) salt marsh manipulation experiment where CO₂ and temperature were actively elevated. We accurately predict annual fine root production across temperature and CO₂ conditions as a function of plant N-demand and soil N-supply. We then investigate the impact of these internal plant-soil feedbacks on efforts to forecast long-term C accumulation. Here, we incorporate the functional balance model into a point-based, soil-cohort model that tracks the evolution of both C and N pools at depth in salt marsh. Our work provides the first model of marsh carbon accumulation that treats root:shoot ratios not as fixed, but instead dynamic, capturing the coupled dynamics of N cycling and C storage. This explicit C,N approach enables new insight into how long-term C storage will respond to elevated temperature and CO₂.