UC Berkeley

Abstract Experimental soil heating experiments have found a consistent increase in soil‐surface CO 2 emissions ( F s ), but inconsistent soil organic carbon (SOC) responses. Interpretation of heating effects is complicated by spatial heterogeneity and soil moisture, nitrogen availability, and microbial and plant responses. Here we applied a mechanistic ecosystem model to interpret heating impacts on a California forest subjected to 1 m deep, 4°C heating. The model accurately simulated control‐plot CO 2 fluxes, SOC stocks, fine root biomass, soil moisture, and soil temperature, and the observed increases in F s and decreases in fine root biomass. We show that a complex suite of interactions can lead to a consistent increase in F s (∼17%) over the 5‐year study period, with very small changes in SOC stocks (<1%). Modeled increases in leaf water stress from soil drying reduced GPP and NPP. The resulting reduction in leaf and fine root allocation increased fine root litter inputs to the soil and reduced root exudation. Soil heating led to about a 50% larger increase in root autotrophic respiration than in heterotrophic respiration, with the heating effect on both these fluxes decreasing over the simulation period. Increased heterotrophic respiration led to increased soil N availability and plant N uptake. These heating responses are mechanistically linked, of magnitudes that can affect ecosystem dynamics, and long‐term observations of them are rarely made. Therefore, we conclude that a coupled observational and mechanistic modeling framework is needed to interpret manipulation experiments, and to improve projections of climate change impacts on terrestrial ecosystem carbon dynamics. Plain Language Summary We used observations and a mechanistic ecosystem model, ecosys , to study how experimental soil warming affects the carbon cycle in a Californian forest. Soil warming is an important response to climate change and can strongly affect ecosystem carbon storage. The model reasonably captured observed surface CO 2 fluxes and vertically resolved soil moisture, temperature, carbon stocks, and fine root biomass. The model also accurately captured the effects of the imposed 4°C soil heating on surface CO 2 fluxes and decreases in fine root biomass. Soil heating dried the soil, particularly near the soil surface, leading to modeled plant water stress and reduced photosynthesis and above‐ and belowground plant growth. This study demonstrates how difficult it is to get a full picture of how warming affects forests from experiments alone—they do not cover sufficiently large areas, last long enough, or capture all the important details about plant and soil interactions. Our modeling work demonstrates the need for more detailed measurements and better models to understand and predict how climate change might alter belowground biogeochemical and plant processes, the carbon cycle, and ecosystem carbon storage. Key Points Modeling of a soil heating experiment showed increased CO 2 emissions consistent with observations, but minimal soil carbon changes Accurate simulation of observed CO 2 fluxes, soil carbon, roots, and soil moisture explained heating impacts on plant water stress Emphasizes the need to combine observational data with modeling to understand heating effects on ecosystem carbon dynamics