Assessing the Potential Impact of Hydraulic Redistribution on Ecosystem Biogeochemical Cycles

Hydraulic redistribution (HR) of water from moist to drier soils, through plant roots, occurs world-wide in seasonally dry ecosystems, with significant impact on landscape hydrology and plant water use documented in both observational and numerical modeling studies. However, HR's effects on microbe-controlled processes sensitive to soil moisture, and thus carbon and nutrient cycling at ecosystem scales, have not been adequately explored and are the focus of this study. We first incorporated HR into the Community Land Model (CLM4.5) to assess its impact on the model performance in simulating water, energy, and system-scale carbon fluxes based on comparison with data from eddy covariance flux towers at temperate and tropical AmeriFlux sites. We then investigate the impact of HR on modeled plant productivity and microbial N-cycling activities, and assess the contribution of the photosynthetic and microbial pathways to the HR impact on ecosystem carbon cycle. Overall, inclusion of HR tends to increase modeled annual ecosystem uptake of CO₂ (or reduce annual CO₂ release to the atmosphere), and the photosynthetic pathway plays a generally more important role than the microbial pathway. Moreover, through enhancing surface and root zone soil moisture, HR is found to suppress fire spread in the model and reduce fire-induced CO₂ emissions. Making use of numerical model and site observational data, this study highlight the important role of coupling between the ecosystem water cycle and biogeochemical cycle.