Relationships between radiation quality and ecosystem carbon and water fluxes

Both the quantity and quality of radiation could be altered by aerosols and cloud, and thus changing ecosystem carbon and water fluxes, as well as their coupling relationship. Although the importance of radiation quantity to ecosystems has been well studied, how radiation quality (characterized by diffuse radiation fraction, the proportion of diffuse to total radiation, K_d) affects ecosystem carbon and water cycles remains unclear due to the lack of diffuse radiation observations. In this work, taking advantage of a newly derived K_d dataset, we comprehensively explored the impact of K_d on ecosystem carbon uptake (net ecosystem exchange, NEE), carbon flux (gross primary production, GPP), water flux (evapotranspiration, ET), and water-use efficiency (GPP/ET, WUE) based on measurements at 201 global FLUXNET sites. We found that diffuse radiation is more efficient to improve NEE, GPP and ET than direct radiation at the same radiation level for all vegetation types, leading to increased ecosystem carbon uptake and water loss with K_d at the low and middle K_d levels. Furthermore, the enhancement of GPP is stronger than ET, leading to improved ecosystem WUE by diffuse radiation. By separating radiation into diffuse and direct components, we found that diffuse radiation is the most important factor for GPP, while ET is more dominated by direct radiation, indicating the complex process underlying the response of WUE to changes in K_d. This research helps improve our understandings of the responses of ecosystem carbon and water cycle to aerosol/cloud changes.