Impact of diffuse radiation on the coupling of carbon and water fluxes in the grassland of northeastern China

Abstract

Solar radiation is a key driver of ecosystem carbon and water fluxes. However, the impacts of radiation quantity and quality on the carbon-water coupling are not well distinguished. In this study, we used simultaneous flux and radiation measurements at two grassland sites in northeastern China to explore the joint impacts of photosynthetically active radiation (PAR) and diffuse fraction (Kd) on carbon and water fluxes and their coupling relationships. Under the low to medium PAR levels (<280 W m−2), gross primary productivity (GPP) and evapotranspiration (ET) increased continuously with Kd but the sensitivity of GPP (8.4%-8.8% per 0.1 increase in Kd) was significantly higher than that of ET (2.2%-5.0% per 0.1 increase in Kd) at both sites. Under the high PAR levels (>280 W m−2), the GPP continued to grow at the southern site but showed limited responses to Kd at the northern site, likely due to the temperature constraint in the latter. Meanwhile, the contribution of evaporation to ET increased under the high radiation conditions, resulting in a decreased ET due to the reduced direct radiation following Kd increment at both sites. Consequently, water use efficiency (WUE) increased with Kd for all radiation levels but showed low sensitivity to PAR changes due to the synchronized GPP and ET responses to PAR. This study unraveled the positive dependence of ecosystem WUE on the increased Kd though with varied sensitivities of GPP and ET under different PAR levels, highlighting the strong impacts of diffuse radiation on ecosystem fluxes over the regions with aerosol pollution and cloud variations.