Responses of gross primary productivity to diffuse radiation at global FLUXNET sites

Diffuse radiation can promote ecosystem gross primary productivity (GPP) and strengthen land carbon sink. However, measurements of diffuse radiation at carbon flux sites are rare, limiting the explorations of diffuse fertilization efficiency (DFE), the percentage changes of GPP per unit diffuse photosynthetically active radiation (PAR). Here, we derive diffuse radiative fraction (Kd) at >200 FLUXNET sites based on an artificial neural network (ANN) and estimate DFE at individual sites around the world. Evaluations show that the ANN on average increases modeling-to-observation correlation coefficients and the Willmott index of Kd by 8% and reduces root-mean-square error by 14.7% compared to 11 empirical models. High Kd of 0.6–0.82 is located at mid-high latitudes and relatively low Kd of 0.46–0.58 is found at subtropical regions. The optimal Kd leading to maximum GPP is estimated to be 0.46 averaged for all FLUXNET sites. In response to 1 W m−2 enhancement of diffuse PAR, daytime GPP increases by 0.44–1.01% for tree species and 0.16–0.94% for non-tree species. Such GPP enhancement by diffuse radiation is on average 2.5 times of that by the same amount of direct radiation.