Effects of diffuse radiation fraction on crop light absorption, light use efficiency and gross primary production on an instantaneous scale in South China.

The fraction of absorbed photosynthetically active radiation (FAPAR), light use efficiency (LUE), and gross primary productivity (GPP) are the key driving factors of crop production and ecological models. Diffuse radiation fraction (DF) has been reported to profoundly affect FAPAR, LUE and GPP, and its impact on a short time scale needs to be emphasized. Based on the field observations at noon local time during 2021-2022 and the Two-Leaf light use efficiency model, this study investigated the magnitudes of the DF effect on the canopy FAPAR, LUE, and GPP for the three different crops (peanut, soybean and corn) on an instantaneous scale in South China. Different from that of peanut and soybean, the FAPAR of corn increased linearly with the rise of DF. The instantaneous LUE of each crop was highly sensitive to DF, and its linear regression slope was greater than 1.0 g C MJ. On average, the DF accounted for around 69-74% of the variations in the instantaneous LUE and 59-64% of the variations in the instantaneous GPP over the entire observation period. The sky conditions with a DF value between 0.45 and 0.66 were favorable for the carbon fixation of the three crops. The linear coupling strength between GPP and PAR under diffuse radiation (DF ≥ 0.5) was stronger than that under direct radiation (DF < 0.5). The results will be helpful in accurate estimating of FAPAR, LUE, GPP and even crop production in both South China and other similar regions.