Biochar application to soils is currently considered as a means of mitigating climate change by sequestering C, which withdraws CO2 from the atmosphere and consequently influences the trend of global climate change. However, only a few studies have been done on surface albedo variations on farmland, it might be true that the application of biochar to soil could induce a radiative forcing by changing the surface albedo. Based on this background, this study aiming at the characterization of seasonal changes in albedo on the farmland both with plants and without plants, and its effect on soil temperature, soil moisture, soil respiration and soil organic carbon fractions. There were 3 major treatments in the experiment with 3 repetition of each major treatment, namely, CK (the control treatment), BC0.5 (biochar applied at 0.5 kg·(m2·a)-1) and BC4.5 (biochar applied at 4.5 kg·(m2·a)-1). Each major treatment contained 2 sub-treatments, which were crop cultivation (+) and non-cultivation (-). The experiment results showed that in the early stage of crop growth (maize seedling stage to jointing stage, wheat seedling stage to winter period), the surface albedo of BC4.5+and BC0.5+significantly decreased compared with CK +(P<0.05). The biggest surface albedo decline rates of BC4.5 + and BC0.5 + in winter wheat season were 23.7%, 17.9% and 44.5%, 44.9% in summer maize season. The leaf area index of each treatment was remarkably correlated with the surface albedo. The shadow produced by the development of crop canopy structure would cover the change of surface color induced by the input of biochar. Under the condition of bare land, the surface albedo of biochar treatment was significantly decreased compared with the control treatment(P<0.05). Compared with the control treatments, the soil CO2 flux of the biochar treatments was significantly increased(P<0.05). With the extension of time, the growth rate of soil CO2 flux of biochar treatment was decreased gradually. The increase from BC4.5 + to CK + was gradually reduced from 276.7% to 36.1%, and the increase of BC4.5-from to CK-was significantly reduced from 163.5% to 39.8%. The increase of soil CO2 flux could be derived from the mineralization of the readily decomposed carbon fractions in the biochar-soil system. The decrease of surface albedo caused by the input of biochar had no direct effect on soil respiration, and the input of biochar could reduce the temperature sensitivity of soil respiration. This result could provide a foundation for the verification of the chemical and biological stability of biochar.
Keywords: biochar; soil organic carbon fractions; soil respiration; surface albedo; temperature sensitivity of soil respiration.