Effects of biochar on spatial and temporal changes in soil temperature in cold waterlogged rice paddies

Soil temperature is an important factor constraining plant growth, large fluctuations of which generally result in low crop yield. However, the investigation of the effect of biochar on soil temperature, especially in cold waterlogged paddies, has been limited. This study investigated the effect of bamboo biochar (BB), rice straw biochar (RB), and rice straw (RS) on temporal variations in soil temperature at different depths, compared with no exogenous organic matter amendment soil as control treatment (CK), in a cold waterlogged paddy in Zhejiang Province, China. A logger with probes was used to record soil temperatures at the depths of 5 cm, 10 cm, and 20 cm at 08:00 h, 14:00 h, and 22:00 h daily from July 1 to Oct 31, 2012. Soil physicochemical properties and rice yield were analyzed after the harvest. The results showed that the application of BB and RB significantly reduced the soil temperature at 5- and 10-cm depths by 0.11 °C-0.21 °C on average, whereas RS significantly reduced soil temperature at a 5-cm depth by 0.19 °C and increased the average temperature at a 20-cm depth by 0.06 °C. Moreover, the average soil temperature related to BB, RB, and RS treatment significantly increased at 08:00 h during the rice-growing season by 0.15 °C, 0.43 °C, and 0.52 °C and significantly decreased at 14:00 h by 0.51 °C, 0.73 °C, and 0.87 °C, respectively, as compared with that in CK. Overall, both biochar and RS reduced the differences between day and night soil temperatures by 0.66 °C-1.39 °C, thereby regulating diurnal soil-temperature fluctuations, especially at a 5-cm depth. The reduced differences were likely attributed to the relatively lower bulk density (8.87-17.1%) and higher water content (9.75-14.3%) in the biochar-amended soil as compared with that in control soil. Furthermore, RB had a significant stronger positive effect on grain yield than BB, because it regulated soil temperature more effectively. These results suggested the feasibility of applying RB to cold waterlogged paddies to improve soil properties and crop yield.