Microalgae-derived hydrochar application on rice paddy soil: Higher rice yield but increased gaseous nitrogen loss

Hydrothermal carbonization represents a promising technique for transforming microalgae into the hydrochar with abundant phytoavailable nutrients. However, the effects of microalgae-derived hydrochars on the gaseous nitrogen (N) loss from agricultural field are still unclear. Chlorella vulgaris powder (CVP) and two Chlorella vulgaris-derived hydrochars that employ water (CVHW) or citrate acid solution (CVHCA) as the reaction medium were applied to a soil column system grown with rice. The temporal variations of nitrous oxide (N₂O) emissions and ammonia (NH₃) volatilization were monitored during the whole rice-growing season. Results showed that CVHW and CVHCA addition significantly increased the grain yield (by 13.5-26.8% and 10.5-23.4%) compared with control and CVP group, while concomitantly increasing the ammonia volatilization (by 53.8% and 72.9%) as well as N₂O emissions (by 2.17- and 2.82-fold) from paddy soil compared to control. The microbial functional genes (AOA, AOB, nirk, nirS, nosZ) in soil indicated that CVHW and CVHCA treatment stimulated the nitrification and denitrification, and inhibited the N₂O oxidation in soil. Notably, CVHW was recommended in the view of improving yield and controlling NH₃ volatilization because no significant difference of the yield-scale NH₃ volatilization was detected between control and CVHW treatment. This study for the first time uncovered that Chlorella vulgaris-derived hydrochars have positive effects on rice N utilization and growth but negative effects on the atmospheric environment.