Could continuous rice cropping increase soil fertility and rice productivity by rice straw carbonized utilization in cold areas? - A 6-year field-located trial

Biochar amendment can benefit rice growth, but the long-term effects of rice straw carbonized utilization (RSCU, biochar, and biochar-based fertilizer) on rice production in cold areas are still unclear. Herein, we conducted a field experiment over 6 years with four treatments: F (conventional fertilization) as the control, RB1 (biochar, 3 t·ha^-1), RB2 (biochar, 6 t·ha^-1), and RBF (biochar-based fertilizer, 0.75 t·ha^-1). We found that rice straw biochar significantly improved soil physical properties by reducing soil bulk density, increasing soil porosity and liquid and gas phases ratio, and enhancing soil aggregate stability. RSCU also increased soil fertility by improving soil organic carbon (SOC), active organic carbon, and soil nutrients (N, P, K) and their availability, as indicated by an increase in soil C:N and a decrease in soil N:P. Moreover, biochar increased soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and enzyme activities. As a result, RSCU increased rice yield, which was positively correlated with soil total porosity, total phosphorus, available potassium, dissolved organic carbon (DOC), easily oxidizable carbon (EOC), labile fraction of organic carbon (LFOC), and urease activity. RB2 had the highest rice yield (5.94% higher than F). Our study suggests that RSCU can synergistically improve the rice straw utilization rate, soil fertility, and rice productivity in cold areas.