Benefits of Controlled-Release/stable Fertilizers Plus Biochar for Rice Grain Yield and Nitrogen Utilization under Alternating Wet and Dry Irrigation

Identifying approaches to producing greater amounts of rice with higher water productivity (WP) and nitrogen use efficiency (NUE) is a major challenge for economical, environmentally friendly rice production. Despite the advantages of controlled-release N fertilizers, few studies have investigated their effects on rice agronomic performance, grain yield, NUE and WP under different irrigation regimes. In this study, a three-year field experiment was conducted in Zhejiang, South China, with two irrigation regimes (flood irrigation, FI; alternating wet and dry irrigation, AWD) and five N application methods (zero N, N0; the conventional N level, PUN100; 80 % of the conventional N level, PUN80; 80 % of the conventional N level applied as controlled-release N fertilizer and urea, plus biochar, CRFN80-BC; and 80 % of the conventional N level applied as stable compound N fertilizer and urea, plus biochar, SFN80-BC) using the indica hybrid rice cultivar Zhongzheyou 1. Compared with FI, AWD significantly increased rice yield, irrigation WP (WPi) and irrigation plus rainfall WP (WPi+r), and the values of these variables under CRFN80-BC and SFN80-BC were significantly higher than those under PUN80 and PUN100 (P < 0.05). Under AWD, compared to those under PUN100, rice grain yield, nitrogen recovery efficiency (NRE) and WPi+r under CRFN80-BC were 6.6 %, 28.5 % and 6.6 % higher, respectively, and 10.6 %, 44.6 %, and 10.6 % higher, respectively, under SFN80-BC on average over the three years. The higher rice yield was accompanied by higher crop growth rate (CGR) and grain "sink" capacity, as indicated by higher effective panicle and grain spikelet numbers and grain-filling rates. Under CRFN80-BC and SFN80-BC, AWD enhanced dry matter (DM) and N accumulation in the rice vegetative organs (functioning as "sources" for the grains) before the heading (HD) stage and the masses, ratios and contributions of DM and nonstructural carbohydrates (NSCs) transferred from the stems/sheaths and of N transferred from the leaves (as "flow") from the rice HD to maturity (MA) stages compared with those in PUN80 and PUN100. These results indicate that maintaining the source-sink-flow balance of carbohydrates and N in rice helps increase rice grain yield and NUE; the significant and positive correlations among these parameters confirm this interpretation. Overall, the results suggest that under AWD, controlledrelease/stable N fertilizers plus biochar can increase the grain yield, NUE and WP of rice.