Urea deep placement in combination with Azolla for reducing nitrogen loss and improving fertilizer nitrogen recovery in rice field

Reducing environmental harm and improving fertilizer use efficiency is critical in Chinese agriculture. Azolla has particular value for paddy rice crop due to its biological nitrogen (N) fixation ability; however, the Chinese rice fields are highly fertilized, which will depress the N fixation of Azolla. Urea deep placement (UDP) reduces floodwater NH4+ and ammonia volatilization losses. To date, few studies have explored the effects of UDP combined with Azolla on the N uptake pattern, N recovery and N loss from 15N-labeled urea. Therefore, a 3-year field experiment was conducted with five treatments (control without urea (CK), urea broadcasting without Azolla (B) or with Azolla (BA), and UDP without Azolla (P) or with Azolla (PA)) in an intensive rice cropping system in the Taihu Region of China. The total 15N recovery (soil-plant system), crop 15N uptake (straw and grain) and 15N loss were assessed, and the ammonia (NH3) loss, rice yield, gross return and net economic benefit were measured. The results showed that the UDP treatments had delayed but long-lasting fertilization effects. The PA treatment resulted in the lowest 15N loss and highest total 15N recovery at harvest due to the prolonged N availability and negligible NH3 loss. The PA, P and BA treatments decreased 15N loss by 47%, 33% and 18% and increased total 15N recovery by 58%, 40% and 22%, respectively, compared to urea broadcasting alone. Almost all labeled Azolla-N was released within two months of which approximately 40% could be absorbed by the crop. Little labeled fertilizer N was absorbed by Azolla during the rice season. In addition, the PA, P and BA treatments increased rice yield and gross return by 13%, 10% and 9%, respectively, and they produced similar net economic benefit compared to urea broadcasting alone. In conclusion, UDP combined with Azolla is a potential strategy to further reduce fertilizer N loss and improve total fertilizer N recovery while maximizing rice yield if it can be widely adopted by Chinese rice farmers. However, widespread adoption will require improved technologies with lower labour requirement for mechanized deep placement of urea and for the propagation of Azolla.