Effect of long-term reduction and deep placement of nitrogen fertilizer on rice yield and soil fertility in a double rice cropping system

Objectives Appropriate reduction of nitrogen fertilization is an important way to deal with serious nitrogen loss and low fertilizer-nitrogen use efficiency in paddy field ecosystem in China, but it is not clear whether and how much of long-term reduction of nitrogen fertilization could maintain rice yield and soil fertility. In this study, we analyzed the yearly variation of double-cropping rice yield, partial factor productivity of nitrogen fertilizer, nutrient contents in rhizosphere soil and soil fertility to discuss the feasibility of long-term nitrogen fertilizer reduction and then provide scientific basis for appropriate nitrogen fertilizer management in a double cropping rice system.

Methods A long-term field experiment of reduction and deep placement of nitrogen fertilizer was conducted in Taoyuan Agro-Ecology Research Station of Chinese Academy of Sciences since 2012. The conventional fertilization was used as control (CF, N 150 kg/hm² in both early and late rice season, using surface broadcast method for the basal and tillering fertilizer), three reduced N application treatments (reducing 30%, 23%, and 16% of the CF, respectively) were setup and most nitrogen fertilizers were made into a ball and dropped into 7 cm deep beside rice root. Grain yields of early and late rice were recorded every year, and the rhizosphere soil was sampled during rice growing stages for measuring organic matter and total and available nutrient contents.

Results Compared with CF, reducing 16%–30% of total N input had no significant effect on the contents of inorganic N and readily available K, but reducing 23% and 30% of total N input significantly decreased available P in rhizosphere soil in the tillering stage of early rice. The NH₄⁺-N and NO₃^–-N contents in rhizosphere soil in the jointing and maturity stages of double cropping rice were increased by 4.26%–109.00% and 2.56%–65.50%, respectively, and the available P and K contents by 3.10%–32.60% and 5.94%–42.40%, respectively, so the supply of NPK nutrition during the two stages of rice growing were stable under the three N-reducing treatments. Based on deep placement of chemical fertilizers, that reduction of nitrogen fertilizer rate with 16%–30% for continuous seven years enhanced partial factor productivity of applied nitrogen, and increased the yields of early and late rice by 4.37%–32.70%, and maintained the stability of soil organic matter and total N, P and K contents.

Conclusions Combined with deep placement, the available nutrient contents in the rhizosphere soil of double rice are higher under reduced N input condition than those under high input but with broadcasting application method. So, continuously reducing 30% of current N fertilizer input will still be able to maintain rice yield and soil fertility of double cropping paddy field in the long run, and increase the partial productivity of nitrogen fertilizers.