Optimization of nitrogen rate and base and topdressing ratio to improve agronomic soil fertility and use efficiency of nitrogen in rice

Objectives Optimizing nitrogen fertilizer and topdressing ratio is an important way to reduce nitrogen fertilizer application and increase fertilizer use efficiency. In this paper, effects of different nitrogen fertilization treatments on soil fertility and nitrogen agronomic efficiency in the typical southern red paddy soil of double cropping rice system were investigated to provide a theoretical basis for sustainable and effective use of nitrogen fertilizers through a comprehensive evaluation.

Methods A 4-season and 2-year field experiment was conducted in the double cropping rice system with the typical southern red paddy soil of China from 2014 to 2015. All treatments were designed as follows: 1) No N fertilizer: T1 treatment; 2) the farmers practice,T2 treatment: N input in early and late rice was N 165 kg/hm² and N 195 kg/hm², the ratio of N in basal∶tillering∶heading stage was 60∶40∶0 and 40∶30∶30 respectively; 3) T3 treatment: the N input in early and late rice was N 135 and 165 kg/hm², the ratio of N in basal-tillering-heading stage was all 40∶30∶30, and 20% of chemical N was replaced by organic N. We measured the rice yield, rice nitrogen content, and the soil microbial biomass carbon and nitrogen, soil pH, organic matter, total N, readily available K and available phosphorus in 0–20 cm soil, calculated the cumulative nitrogen use efficiency and the agronomic efficiency of nitrogen fertilizer, and evaluated the soil fertility and fertilization modes by Nemero index and grey relational grade, respectively.

Results 1) The soil fertility results showed that the sequence from high to low of the integrated fertility indexes (IFI) values was T3 > T2 > T1, and the IFI value of the T3 treatment was 2.34% higher than that of T2 treatment. Compared with the T2 treatment, the contents of soil microbial biomass carbon and soil microbial biomass nitrogen in T3 treatment were enhanced by 4.37%–25.39% and 17.85%–29.24% separately. 2) The results of nitrogen agronomic efficiency showed that the nitrogen agronomic efficiency and apparent utilization of the T3 treatment from 2014 to 2015 were increased significantly by 29.66% and 28.82% (P < 0.05), compared with the T2 treatment. The annual rice yield of T2 and T3 treatments had no significant difference in 2014, while that of T3 treatment was improved significantly by 5.26% (P < 0.05) in 2015. The total rice yield of the T3 treatment from 2014 to 2015 was increased by 2.38% compared with the T2 treatment. 3) Based on the correlation degree analysis of the soil nutrient index, soil microbial index and nitrogen agronomy effect index, the correlation degrees of the T3 treatment in 2014 and 2015 were 0.9999 and 1.0000 respectively, which were the highest and indicated it was the best in the comprehensive evaluation of the agronomic effects on nitrogen fertilizer and the effect on soil fertility. The result showed that optimized nitrogen application rate and topdressing ratio could improve nitrogen fertilizer application and fertilizer utilization efficiency.

Conclusions The results of the 4-season and 2-year field experiment showed that 20% chemical nitrogen fertilizer less than local farmers practice and 20% nitrogen of the organic manure replaced the chemical nitrogen fertilizer and increasing the proportion of nitrogen fertilizer in heading stage could ensure the sustainability of soil fertility, nitrogen utilization efficiency and rice yield effectively.