Effects of postponing nitrogen application on rice and carp growth and paddy field water quality in rice-fish co-culture system

Objective This study aims to explore the effects of postponing nitrogen fertilization in the rice-fish co-culture system on rice (Oryza sativa) yield and grain quality, nitrogen utilization rate, fish survival rate, fish yield, fish meat quality, as well as paddy field water quality. This research would provide a theoretical basis for stabilizing rice and fish yields and improving nitrogen utilization efficiency in rice fields under the rice-fish co-culture system.

Method The study used hybrid rice ‘Taifengyou 208’, conventional rice ‘19 Xiang’ and Cyprinus carpio variety ‘Ruyuanshili 1’ as experimental materials. Based on a total nitrogen application rate of 150 kg·hm⁻² for early season and 180 kg·hm⁻² for late season, three experimental treatments were designed: No nitrogen fertilizer application (M0), one-time basal nitrogen fertilizer application (M1, the top dressing fertilizer was not added), and postponing nitrogen application (M2, the mass ratio of basal fertilizer and top dressing fertilizer was 7∶3).

Result Compared to the M1 treatment, M2 treatment was more beneficial for improving the rice tiller numbers, leaf SPAD values and nitrogen absorption and utilization efficiency (RE_N) at heading stage. The RE_N of M2 treatment increased 25.82%−91.11% and 62.74%−67.35% in early and late rice growing seasons respectively compared to M1. There was no significant difference in milling quality and appearance quality of rice between two nitrogen application treatments. M2 treatment significantly increased the paddy water electrical conductivity, total nitrogen, ammonia nitrogen, and nitrate nitrogen contents at rice panicle initial stage (P < 0.05), while significantly reduced the crude fat content in carp meat (P < 0.05). However, it had no significant effect on the survival rate or yield of field fish.

Conclusion In the rice-fish co-culture system, postponing nitrogen application can stabilize rice and carp yield, improve nitrogen use efficiency, and reduce crude fat content of carp meat, with no significant effect on rice quality or the protein and total amino acid contents of carp meat.