Objectives We explored the effects of organic fertilizer proportions in total N input on the rice yield and soil fertility in short-term, in order to propose a reasonable organic fertilizer ratio for improving soil fertility and rice yield simultaneously.

Methods A localized field experiment was conducted successively for five years from 2017 to 2021 under double rice systems in Yichun City, Jiangxi Province. The experimental scheme was composed of no fertilizer control (CK), sole chemical NPK fertilizers (CF), organic N (pig manure) replacing 25%, 50%, 75% and 100% of chemical N input (denoted as 25%ON, 50%ON, 75%ON, and 100%ON), all the fertilizer treatments had the same total N, P and K input. The grain yields of early and late rice were recorded every year at harvest. At harvesting of late rice in 2021, tillage layer soil samples were collected for the determination of 15 physical, chemical properties, and biological properties indicators. The soil integrated fertility index (IFI) of treatment was calculated and the importance of organic and chemical fertilizer on IFI and yield was assessed by partial least squares path model (PLS-PM).

Results Compared to CF treatment, organic fertilizer promoted the soil physical, chemical and biological properties, and the effect was enhanced with the increasing of organic N proportion. Soil organic matter and β-glucosidase activity were significantly correlated with the soil chemical and physical indicators. Among the tested indicators, total and available P content had the highest variable coefficient values, showing their sensitivities to the input of organic fertilizer. All the fertilization treatments improved the IFIs, and the four organic replacement treatments did better than CF treatment. The IFIs increased with the enhancing of organic N proportion, while the grain yield effect was on the contrary. The 25%ON and 50%ON treatment were recorded similar annual yield with CF, while 75%ON and 100%ON treatments were reordered significantly lower annual yield. PLS-PM results showed that both chemical and organic fertilizers had significant effect on the IFIs, and the effect of organic fertilizer was higher than that of chemical fertilizer; organic fertilizer had no significant effect on rice yield, while inorganic fertilizer had significant effect.

Conclusions Chemical fertilizer plays a decisive role for maintenance of rice yield, but limited effect on soil fertility. Whereas organic fertilizer shows no obvious effect on rice yield in short term, but contributes greatly to the improvement of soil fertility. The proportion of organic fertilizer nitrogen should be controlled within 50% for maintaining high rice yield and improving soil fertility simultaneously.