Objectives The combined application of organic and inorganic fertilizers can improve crop yield, nutrient utilization rate and soil fertility. Therefore, the systematic study on the variation characteristics of soil nutrient content and nutrient stoichiometric ratio under different organic substitution ratios can provide a theoretical basis for the efficient replacement of chemical fertilizers by organic fertilizers.

Methods In 2017, an organic substitution positioning test was carried out in Gao an City, Jiangxi Province. The test set up six treatments: no fertilization control (CK), single application of nitrogen, phosphorus and potassium fertilizers (CF), and organic nitrogen substitution of 25% (25%OM), 50% (50%OM), 75% (75%OM), and 100% (100%OM) chemical fertilizer nitrogen. After the harvest of early rice in 2023 and 2024, the topsoil was collected to determine the total and effective nutrient content, microbial biomass carbon, nitrogen and phosphorus content, and the key extracellular enzyme activity of carbon, nitrogen and phosphorus nutrient cycling. At the same time, the yield, biomass and nutrient content of early rice were measured, the nutrient utilization rate of crops was calculated, and the regulation of stoichiometric characteristics on the yield of early rice was analyzed.

Results Compared with CF treatment, there was no significant difference in early rice yield, aboveground nutrient uptake, nitrogen and phosphorus nutrient recovery, agronomic efficiency and partial productivity between 25%OM and 50%OM treatments. The contents of easily oxidizable organic carbon (EOC), mineral nitrogen (Nmin), available phosphorus (AP) and available potassium (AK) in soil at maturity stage were significantly increased, and EOC:AP and Nmin:AP were decreased. Partial least squares equation model analysis showed that soil nutrients (Nmin, AP, EOC) had a significant negative impact on early rice yield (P<0.01), while biological characteristics (leucine aminopeptidase activity LAP, microbial mass nitrogen MBN, microbial mass phosphorus MBP), stoichiometric ratio (soil total nitrogen to total phosphorus ratio SOC∶TP, organic carbon to total phosphorus ratio SOC∶TN, organic carbon to total nitrogen ratio SOC∶TN, enzyme activity carbon-phosphorus ratio EC∶P, microbial mass nitrogen-phosphorus ratio MBN∶MBP) had a significant positive effect on early rice yield (P<0.01).

Conclusions Under long-term organic substitution, maintaining the organic substitution ratio of no more than 50% can effectively stabilize yield and improve soil fertility. Conversely, too high proportion (>50%) tends to impede nutrient release and subsequently diminish yields. Nutrient stoichiometry, particularly those related to phosphorus, exhibits a significant positive effect on early rice yield. On the other hand, soil nutrients such as Nmin, AP, and EOC demonstrate a significant negative impact on early rice yield. Therefore, when implementing organic substitution in red paddy soil for early rice cultivation, it is crucial not only to maintain an appropriate ratio but also to carefully consider the effects of soil nutrient levels and phosphorus-related nutrient stoichiometric ratios on crop yield.