Objectives The combined application of organic and inorganic fertilizers can improve crop yield, nutrient utilization efficiency and soil fertility, while the non-reasonable combination of organic and inorganic fertilizers can cause the high negative effects on yield or environment. In order to the synergistic relationship of crop nutrients demand and soil nutrients change under the organic and inorganic fertilizers combination, 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 with organic fertilizers.

Methods In 2017, an organic substitution positioning trial was carried out in Gaoan City, Jiangxi Province. Six treatments were established: 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) of total chemical fertilizer nitrogen. After early rice was harvested in 2023 and 2024, topsoil samples were collected to determine total and available nutrient contents, microbial biomass carbon, nitrogen and phosphorus contents, and the key extracellular enzyme activity involved in carbon, nitrogen and phosphorus cycling. At the same time, early rice yield, biomass and their nutrient contents were measured to calculate nutrient utilization efficiency. The effects of stoichiometric characteristics on early rice yield were analyzed.

Results Compared with CF treatment, there was no significant difference in early rice yield, aboveground nutrient uptake, nitrogen and phosphorus nutrient recovery efficiency, agronomic efficiency and partial factor productivity in 25%OM and 50%OM treatments. The contents of easily oxidizable organic carbon (EOC), mineral nitrogen (Nmin), available phosphorus (AP) and readily available potassium (AK) in soil at maturity stage were significantly increased, and EOC:AP and Nmin:AP were decreased. Partial least squares regression 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 (LAP) activity, microbial biomass nitrogen(MBN), microbial biomass phosphorus (MBP), stoichiometric ratio soil total nitrogen to total phosphorus ratio(TN:TP), organic carbon to total phosphorus ratio(SOC:TP), organic carbon to total nitrogen ratio(SOC:TN), enzyme activity carbon-phosphorus ratio E_C:P, microbial biomass nitrogen-phosphorus ratio (MBN:MBP) had significant positive effects 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, excessive substitution (>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.