Rice cultivation is crucial in China, contributing significantly to national grain production. Adequate nitrogen (N) supply is vital for high rice yields, but over-application leads to low N use efficiency, environmental problems, and N losses. Fertilizer N can be absorbed directly by rice or temporarily immobilized by biotic and abiotic processes before release to meet crop demand. These processes are influenced by management practices, such as adding urease and nitrification inhibitors, and returning organic materials. Improving immobilization of fertilizer N through adding decomposable carbon (C) is an effective approach to retain N and minimize losses. Studies show that available C inputs temporarily enhance microbial immobilization of N, increasing soil microbial nitrogen (MBN). However, flooded paddy soils exhibit different responses due to ammonium N volatilization and/or nitrate leaching. Straw return generally stimulates immobilization, altering fertilizer N conservation and supply. The influence of inhibitors and straw on fixed ammonium (FA) in flooded paddy soils remains understudied. While inhibitors generally improve yield and N use efficiency, their impact in combination with straw return is not fully understood. This study aimed to clarify how inhibitor and straw additions affect fertilizer N dynamics in a paddy soil-rice system using ¹⁵N tracing. The hypotheses were: (1) straw decreases N uptake and yield but stimulates microbial growth and N storage; (2) inhibitors improve urea-N retention abiotically in the absence of C; and (3) combined inhibitors and straw alleviate the negative effects of straw alone.

Existing research demonstrates that rice yield is strongly linked to nitrogen uptake, particularly during critical growth periods. Over-application of nitrogen fertilizers leads to environmental concerns due to inefficient utilization and losses through volatilization and leaching. The addition of decomposable carbon, such as rice straw, has been shown to temporarily increase microbial immobilization of nitrogen, resulting in a higher soil microbial nitrogen (MBN) content. Studies on the effects of nitrification and urease inhibitors have yielded variable results, with some showing improved yield and N use efficiency, while others reported yield decreases when rice residue was returned, potentially due to inadequate late-season N supply. The interaction between straw addition, inhibitor application, and the dynamics of fixed ammonium (FA) in flooded paddy soil systems requires further investigation to optimize nitrogen management strategies for rice cultivation.

A pot experiment was conducted in a net house from May to October 2018 using 45 pots (5 treatments × 3 sampling times × 3 replicates) containing 3 kg of air-dried Alfisol. Five treatments were implemented in a completely randomized design: (1) C (control, no nitrogen); (2) U (¹⁵N-labeled urea); (3) US (¹⁵N-urea + rice straw); (4) UI (¹⁵N-urea + urease and nitrification inhibitors); (5) UIS (¹⁵N-urea + urease and nitrification inhibitors + rice straw). ¹⁵N-labeled urea was applied at a rate equivalent to 318 kg N ha⁻¹, with superphosphate calcium and potassium chloride applied at rates of 212 kg P ha⁻¹ and 318 kg K ha⁻¹, respectively. Inhibitors (PPD, NBPT, and DMPP) were applied at specific rates relative to urea-N. Rice straw (C/N ratio of 63) was applied at 10.6 t ha⁻¹. Rice (Meifeng 9) was transplanted and managed according to local practices. Soil and rice plant samples were collected at seedling, tillering, and maturation stages. Soil analyses included pH, organic C, total N, P, K, CEC, particle size distribution, ammonium, MBN, FA, DON, and TDN. Plant samples were analyzed for total N and ¹⁵N content. Statistical analyses (one-way and two-way ANOVA, Duncan’s method) and structural equation modeling (SEM) were performed using Excel, SPSS 16.0, and Amos 7. The amount of urea-derived N in soil pools was calculated using the ¹⁵N isotopic enrichment.

Total soil N did not differ significantly across treatments (P > 0.05), except for UIS showing increased total N at maturation. Urea-N recovery varied significantly among treatments and sampling times. UI showed the lowest recovery rate (18.3%, 28.4%, and 39.4% lower at maturation compared to U, US, and UIS, respectively). Straw addition increased urea-N retention, particularly at the tillering stage. UIS alleviated this retention compared to US. Urea-derived NH₄⁺-N decreased with rice growth except for UIS, which initially increased. At seedling, U and UI had higher ¹⁵NH₄⁺ than US and UIS. Straw addition (US, UIS) significantly increased urea-N recovery in MBN, with increases of 77.9% and 122.7% at seedling and 191.9% and 269.6% at tillering compared to U. At maturation, U had higher MBN than other treatments. FA was highest at seedling (around 10% recovery) then decreased over time. UI had the highest FA at seedling. Urea-derived DON was not significantly different among treatments except for US at tillering. US resulted in significantly reduced rice yield and biomass compared to other treatments, with higher urea-N loss. UIS alleviated these reductions. UI showed the highest yield, biomass, and N uptake with the lowest unaccounted-for N. SEM analysis revealed significant correlations between urea-derived N pools.

The results confirm that straw addition (Hypothesis 1) initially decreases N uptake and yield due to microbial immobilization of N, leading to competition with rice. However, the enhanced MBN and DON suggest a long-term benefit of increased N retention. The significant FA pool emphasizes its role in N retention and supply, with release occurring later in the growing season. Inhibitor application (Hypothesis 2) alone improved N use efficiency and reduced losses, aligning with previous meta-analyses. The combined application of inhibitors and straw (Hypothesis 3) mitigated the negative effects of straw on yield and N uptake, likely due to the delayed release of NH₄⁺ by urease inhibitors, allowing better synchrony between N supply and rice demand. While UIS increased overall N retention, UI showed the highest N uptake by rice, indicating a trade-off between N retention in the soil and N uptake by the crop. The unaccounted-for N may be attributed to ammonia volatilization, enhanced by increased dissolved nitrogen levels in treatments with straw.

This study demonstrates that the combined application of inhibitors and straw (UIS) is a promising strategy for improving nitrogen use efficiency in paddy rice systems. While straw initially reduces plant-available N, leading to lower yields, the long-term implications may be positive due to the increased organic N stores. Inhibitors help to better synchronize N supply with crop demand, leading to higher yields and reduced N losses. However, further research is needed to optimize straw application rates and assess long-term sustainability and economic feasibility under field conditions.

The study was conducted in a pot experiment, which might not fully replicate field conditions. Factors like leaching losses, which are important in field scenarios, were not considered in the controlled environment of the pot experiment. Furthermore, the economic benefits and long-term effects of the proposed management strategy need further evaluation through field-scale studies spanning multiple growing seasons.