To investigate the effects of long-term biochar application on different forms of potassium (K) content in maize rhizosphere soil and maize growth, two biochar application rates (B0: 0 t ha⁻¹ yr⁻¹, B1: 2.625 t ha⁻¹ yr⁻¹) and two K fertilizer application rates (K0: 0 kg ha⁻¹ yr⁻¹, K1: 60 kg ha⁻¹ yr⁻¹) to create four treatments (B0K0, B0K1, B1K0, B1K1). In this long-term field trial, we investigated various forms of K in the maize rhizosphere soil, together with soil physicochemical properties and maize growth indicators. Results indicate that biochar significantly increased microbial biomass carbon (MBC), cation exchange capacity (CEC), and electrical conductivity (EC) in the rhizosphere soil, while also improving rhizosphere soil pH. Compared with the treatment without biochar, biochar application significantly increased the content of water-soluble potassium (WSK), exchangeable potassium (EK), and non-exchangeable potassium (NEK) in the rhizosphere soil by 18.57% (2021) and 11.18% (2022), 13.49% (2021), and 11.43% (2022), 14.65% (2021), and 17.06% (2022), respectively. Maize roots were more developed, and plant height, stem diameter, and leaf area index were significantly increased. With above-ground dry weight and K uptake significantly increasing by 13.87% (2021) and 12.04% (2022), and 41.84% (2021) and 43.87% (2022), respectively. Compared with B0K0, the B1K1 treatment—which combined biochar with K fertilizer—exhibited the highest K content in all forms within the rhizosphere soil, along with the greatest maize aboveground dry weight and K uptake. This study demonstrates biochar’s potential in meeting crop root K demands, laying the foundation for its application in enhancing soil K fertility.

To elucidate the synergistic effects of nitrogen application rate and fertilization method on the yield and quality of high-quality japonica rice, a two-year field experiment (2024 and 2025) was conducted using Koshihikari with three fertilization methods—farmers’ practice (FP), light and simplified fertilization (LSF), and precise quantitative side-deep fertilization (PQSF)—and five nitrogen rates (0, 45, 90, 135, and 180 kg ha⁻¹). In 2024, comprehensive traits were evaluated, including grain-filling dynamics, protein fractions, panicle traits, milling quality, multidimensional quality, and yield. In 2025, only core yield and quality traits were measured for validation. Superior grains had higher filling rates and earlier initiation, whereas inferior grains showed delayed initiation and later peak timing; however, high nitrogen shortened effective filling duration and exacerbated delayed filling in inferior grains. Protein accumulation was closely associated with grain-filling parameters, with glutelin and prolamin showing more persistent associations than albumin and globulin. Milling quality was optimal at 90 kg ha⁻¹ (N90), whereas appearance and eating quality deteriorated with increasing nitrogen; springiness and adhesion force peaked at N45. Under high nitrogen, PQSF sustained inferior grain filling and mitigated the negative effects of excessive storage protein accumulation on eating quality. LSF achieved the highest yield (approximately 8900 kg ha⁻¹) within 135–153 kg ha⁻¹, whereas PQSF combined with 135 kg ha⁻¹ provided a quality-oriented strategy, increasing taste value by 3.00 and 2.33 points and reducing chalkiness percentage by 1.70 and 0.25 percentage points relative to FP and LSF, respectively, while maintaining lower hardness. PCA and hierarchical clustering analyses revealed that nitrogen rate established the fundamental framework for yield and quality, while fertilization method provided secondary regulation. The core yield and quality findings were consistent across both years, confirming the robustness of the conclusions and providing a basis for precise nitrogen management of high-quality japonica rice in the Liaodong coastal rice region; however, the mechanistic insights were mainly derived from 2024 and require further multi-year validation.

As an organic soil amendment, biochar can effectively increase the contents of soil nutrients such as nitrogen (N), phosphorus (P), and potassium (K). However, few studies have focused on the effects of biochar on K fractions and contents in maize rhizosphere soil. To investigate the effects of biochar on different K fractions in maize rhizosphere soil and on maize growth, four treatments were established in this experiment: B₀K₀ (control, no biochar and no K fertilizer), B₀K₁ (no biochar, K fertilizer at 60 kg ha⁻¹ yr⁻¹), B₁K₀ (biochar at 2.625 t ha⁻¹ yr⁻¹, no K fertilizer), and B₁K₁ (biochar at 2.625 t ha⁻¹ yr⁻¹ combined with K fertilizer at 60 kg ha⁻¹ yr⁻¹). Results indicate that biochar significantly increased microbial biomass carbon (MBC), cation exchange capacity (CEC), and electrical conductivity (EC) in the rhizosphere soil, while also improving rhizosphere soil pH. Compared with the treatment without biochar, biochar application significantly increased the content of water-soluble potassium (WSK), exchangeable potassium (EK), and non-exchangeable potassium (NEK) in the rhizosphere soil by 18.57% (2021) and 11.18% (2022), 13.49% (2021) and 11.43% (2022), 14.65% (2021) and 17.06% (2022), respectively. However, the increases in different K fractions were not accompanied by significant changes in total K (TK) content across the two years. In addition, biochar application significantly improved maize root development, plant height, stem diameter, and leaf area index. Meanwhile, aboveground dry weight and K uptake increased significantly by 13.87% (2021) and 12.04% (2022), and 41.84% (2021) and 43.87% (2022), respectively. Compared with B₀K₀, the B₁K₁ treatment—which combined biochar with K fertilizer—exhibited the highest K content in all forms within the rhizosphere soil, along with the greatest maize aboveground dry weight and K uptake. This study demonstrates biochar’s potential in meeting crop root K demands, laying the foundation for its application in enhancing soil K fertility.