Crop rotation systems profoundly influence soil phosphorus (P) dynamics through physicochemical and microbial interactions. The mechanisms regulating P availability under various rotational practices remain poorly understood. This five-year field experiment investigated the effects of four rotation systems (WM: wheat–maize; WP: wheat–peanut; WS: wheat–soybean; MV: maize–hairy vetch) on soil P fractions, phosphatase activities, P-cycling gene abundance, and their interactions with soil properties. The WM rotation substantially reduced soil pH (6.29) while increasing labile P fractions (Ca₂-P) and moderately labile P (Al-P, Fe-P, and Ca₈-P), which was attributed to enhanced acid phosphatase activity. The WP rotation elevated soil pH (8.13) but reduced P availability due to calcium–P immobilization. The MV rotation stimulated microbial P cycling, exhibiting the highest phoD (2.01 × 10⁶ copies g⁻¹) and phnK (33,140 copies g⁻¹) gene abundance, which was linked to green manure-induced microbial activation. Redundancy analysis identified soil pH, total nitrogen, and stoichiometric ratios (C/N and N/P) as key shared drivers of P fractions and enzymatic activity. Partial least squares path modeling (PLS–PM) indicated that crop rotation directly regulated P availability through pH modulation (r = −0.559 ***) and the C/N ratio (r = 0.343 ***) while indirectly regulating P fractions through phosphatase activity. Lower C/N ratios (<10) across all rotation regimes amplified the carbon limitation in the process of P transformation, indicating that exogenous carbon inputs and appropriate stoichiometry in the soil should be optimized. The results of this study inform the selection of suitable crop rotation patterns for sustainable agriculture.

Controlled-release potassium sulfate (CPS) and deep banding fertilization are promising strategies for improving crop productivity, but their combined effects on soybean growth remain unclear. In this study, field experiments were performed over two growing seasons (2023–2024) to evaluate how potassium (K) fertilizer types (potassium sulfate and CPS) and fertilization methods (broadcast incorporation and deep banding) influence soil microbial composition, soil enzyme activity, and soybean growth characteristics. The results indicate that deep banding and CPS application consistently increased soybean yield, K use efficiency, and nitrogen use efficiency. Relative to broadcast incorporation, the application methods of deep banding increased soybean yield by 1.1%–2.7% in 2023 and 3.7%–3.8% in 2024, while the application of CPS increased soybean yield by 6.0%–7.7% and 9.6%–9.7% compared with potassium sulfate (PS) application. Additionally, deep banding of CPS increased soil protease, urease, acid phosphatase, and phosphodiesterase activities and improved plant enzyme activities and hormone contents. Microbial analyses showed that CPS combined with the deep banding method (DCPS) enriched several microbial taxa ( Bradyrhizobium , Lysobacter , Rhizophagus , and Funneliformis ) potentially associated with nutrient cycling and was associated with more connected genus-level microbial co-occurrence networks. Structural equation modeling further indicated that microbial diversity, soil fertility, enzyme activity, and plant growth characteristics were jointly linked to soybean productivity. These results suggest that DCPS is a promising K management strategy for improving soybean productivity and nutrient use efficiency in the North China Plain.

Apple cultivation faces soil acidification and pollution due to excessive fertilization, compounded by a scarcity of potassium (K) fertilizers. Polyhalite, a natural multi-nutrient mineral, offers a potential sustainable alternative. Therefore, a three-year field experiment was conducted, comprising a no-potassium control (CK), two conventional potassium fertilizers (sulfate of potash-based and muriate of potash-based), and six polyhalite compound fertilizer treatments (with different basal and topdressing strategies), to evaluate their effects on apple growth and soil fertility. Results showed that the single topdressing application of potassium chloride-type polyhalite compound fertilizer (T6) achieved the highest yield in the final year, which was 10.11–28.03% higher than the other potassium-applied treatments. It also achieved the highest fruit vitamin C and soluble solids content (9.53 mg 100 g⁻¹ and 13.27%, respectively). The T6 treatment demonstrated the best performance in terms of agronomic efficiency and partial factor productivity of potassium fertilizer, reducing fertilizer waste and loss. Furthermore, the T6 treatment effectively increased soil pH, available potassium, and exchangeable calcium levels, thereby improving soil fertility. Thus, polyhalite proves effective in replacing conventional K fertilizers, with the single topdressing of MOP-type polyhalite compound fertilizer (T6) offering the most comprehensive agronomic and environmental benefits.