Biochar, Organic Fertilizer and Bacillus Treatment Combinations Improve Lettuce Phosphorus Nutrition, Rhizosphere Phosphatase Activity and Growth in a Calcareous Alkaline Soil

Calcareous alkaline soils restrict phosphorus (P) nutrition, while greenhouse lettuce requires rapid nutrient supply during a short production cycle. A two-season randomized complete block experiment compared eight treatments: control (C), biochar (BC; 3.0 t ha−1), fermented organic fertilizer (OF; 3.0 t ha−1), a four-species Bacillus consortium (Bac), BC + OF, BC + Bac, OF + Bac, and BC + OF + Bac. BC + OF + Bac produced the highest yield in both 2025 and 2026 (58.09 and 59.60 t ha−1). Compared with the control, this treatment increased yield by 15.5–17.2%, leaf P by 30.4–33.3%, root P by 31.1–35.7%, and rhizosphere alkaline phosphatase (ALP) activity by 158.8–186.6%, while acid phosphatase (ACP) activity declined by 23.4–25.5%. Combined treatments also generally increased vegetative growth and leaf macro- and micronutrient concentrations. Because BC and OF supplied nutrients directly and the combination treatments underwent 14-day pre-incubation procedures not shared by all single-component treatments, the results represent the performance of complete treatment packages rather than proof of native-soil P mobilization or ingredient synergy. The BC + OF + Bac package therefore showed strong agronomic potential for lettuce in this calcareous alkaline soil, while nutrient-balanced and formulation-standardized experiments are required to resolve the underlying mechanisms.