Objectives Significant differences exist in phosphorus (P) utilization strategies between gramineous and leguminous crops. An in-depth understanding of how these crops respond to different phosphorus levels is crucial for improving phosphorus use efficiency. Therefore, this study compared the growth response characteristics and rhizosphere phosphorus mobilization strategies of representative gramineous and leguminous crops under different phosphorus supply levels.

Methods A greenhouse pot experiment was conducted using foxtail millet (Setaria italica), a gramineous crop, and chickpea (Cicer arietinum), a leguminous crop. Low phosphorus soil was used as the growth medium, and six KH₂PO₄ application rates were set: 0, 10, 25, 50, 100, and 200 mg/kg soil(designated as P0, P10, P25, P50, P100, and P200), forming a continuous phosphorus gradient from severe deficiency to sufficient supply. Foxtail millet was harvested at 38 days after sowing (heading stage), and chickpea was harvested at 60 days after sowing (flowering stage). Aboveground and belowground biomass, and root/shoot ratio of the crops were measured and analyzed. Rhizosphere soil was collected to determine pH, acid phosphatase activity, and carboxylates content.

Results The two crops exhibited markedly different responses to soil P availability. Foxtail millet was extremely sensitive to soil phosphorus levels. with increasing P application, its aboveground biomass increased exponentially, with the P200 treatment producing a 140-fold increase relative to P0. In contrast, chickpea responded much more moderately, showing only a gradual linear increase in biomass, with a maximum increase of 40% in P200 compared with P0. Under low phosphorus stress (P0−P25), foxtail millet secreted carboxylates to acidify the rhizosphere soil, and the pH in P0 and P10 treatments was significantly lower than that in other P gradient treatments. Under low phosphorus conditions (P0−P25), total rhizosphere carboxylates content of foxtail millet remained above 4000 nmol/g soil, approximately 279 times higher than that of chickpea. However, under high phosphorus conditions (P50−P200), total rhizosphere carboxylates content in foxtail millet declined sharply to below 20 nmol/g soil. In contrast, rhizosphere pH and carboxylates exudation in chickpea showed no significant changes among all phosphorus level treatments, showing relatively “insensitive” rhizosphere chemical regulation characteristics.

Conclusions Foxtail millet adopts a “sensitive” rhizosphere phosphorus mobilization strategy, while chickpea exhibits a “conservative” strategy to cope with changes in soil phosphorus levels. When phosphorus is deficient, foxtail millet mobilizes soil phosphorus by secreting large amounts of carboxylates and protons. Once external phosphorus supply is sufficient, it immediately reduces carboxylates secretion and uptake a large amount of soil phosphorus to achieve explosive growth. In contrast, chickpea is insensitive to changes in soil phosphorus supply, and its growth strategy focuses more on maintaining the stability of internal phosphorus concentration, with stronger tolerance to low phosphorus stress. The application of phosphate fertilizer does not significantly promote its biomass accumulation. Therefore, improving the efficiency of phosphate fertilizer requires full consideration of the phosphorus utilization strategies of crops.