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 activation 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, as test materials. Low phosphorus soil was used as the background soil, and six KH₂PO₄ application rates were set: 0, 10, 25, 50, 100, and 200 P mg/kg soil, forming a continuous phosphorus supply gradient from severe phosphorus deficiency to sufficient phosphorus. Foxtail millet was harvested 38 days after sowing (heading stage), and chickpea was harvested 60 days after sowing (flowering stage). Aboveground and underground 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 There were significant differences in the responses of the two crops to soil phosphorus levels. Foxtail millet was extremely sensitive to soil phosphorus levels. with increasing P application, its aboveground biomass increased exponentially, reaching a maximum of 140-fold. In contrast, chickpea’s response to soil phosphorus levels was relatively gentle, with only a slow linear increase in biomass and a maximum increase of 40%. 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), the total carboxylates content in the rhizosphere of foxtail millet was maintained above 4000 nmol/(g, soil), and the total content of secreted carboxylates was about 279 times higher than that of chickpea. However, under high phosphorus conditions (P50−P200), the total carboxylates content in the rhizosphere soil of foxtail millet dropped sharply to below 20 nmol/(g, soil). In contrast, the rhizosphere pH and carboxylates secretion of 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 activation strategy, while chickpea exhibits a “conservative” one to cope with changes in soil phosphorus levels. When phosphorus is deficient, foxtail millet activates soil phosphorus by secreting a large amount 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.