Maize/soybean intercropping promoted activation of soil organic phosphorus fractions by enhancing more phosphatase activity in red soil under different phosphorus application rates

Rational intercropping is crucial for improving phosphorus (P) uptake and utilization. This study aimed to investigate the effects of intercropping on the activation of soil P fractions and available P in acid soil under different P application rates. Field experiments were conducted to investigate the effects of maize intercropping with soybean at different P application rates (0, 60, 90, and 120 kg P ha−1) on soil P fractions and P pool turnover over two consecutive years. Intercropping of maize and soybean showed an advantage in increasing P uptake and reducing apparent P balance at all P application rates, and promoting soil P pool activation. Compared with monoculture maize, intercropping significantly increased maize P uptake by 43.6–74.3% and 45.5–76.8% in consecutive years, reduced apparent P balance by 17.1–33.4% and 19.9–32.4% in those years. Additionally, intercropping maize increased labile P pools by 32.5–38.4% and 14.4–82.1% over consecutive years and reduced non-labile P pools by 7.4–10.9% and 6.6–11.6% compared with monoculture maize. Moreover, intercropping depleted NaOH-Po, conc. HCl-Pi, conc. HCl-Po and Residual-P fractions, and increased Resin-P, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi, and 1 M HCl-Pi fractions compared with monoculture maize. Resin-P, NaHCO3-Pi, NaHCO3-Po increased by 4.3–41.2%, 21.1–84.6%, and 9.7–98.8%, respectively. Furthermore, intercropping at different P application rates significantly increased acid phosphatase activity (ACP) by 13.8–27.1% and 9.5–13.4%, and significantly increased alkaline phosphatase activity (ALP) by 21.2–42.6% and 19.9–28.6% in those years. Structural equation modelling (SEM) showed that both ACP and ALP played a crucial role in increasing available P directly or indirectly through their effects on organic P turnover. These results demonstrate that intercropping maize with soybean increases soil P bioavailability by transforming NaOH-Po and conc. HCl-Po into soluble P (Resin-P and NaHCO3-Pi) by facilitating the accumulation of soil phosphatase activity.