Objectives Drought and phosphorus (P) deficiency seriously affect soil P availability and restrict crop growth. This study aimed to reveal the effects of fulvic acid (FA) on soil P availability, maize growth and P uptake under drought stress with different P levels, in order to provide a theoretical basis for P-utilization efficiency and agricultural development in arid regions.
Methods A pot experiment with three factor-three level design was conducted using maize in a loess soil. The low, medium, and high P levels were 30 (P30), 60 (P60), and 120 mg/kg (P120); the normal (W1), mild drought (W2), and moderate drought (W3) water conditions were 75%, 60%, 45% of field capacity, respectively; and the three application rates of FA were 0 (FA0), 50 (FA1), and 100 mg/kg (FA2), respectively. The maize was harvested after 60 days of growth for measurement of plant height, biomass and P concentration. At the same time, soil pH, Olsen-P, P fraction concentration, alkaline phosphatase activity and organic acid in rhizosphere were determined.
Results FA enhanced soil P availability under drought conditions, and its effect was related to P application and water conditions. At P60W2 and P120W2, FA1 and FA2 treatments significantly reduced soil pH by 0.14−0.21 units through acidification effects, resulting in 15%−19% and 44%−47% increases in soil Olsen-P and labile P concentration, respectively. At P60W3, FA2 treatment increased tartrate and Olsen-P concentration. Moreover, there was a positive (P<0.05) correlation between tartrate and Olsen-P concentration, indicating that fulvic acid could improve soil P availability by promoting the secretion of organic acid in rhizosphere. Under drought conditions at any P treatment level, FA had no significant effect on soil alkaline phosphatase activity. At P30W2, FA1 and FA2 treatments reduced various parts of maize biomass (35%−39%) and plant P accumulation (20%−38%). However, at P60W2 and P120W2, FA promoted maize growth and P uptake. At P60W2, applying FA2 increased P-uptake efficiency by 38%. At P120W2, applying FA1 increased maize plant height by 10%, and applying FA2 increased root P concent by 12%.
Conclusions Under drought and medium to high soil P conditions, the application of fulvic acid can drive the transformation of soil P forms through acidification effects and the regulation of organic acid in rhizosphere, increasing soil Olsen-P and labile P concentration, and thus improving soil P availability. However, only under mild drought conditions can the application of fulvic acid promote maize growth and P uptake.
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