Phosphorus (P) is an essential macronutrient for plant growth, involved in the composition of important structures, cell energy transfer, and various metabolic processes (Al-Juthery et al., 2021; Hu et al., 2023). The application of P fertilizers is important in ensuring food security (Langhans et al., 2022). The yield of three major grain crops (rice, wheat and maize) increased by 16% in China in response to mineral P fertilizer applications (Gong et al., 2022). However, excessive P fertilization occurs regularly (Sattari et al., 2014; Li et al., 2016). The use efficiency of large amounts of P fertilizer applied is generally less than 20% in the first cropping season due to the adsorption/fixation of P by soil (e.g., Simpson et al., 2011; Zhang et al., 2019), which has resulted in the accumulation of P in soils and even leaching/runoff of P to waters, leading to severe environmental problems, such as water eutrophication (Zhu et al., 2018; Yan et al., 2021). Furthermore, phosphate rock (mined from the ground and used in the production of P fertilizers) is a non-renewable resource made by natural processes over geological timeframes. Despite some new discoveries of phosphate rock deposits (e.g., in Norway in 2018) that are quite deep and therefore technically challenging and expensive to exploit, the inescapable inference is that the humanity will run out of phosphate rock to mine for producing P fertilizers over the course of the next few generations (cf. Geissler et al., 2015). Therefore, it is necessary to improve P-use efficiency (PUE) while increasing crop yield for achieving sustainable P management and developing green agriculture.

Soil water significantly affects P availability because (i) the movement of P is mainly through diffusion (with a diffusional pathway lengthening significantly due to tortuosity under low soil water contents) and (ii) the P biogeochemical cycles, including sorption/desorption and precipitation/dissolution, are highly sensitive to soil water content (Plett et al., 2020; Maharajan et al., 2021). Hence, low soil water content may significantly reduce P accumulation by plants and grain yield (e.g. by 40% and 60%, respectively, in wheat; He et al., 2017).

Climate change resulting from global warming is increasing precipitation in some, but exacerbating aridity in other, parts of the world. Aridity leads to water stress, reducing crop yield and agricultural efficiency (Shayanmehr et al., 2022; Kim et al., 2023). However, under water stress conditions, maize root growth was stimulated [the root/shoot ratio and specific root length increased, and the inorganic phosphate transporter genes were up-regulated (Zhan et al., 2019)]. Plants allocate more biomass to roots to improve the soil exploration and increase P uptake under P-deficiency conditions (Lambers et al., 2006). Within a certain range of reduced soil water content, P-uptake efficiency and total P uptake would increase due to the root system alterations. However, decreased P diffusion in partially dry soil would counteract the beneficial effects of the improved root system on total P uptake (Bauw et al., 2019).

Different P fertilizers vary in their impact on crop growth and nutrient-use efficiency because of their specific physical and chemical properties. These properties, which control the immediate reaction of P fertilizers with soil surface or solution, primarily include the phosphate ion concentration and its speciation, the form and concentration of the accompanying cation and the pH of the solution. The reactions of the different P fertilizers and their fertilizer solutions with soils can be categorized as follows: (1) triple superphosphate (TSP) and single superphosphate (SSP); (2) monoammonium phosphate (MAP) and diammonium phosphate (DAP); (3) ammonium polyphosphates (APP) (Hedley and Mclaughlin, 2005). Based on these specific properties, the fertilizer efficiency and agronomic effectiveness of P fertilizers also vary significantly across different soil environments. For example, the application of acid or neutral fertilizers, including MAP, APP, and urea phosphate (UP), resulted in higher maize yield and economic benefits in the calcareous soil compared with alkaline fertilizers (e.g. DAP) (Wang et al., 2024b; Tian et al., 2024). By contrast, in acid soil, the maize P-uptake efficiency and partial factor productivity of calcium magnesium phosphate (CMP) and DAP were higher than those of SSP, MAP and APP (Zhao et al., 2021). APP reduced the fixation/sorption and increased the desorption/dissolution of P due to the simultaneous presence of soluble orthophosphate, slowly hydrolyzed pyrophosphate and more condensed P forms (Yuan et al., 2022).

Currently, the two high-grade P fertilizers (MAP and DAP) are predominantly used in agricultural production in China because of their perceived yield-increasing effect, even though these fertilizers are associated with increasing soil residual P and exacerbated P losses (Gong et al., 2022). The production of DAP and MAP has accounted for about 80% of the total P fertilizer production in China since 2014 (Zhang et al., 2017). Low-grade P fertilizers such as SSP and CMP are gradually declining on the market because their low nutrient content is not favored by farmers. New fertilizers (such as APP, UP, and other slow-release P fertilizers) have not been widely accepted due to their high prices, which significantly limited the transformation and upgrade of P fertilizer industry as well as the development of green agriculture. The research on the use efficiency of different P fertilizers in different regions or environmental conditions is urgently needed to provide scientific support for the efficient utilization of existing P fertilizers and the formulation of new P fertilizers. Current research is mainly focused on P application rates under various crop-producing conditions, but the studies comparing different P fertilizers for PUE via influencing root growth and rhizosphere processes are relatively rare, and this paucity is even more obvious regarding the interaction of P fertilizer types and water supply.

This study was aimed at investigating the effects of two widely used P fertilizers (MAP, DAP) and two new P fertilizers (APP, UP) on the root and rhizosphere processes of maize under two contrasting water supplies to test the following hypotheses: (1) different P fertilizers would differentially influence maize growth and P uptake by altering root morphology and rhizosphere processes, and this effect would be modified by soil water supply; and (2) optimizing the interaction between P fertilizer type and soil water content can increase PUE and maize growth, depending on the properties of P fertilizers.

Exploring the effects of different P fertilizers on the root growth and P uptake of maize in dependence of soil water content provides important evidence for achieving the precise and efficient use of P fertilizers to underpin sustainable P management. The present study indicated that the root morphological and physiological traits of maize exhibited differences in response to the interaction between P fertilizers and water supply, which were associated with improved nutrient uptake and stimulated maize growth.

APP maintained the highest soil Olsen-P concentration (especially in watered treatments) compared with other fertilizers (Figure 1). Given that APP contains orthophosphate (ortho-P, H₂PO₄^-, HPO₄^2-, PO₄^3-) and polyphosphates (poly-p, P_nO_3n+1^(n+2)-, n ≥ 2), it can quickly provide ortho-P in the early stage of maize growth, and continuously provide available P from poly-P in the later stage. The poly-P can slowly hydrolyze into ortho-P that can be directly absorbed by plants (McBeath et al., 2007; Zhou et al., 2023). Moreover, the poly-P can also decrease P fixation and increase soil available P by preventing Ca-P formation or retarding the ripening of Ca-P to more stable mineral in calcareous soils (Gao et al., 2020; Yuan et al., 2022). Hence, poly-P can improve P availability in soil and P uptake by crops compared with traditional ortho-P fertilizers (Weeks and Hettiarachchi, 2019; Erel et al., 2023), such as MAP and DAP, because of ready fixation of ortho-P by soil (Rajput et al., 2014).

In calcareous soils, the phosphate from UP takes time to transform into the P forms that can be absorbed by plants, leading to insufficient supply of P in the early stage of maize growth (Ji et al., 2021). A temperature decrease caused by UP dissolution (Supplementary Figure S1) (Long et al., 2022) might further prolong the transformation time and also slow the germination of seeds compared with other P fertilizers if the fertilizer is close to seeds (Sbrussi and Zucareli, 2014). Based on the above results, it is recommended that the combined application of readily-available P and slow-release P like APP should be further tested to effectively reduce P fixation and improve PUE.

The major root morphology traits (e.g., total root length in MAP and APP, root diameter in MAP) or physiological traits (rhizosphere soil pH in DAP, APase acitivity in rhizosphere soil in MAP and DAP) of maize treated with MAP, DAP and APP differed in the two contrasting soil water conditions, but not in case of UP (Figures 3, 4). This may be due to the presence of NH₄⁺-N in MAP, DAP and APP. Crops release protons to the rhizosphere in the process of absorbing NH₄⁺-N, which not only acidified the rhizosphere, but also accelerated the cell divisions and cell wall elongation of root meristematic cells, promoting root proliferation (Bloom et al., 2002; Lin et al., 2021). Longer roots mean increased root-soil contact area, thus improving root uptake of available nutrients from soils (Jing et al., 2022). Although urea (the N form in UP) can also hydrolyze to produce NH₄⁺-N, thereby influencing root growth, its complete hydrolysis to release NH₄⁺-N requires up to four days even in fertile soil with 18% soil water content (Skiba and Wainwright, 1984). Moreover, the high pH of calcareous soils further restricts this conversion. Root proliferation was greater with a combined supply of P and NH₄⁺-N than with other N forms, such as nitrate or urea (Jing et al., 2012), which aligns with the findings in this study. Specifically, MAP and APP showed a significant increase in total root length of maize in the watered compared with the dry treatments (Figure 3). Maize allocated relatively more photosynthates to shoots under the conditions favoring efficient uptake of nutrient by roots (Poorter and Nagel, 2000; Nielsen et al., 2001; Zhang et al., 2023), resulting in an increase in shoot biomass and decreased root/shoot ratio in the treatments with MAP and APP (Figure 2). The application of DAP significantly reduced the rhizosphere soil pH in the watered compared with the dry treatment (Figure 4A), likely due to its high NH₄⁺-N content under optimal soil water content, whereby soil acidification may enhance the P availability in the rhizosphere soil, thus increasing the P uptake by maize (Figure 5). However, there was no difference in the root/shoot ratio of DAP in the two watering treatments, even though the root and shoot biomass significantly increased in the watered treatment (Figure 2). It indicated that the nutrient uptake efficiency of roots was lower in the DAP than the MAP and APP treatments, although DAP had the highest proportion of NH₄⁺-N. An excessively high level of NH₄⁺-N can cause ammonium toxicity, which is detrimental to plant and root growth (Hachiya et al., 2021; Zhang et al., 2021). Another possible reason for this is that the high pH of DAP solution in the early stage of dissolution weakens the positive effect of rhizosphere acidification on root growth and increases the formation of insoluble Ca phosphates in calcareous soil (Hedley and Mclaughlin, 2005). By contrast, the acidic fertilizers exhibited higher root and fertilizer efficiency in calcareous soil (Wang et al., 2024b; Tian et al., 2024), which was consistent with the results of the present study. However, compared with other P fertilizers, the maize shoot and root biomass and PUE of UP showed the smallest increase in the watered vs. dry treatments despite the UP solution having the lowest pH of all fertilizers tested (Figures 2, 5). This further demonstrates that the fertilizer efficacy is subject to the collective influence of factors such as nutrient types, forms, and pH (Hedley and Mclaughlin, 2005). And the results might suggest that the maize root growth was more effective than the fertilizer pH in regulating the responses to environmental and nutrient changes, emphasizing the importance of engineering the rhizosphere microenvironment and the soil macro-environment to promote nutrient-use efficiency and crop growth as opposed to relying solely on high fertilizer input (Wang et al., 2020; Wang et al., 2024a). In summary, among the four P fertilizers evaluated, APP demonstrates higher agronomic performance. However, due to high raw material costs and substantial energy consumption, APP products are priced 1.7 to 2.1 times higher than MAP and DAP, which limits their widespread adoption (Lin and Yin, 2014; Kang and Chu, 2018). A key challenge is to achieve low-cost, continuous and large-scale production of APP. In recent years, some progress has been made in reducing APP production costs through the use of low-cost feedstocks such as wet-process phosphoric acid and its by-product raffinate, alongside process optimizations that reduce material requirements. The resulting low-cost and high-quality products have established a foundation for large-scale production and widespread promotion of APP (Li, 2025).

The shoot biomass (Figure 2) and PUE of maize (Figure 5) treated with four P fertilizers were significantly higher in the watered than the dry treatments, suggesting that the nutrient uptake by roots decreased in the dry treatment, limiting maize growth. One reason is that low soil water content can reduce the P dissolution and diffusion in soil, leading to a decrease in P availability (Plett et al., 2020; Maharajan et al., 2021). Moreover, aridity also directly affects functioning of the root system, reducing hydraulic conductivity and stomatal conductance and restricting root-to-shoot nutrient transport (Lefebvre et al., 2011). The root physiological response to P stress (higher APase activity in the rhizosphere soil in the CK than in any fertilizer treatment) and water shortage in the MAP and DAP treatments (higher APase activity in the rhizosphere soil in the dry compared with the watered treatments) would have likely resulted in increased P availability from organic P sources (Figure 4B). Similar results were also found in the previous studies (Xia et al., 2020; Jing et al., 2022). By contrast, the response to P stress also included an increase in exudation of protons and strong acidification of the rhizosphere soil in the previous studies (Liu et al., 2004; Shen et al., 2011), but in the present study the rhizosphere soil acidification was similar in the CK and most fertilizer treatments (except DAP) in the watered conditions, but this acidification was greater in the CK compared with the MAP, DAP and UP treatments under water shortage (Figure 4A). A possible reason for these findings might have been the interactive effects between P fertilizer and water supply. Exacerbated aridity can increase soil pH, which would be associated with depressed nutrient mobility and availability (Suriyagoda et al., 2014; Zhan et al., 2019) as well as decreased proton release accompanying low cation uptake rates (Bloom et al., 2002; Yan et al., 2002). In conclusion, the root growth and rhizosphere processes can effectively be regulated by using specific P fertilizers to suit specific soil conditions (such as low soil water content), thus affecting crop growth and uptake of fertilizer nutrients. Precise application of P fertilizers and developing new P-based smart fertilizers should be implemented to enhance sustainability of agricultural production based on better understanding of the interactive soil-crop-environment system to achieve the high crop yields with efficient use of P fertilizers and small environmental footprint.

In the present study, MAP, DAP and APP significantly promoted maize shoot growth and PUE in the watered treatments by modifying the root morphology and physiological traits. The best agronomic performance was observed with APP, followed by that of MAP. In addition, APP maintained a high soil Olsen-P concentration in both watered and dry treatments. The responses of maize root growth and P availability differed depending on the combinations of P fertilizer and water supply, emphasizing the necessity to coordinate the choice of P fertilizer with the interactive crop-soil-environment systems to improve both nutrient uptake by roots and the fertilizer use efficiency for sustainable crop production.