Poly-γ-Glutamic Acid Rhamnolipid Ester (APAA) enhances phosphorus fertilizer efficiency: Mechanisms and field application

Phosphorus (P) is an essential nutrient for crop growth, but the global utilization rate of P nutrient is less than 20%. Issues such as soil fixation, low root absorption efficiency, and environmental loss lead to significant P nutrient waste and water pollution. APAA addresses these problems by activating the soil phosphorus pool and enhancing root absorption, achieving a 10% reduction in phosphorus fertilizer usage and a 10% increase in yield. 

How APAA Enhances Phosphorus Fertilizer Efficiency

• Increasing Soil Available Phosphorus Content

APAA is rich in hydroxyl and carboxyl functional groups, which can chelate metal cations such as iron, aluminum, and calcium in the soil, preventing phosphorus fixation and releasing insoluble phosphorus that has been fixed. This transforms ineffective phosphorus into an available form. 

APAA stimulates root systems to secrete organic acids and phosphatases, while also activating the activity of phosphate-solubilizing bacteria. This promotes the continuous release of organic and mineral phosphorus in the soil, forming a long-lasting phosphorus supply mechanism.

Figure 1 Methods of APAA improve content of available P in soil

• Increasing Root Surface Area

In low-phosphorus environments, APAA regulates the flavonoid metabolism pathway, enhancing the binding ability of auxin transport proteins (PIN) to RNA polymerase while weakening the interaction between growth-regulating factors (GIFs) and RNA polymerase. This regulatory mechanism significantly promotes the expression of the core transcription factor PLT in the root apical meristem, stimulating root growth and improving phosphorus absorption efficiency. 

Under low-phosphorus conditions, APAA further activates the expression of phosphorus starvation-inducible genes (PSI), which significantly enhances the activity of phosphorus transporter proteins (PHTs), improving phosphorus utilization efficiency.

Figure 2 APAA increases root surface area 

Test and Trial about APAA enhance Phosphorus Fertilizer Efficiency

• Leaching Test of APAA enhancing Diammonium Phosphate (DAP) efficiency

Treatments: CK (DAP), DAP+ Competitor Product , DAP + APAA

Application: Base application, 300 kg /ha

Measurement: Measure available phosphorus content in leachate (12 h, 36 h, 6 d)

Figure 3 Schematic diagram of the leaching device and the difference in effective phosphorus content in leachate between different treatments 

Results: APAA reduces the leaching of available phosphorus and increases the content of available phosphorus in the soil. 

• APAA --- Phosphorus Fertilizer Efficiency Enhancement Test on Corn

Trial: Effect of APAA-Enhanced Phosphorus Fertilizer on Corn

Treatments: CK (DAP), T1 Competitor (Enhanced DAP) , T2 (DAP + APAA)

Application: Base fertilizer, 825 kg/ha of DAP and 225kg/ha of potassium sulfate

Measurement: Investigate above ground and underground growth differences at the jointing stage; measure yield at harvest

Figure 4 The effect of different treatments on the overall growth of corn at the jointing stage

Figure 5 The effect of different treatments on the individual plant growth of corn at the jointing stage

Figure 6 The effect of different treatments on the aboveground indicators of corn at the jointing stage

Figures 4-5 show the effects of different treatments on the overall growth and individual plant growth of corn in the field. Overall, the corn treated with APAA (T2) showed the best growth. As shown in Figure 6, the corn treated with APAA (T2) performed the best in all above ground indicators. Plant height, stem diameter, SPAD value, and above ground fresh weight increased by 32.9% and 10.9%, 33.3% and 18.4%, 3.27 and 3.59, and 70.5% and 20.0%, respectively, compared to CK and T1.

Figure 7 The effect of different treatments on the root system of corn at the jointing stage

Figure 8 The effect of different treatments on various root indicators of corn at the jointing stage

Figure 8 The effect of different treatments on various root indicators of corn at the jointing stage Figure 7 shows root scans of corn under different treatments. It is evident that the corn treated with APAA (T2) has more developed roots and a higher number of root hairs. As seen in Figure 8, the root fresh weight of the APAA-treated corn (T2) is the highest at 5.0 g, which is 51.5% and 6.4% higher than CK and T1, respectively. The number of primary roots in the APAA treatment (T2) is the same as in T1, both higher than in the CK treatment. Additionally, the total number of roots and the total root surface area are highest in the APAA treatment (T2), with values of 3,596 and 12,188 mm², respectively.

Figure 9 The effect of different treatments on corn at harvest stage

Figure 10 The effect of different treatments on cob length and cob diameter of corn

Table 1 The effect of different treatments on related indicators of corn at harvest stage

Table 2 The effect of different treatments on corn yield and profit

Figure 9 shows the effects of different treatments on the corn's growth at harvest and the cob characteristics. Combined with Figure 10, it is evident that the APAA treatment (T2) achieved cob length and cob diameter of 19.8 cm and 47.0 mm, both superior to CK and T1. As shown in Table 2, at harvest, the stem diameter, 100-grain dry weight, leaf area, and leaf dry weight in the APAA treatment (T2) were all better than in CK and T1. The final yield measurement revealed that the corn treated with APAA (T2) achieved a yield of 14100 kg/ha, an increase of 11.1% and 9.2% compared to CK and T1, respectively. Additionally, after adding APAA, the net income per ha increased by CNY2835 and CNY2370 compared to CK and T1 (Table 2). 

Conclusion 

APAA significantly enhances phosphorus fertilizer utilization through mechanisms such as chelating soil metal ions, activating phosphate-solubilizing bacteria, and enhancing root absorption. Field trials on corn have shown that adding APAA to diammonium phosphate has a noticeable promoting effect on corn growth. Overall, during the seedling stage, the primary improvements were in plant height and the number of root hairs. At harvest, the main effects were seen in the size of the corn cobs and yield, achieving both increased production and income.