A comprehensive evaluation of the optimum amount of phosphate fertilizer for drip irrigation of cotton under mulch based on root morphology, physiology, and mycorrhizal symbiosis

Cotton (Gossypium hirsutum L.) has been a valuable economic crop in arid Xinjiang, whose cotton production accounts for 90.2 % of the grand total in China. Low phosphorus (P) bioavailability brings severe restrictions for cotton production in arid regions as a result of P precipitation caused by high soil pH and Ca²⁺ content. Therefore, how to utilize the biological potential to improve the efficiency of P utilization has become a hotspot for international research.To learn more about P bioavailability in cotton from the perspectives of root morphology, rhizosphere physiology, and mycorrhizal association, especially the synergistic effect of these three under the optimal P input on P bioavailability. A 2-year, split-plot field experiment was conducted consecutively from 2016 to 2017, in which the main plots contained three cotton varieties (XLZ57, XLZ19, and XLZ13) and the subplots were treated with five P levels (0, 75, 150, 300, and 450 kg P₂O₅ ha⁻¹). Optimal P input (P fertilizer application: 75–150 kg P₂O₅ ha⁻¹ or soil available P content in topsoil: 11–25 mg kg⁻¹) was found to not only improve the distribution of root system and mycorrhiza in soil but also promote the secretion of protons and alkaline phosphatase in the rhizosphere, leading to higher P uptake and cotton yield. Although high P input (300–450 kg P₂O₅ ha⁻¹) increased soil available P content, it inhibited root growth, mycorrhizal infection and phosphatase activity, thus reducing P uptake and product. To obtain a relatively high yield (5500–6500 kg ha⁻¹ unginned cotton) and high P accumulation (120–130 kg P₂O₅ ha⁻¹), an ideotype cotton root/rhizosphere should be characterized by high root length density (4–5 m 1000 cm⁻³), large hyphal density (15–18 m g⁻¹), and greater exudation of protons and alkaline phosphatase (60–70 μg g⁻¹ h⁻¹) in topsoil, as well as a large microbial P (MBP) value (25–28 mg kg⁻¹).Compared to mycorrhizal symbiosis (reflected by hyphal density), rhizosphere secretion of protons and alkaline phosphatase (rhizosphere physiology) and root length density (architecture) pose greater contributions to higher rhizosphere P availability and cotton P uptake. Moreover, the rhizosphere process and P use efficiency (PUE) of the P-efficient cultivar (XLZ19) were higher compared to the P-inefficient one (XLZ13).The results suggest that maximizing root/rhizosphere efficiency under optimal P input may improve cotton productivity and P uptake efficiency in mulched cotton fertigation systems in arid and semi-arid areas.