Synergistic effects of humic acid and phosphate fertilizer facilitate root proliferation and phosphorus uptake in low-fertility soil

Background and aims Humic acid-enhanced phosphate fertilizer (HAP) is an efficient, widely used fertilizer in China produced by incorporating trace amounts of humic acid (HA) into conventional phosphate fertilizer (CP). However, the mechanism of its increased efficiency has remained unclear, limiting its optimization. Methods Maize (Zea mays L. cv ZD 958) was cultivated in root boxes to evaluate the effects of localized HAP, CP, or HA on biomass production, phosphorus (P) uptake, distribution of roots and available P in soil, and activity of C-acquiring and P-acquiring enzymes. Meanwhile, the effects of HAP, CP, and HA in fertosphere were isolated through their uniform application in confined growth tubes and quantifying root traits. Results HAP significantly increased available P content and soil area with high P content, and elevated the ratio of soil C- and P-acquiring enzyme activity. Total maize biomass and root distribution area treated with localized HAP were 12.9% and 29.7% higher than that with local CP application, and root biomass was positively correlated with soil available P. Uniform HAP supply significantly increased the maize biomass and P uptake compared to CP uniform treatments, and the biomass and P uptake were highly correlated with root length. HA alone had no impact on above indices. Conclusions The presence of HA in HAP increased the area and strength of P supply, and P in HAP guaranteed the adequate nutrient supply for maize enabling HA to stimulate growth. Thus, efficacy of HAP was resulted from the synergistic effects of HA and P fertilizer.