Mechanism of middle and low molecular weight humic acids in promoting phosphorus fertilizer uptake efficiency and yield of winter wheat

Objectives This study investigated the effect of winter wheat yield and phosphate utilization efficiency applied with different molecular weight humic acid phosphate fertilizers, providing theoretical and data reference for the research and application of humic acid phosphate fertilizer.

Methods Humic acids with molecular weight > 100 kDa (high), 10–100 kDa (middle) and < 10 kDa (low) were made from weathered coal by ultrafiltration classification. Humic acid-phosphate fertilizers were prepared by adding mixed, high, middle and low molecular weight humic acids in phosphoric acid and KOH, recorded as PHA, PHA_H, PHA_M and PHA_L in turn, and the prepared P fertilizer without adding humic acid was used as control (P). Soil columns of 100 cm × 25 cm were used to carry out a wheat pot experiment, and the application amount of P₂O₅ in all the five treatments was 0.1 g/kg soil. After wheat harvest, the wheat yield, yield components were investigated, and the plant and grain P contents were measured, the contents of available P in 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm soil layers were analyzed.

Results 1) There was no significant difference in wheat grain yields among the HA, HA_H, HA_M, HA_L and CK treatments. Compared with common P treatment, the wheat yields in PHA, PHA_M and PHA_L treatments were significantly increased by 14.73%, 18.84%, and 21.37%, and there was no significant among the three treatments of humic acid-phosphate fertilizers, while the yield increase in PHA_H was not significant. The 1000-grain weight in PHA_L treatment was significantly higher than that in common P fertilizer treatment, those in the others were not significant. 2) Compared with common P fertilizer treatment, the P uptakes in wheat straw of humic acid-phosphate fertilizer treatments were not increased significantly, where the uptakes in wheat grain in PHA, PHA_M and PHA_L treatments were significantly increased by 14.97%, 19.45%, and 22.68%, respectively, while that in PHA_H was not significantly increased. 3) Compared with common P fertilizer treatment, the P partial productivity and agronomic efficiency in PHA, PHA_H, PHA_M, PHA_L treatments were increased by 14.71%, 6.01%, 18.82%, 21.35% and 14.95%, 1.66%, 20.18%, 23.03%, and the P recovery efficiency were increased by 2.93, 0.51, 4.52, 5.41 percentage points, respectively. Wheat grain yield and P recovery efficiency were positively correlated with alkyl carbon, carboxyl/carbon alkyl amide, alkyl carbon of humic acids, while negatively related to the aromatic carbon, aromatic C―O. 4) There was no significant difference in available P contents in 0–20 cm, 20–40 cm, 40–60 cm soil layers among PHA, PHA_H, PHA_M, PHA_L and P treatments.

Conclusions Under the condition of field soil column cultivation, when only humic acid is used, the wheat yield can not be increased obviously. Wheat grain yield and P recovery efficiency are positively correlated with alkyl carbon, carboxyl oxygen/carbon alkyl amide, alkyl carbon of humic acids. As the low molecular weight humic acid-phosphate fertilizer contains more active functional groups than the middle and high molecular weight ones, it is more effective in promoting the P uptake and yield, while the yield and P efficiency effects of high molecular one is not significant.