Temporal dynamics of gradients of phosphorus, ammonium, pH, and electrical conductivity between a di-ammonium phosphate band and wheat roots

Abstract

Our previous studies have demonstrated that di-ammonium phosphate (DAP) banded into soil led to a rise in pH and EC, and concentrations of NH₄⁺ and bicarbonate-extractable phosphorus around the band. When these chemical gradients develop too close to plant roots, they may cause toxicity and stop root growth. However, the temporal dynamics of gradient development and effects on plants are not known. In this study, wheat was grown in specially constructed pots, with DAP banded 2.5 cm below the seed and either 1 or 2.5 cm horizontally from seed. Plants were harvested at different growth stages to establish the temporal variation in soil chemical gradients between the fertiliser band and the plant roots.

Visual toxicity symptoms and decreased root and shoot dry weight and root length in the plants growing with DAP banded 1 cm horizontally away from seed were obvious 14 days after sowing, but disappeared after 6 weeks. Plants in that treatment had a significantly lower number of heads than control plants, implying that there was a harmful effect on reproductive growth and yield with DAP banded too close to wheat seed.

The pH increased by one unit and EC increased 2-fold in the soil section containing a DAP band 14 days after sowing. By Day 63, the pH peak disappeared and the EC peak in the 1-cm horizontal distance banding treatment shifted toward soil sections containing the highest root density (4 cm away from the seed on the side opposite to where fertiliser was banded). In the 2.5-cm banding treatment, the EC peak disappeared 42 days after sowing. The ammonium peak was measured in the soil section containing the fertiliser band in the 1-cm banding treatment on Day 14, but the peak shifted to the section with the highest root density by Day 42, and disappeared completely by Day 63. The 2.5-cm banding treatment resulted in a lower ammonium peak compared with the 1-cm treatment after 14 and 42 days, and the highest concentration was found in the section with the highest root density by Day 63. Phosphorus concentration gradients were similar to those of ammonium, except that less depletion and movement of P occurred during the 63-day experiment.

In conclusion, the chemical gradients around the fertiliser band show strong temporal and spatial dynamics. However, toxicity caused by close banding and hence high concentrations of ammonium in the initial growth stage results in a decreased wheat yield potential.