Evaluating colloidal phosphorus delivery to surface waters from diffuse agricultural sources

Area of Science:

• Soil Science
• Environmental Chemistry
• Agricultural Science

Background:

• Phosphorus (P) loss from agricultural soils is a major environmental concern.
• Understanding P transport mechanisms in different hydrological pathways is crucial for mitigating water pollution.
• Colloidal particles play a significant role in nutrient transport, including phosphorus.

Purpose of the Study:

• To investigate colloid-facilitated phosphorus delivery from agricultural soils.
• To develop and validate a method for assessing soil colloidal phosphorus.
• To determine the contribution of different particle-size fractions to phosphorus transport in runoff.

Main Methods:

• Laboratory and field experiments were conducted.
• A soil colloidal phosphorus test was developed, relating turbidity to colloidal P (H2O-CaCl2 extracts).
• Randomized plot experiments evaluated phosphorus transport in different particle-size fractions in agricultural runoff.

Main Results:

• A strong correlation was found between soil extract turbidity and total phosphorus (TP), and between turbidity and colloidal P.
• Fine clay (<2 microm) showed a weak positive relationship with the colloidal P fraction.
• Subsurface flow analysis revealed significant TP and reactive phosphorus (RP) transport associated with <2 microm and <0.001 microm fractions.
• TP concentrations in runoff were higher in P-amended plots, particularly for >0.45 microm fractions, suggesting fertilizer transfer via mineral particles and organic colloids.

Conclusions:

• Colloids are significant carriers of phosphorus in agricultural soils.
• Drainage water can mobilize colloids and associated phosphorus during rainfall events.
• Understanding particle-size fractions is key to managing phosphorus loss from agricultural landscapes.