Phosphorus exchangeability and leaching losses from two grassland soils

Area of Science:

• Soil Science
• Environmental Chemistry
• Agricultural Hydrology

Background:

• Phosphate phosphorus (P) sorption in soils is strong, but preferential flow can enhance its transport.
• Flood irrigation intensifies preferential flow, potentially leading to significant solute losses.
• Understanding P mobility under irrigation is crucial for managing water quality and soil fertility.

Purpose of the Study:

• To investigate phosphorus losses induced by flood irrigation in a lysimeter study.
• To determine the role of preferential flow pathways in P transport and retention.
• To assess the influence of soil layers with varying P-fixing capacities on overall P mobility.

Main Methods:

• Conducted a lysimeter study simulating flood irrigation conditions.
• Performed detailed soil chemical analyses to assess P distribution and mobility.
• Utilized a dye tracer to identify preferential flow pathways and sampled soil accordingly.
• Analyzed soil organic carbon, P content, and the leaching of Al- and Fe-oxides.

Main Results:

• Phosphorus was mobile in the topsoil but restricted in the subsoil due to higher P-fixing capacity.
• Preferential flow areas showed higher exchangeable P, linked to organic carbon accumulation and reduced P sorption.
• Despite preferential flow, outflowing water had low P concentrations, indicating retention in deeper soil layers.
• The subsoil's high P-fixing capacity was the primary factor limiting significant P losses.

Conclusions:

• Preferential flow alone is insufficient to cause substantial phosphorus loss, even in P-rich topsoils.
• Subsoil P-fixing capacity plays a critical role in retaining phosphorus and preventing large losses.
• Assessing potential P losses solely based on topsoil mobility is unreliable and may underestimate retention.