Phosphorus Sorption and Desorption From Ephemeral Gully Soils.

Phosphorus sorption and desorption from ephemeral gully soils

J. Coover and N.O. Nelson

Rationale

Erosion from ephemeral gullies can be a major source of soil loss from some fields.  The soil loss represents a deeper fraction of soil then from sheet and rill erosion.  This subsurface soil has differing characteristics from the surface soil that can affect phosphorus (P) sorption and can cause a change in the form of P leaving the field.  Best management practices that limit ephemeral gullies can impact the forms of P in runoff.

Objective

The objectives of this study are to 1.) determine the P sorption and desorption related soil parameters for ephemeral gully soils, and 2.) when soil depth fraction are mixed, determine the impact to equilibrium phosphorous concentration at zero point sorption (EPC₀).

Methods

Field samples of 3 fields near McPherson, KS were taken at 0-2, 2-5, 5-15, and 15-30 cm within and along ephemeral gully banks and channels.  The soils were tested for ammonium-oxalate extractable Al+, Fe and P, anion exchange resin P, equilibrium P concentration at zero net sorption (EPC₀), water soluble phosphorus, texture, pH, and total P.

Current Results

Preliminary results are showing a drop in EPC₀  within deeper soil factions within the three fields.  Am-Ox extractable Fe_ox and Al+_ox remains relatively unchanged through the profile while the oxide bound P decreases with depth.  Anion exchange resin P also decreases quickly with soil depth.  A surface and subsoil mixing test showed that EPC₀ drops quickly when subsoil is mixed with surface soil due to its high buffering capacity and low EPC₀.

Conclusions

Changes to field management that reduce the loss of subsurface soil can affect the sorbtive abilities of sediment being removed from field.  By reducing the amount of sorbtive subsoil within erosion sediment it is possible to increase the EPC₀ of eroded sediment and the soluble reactive phosphorus (SRP) in runoff solution, although reducing the total P losses.  SRP, being bioavailable, assists in aquatic plant growth and algae blooms which can lead to eutrophication within watershed basins