Phosphorus Loss from Ephemeral Gully Erosion.

Erosion from ephemeral gullies (EG) can be a major source of soil and phosphorus (P) loss from fields and can cause environmental damage. Because EG erosion removes soil from deeper in the profile, soil lost from EG erosion can have P sorption characteristics different than that of soil lost from sheet and rill erosion.  Therefore EG erosion can influence both the form (dissolved P vs. particulate P) and quantity of P leaving a field. However, very few studies have quantified the impact of EG erosion on dissolved and total P loss.

The objectives of this study are to i) determine the relative contribution of EG erosion to the total P loss from agricultural fields and ii) estimate the relative impact of EG erosion on the dissolved P concentration in runoff from agricultural fields. Soil samples from 2 fields near McPherson, KS were taken from landscape positions affected by EGs and in the contributing watersheds at 0 to 2, 2 to 5, 5 to 15, and 15 to 30 cm deep. Soils were analyzed for equilibrium P concentration at zero net sorption (EPC₀), anion exchange extractable P (AEP), and total P. Soil loss from the EG was determined using surveying methods and soil loss from sheet and rill was determined using the Water Erosion Prediction Project (WEPP) model.

Sediment loss was 0.6 and 9.8 Mg ha^-1 with 65 and 96% coming from EG erosion. Phosphorus loss was 0.09 and 1.96 kg ha^-1, with 48 and 93% coming from the EGs. Although much of the P loss was from EG erosion, EGs contributed relatively less P compared to sediment due to the P stratification and enrichment of surface soils eroded through sheet and rill erosion. Sediment lost from EG erosion had low EPC₀ and acted as a sink for dissolved P released from sediment eroded through sheet and rill erosion. Therefore, the estimated dissolved P concentration in runoff from the field with high EG erosion would increase from 0.0034 to 0.0204 mg L^-1, or by 600%, if soil loss from the EG erosion was eliminated.

Best management practices (BMPs) such as grass waterways that limit EG erosion could have a side effect of increasing dissolved P in runoff, which could increase P bioavailability. Dissolved P, being bioavailable, assists in aquatic plant growth and algae blooms which can lead to eutrophication within receiving waters. Therefore BMPs should be combined, such as grass waterways with conservation tillage and subsurface P application, to fully control P loss from agricultural field.