|HONGTHANAT, NAJPHAK - Iowa State University|
|THOMPSON, MICHAEL - Iowa State University|
Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2011
Publication Date: 5/1/2011
Citation: Hongthanat, N., Kovar, J.L., Thompson, M.L. 2011. Sorption indices to estimate risk of soil phosphorus loss in the Rathbun Lake watershed, Iowa. Soil Science. 176:237-244.
Interpretive Summary: Livestock grazing in pastures has been identified as a possible factor contributing to sediment and phosphorus (P) delivery to Rathbun Lake and associated water bodies in southern Iowa. We collected seven representative soil materials from areas adjacent to pasture streams within the 143,000-ha watershed, and estimated the risk of P loss, using three P loss indices. In general, most of selected soils had low risk of P loss as indicated by soil test P via Mehlich-3 extraction (PM3) and degree of P saturation (DPS). However, equilibrium phosphorus concentration (EPC) values, which provide an estimate of P retention or release by soils and sediments, indicated that four of the seven soils could behave as sources by releasing P to the water if they become suspended in stream water. The risk was greater in summer months when P concentrations in stream water tended to be lower. Based on the DPS indices, we were able to divide the soils into low-, intermediate-, and high-risk categories. Soils in the high-risk category would be more susceptible to P loss, suggesting that more intensive management of these areas is required. Two of the seven soils were categorized as low risk by all three of the indices we used. We found that iron-containing minerals and organic matter in the soils had some control over the release of P to stream water from material eroded from these sites. The results of this work will contribute useful information to cattle producers, local environmental groups, and Cooperative Extension and NRCS personnel interested in reducing the negative effects of livestock grazing on water quality.
Technical Abstract: To rank and better understand the risk of P loss from potentially erodible soil materials in the Mollisol-dominated watershed of Rathbun Lake in southern Iowa, we sampled seven representative soil materials at four floodplain sites. We compared the samples by using a variety of characteristics and indices, including particle size distribution; total P, C, and N; P sorption indices;equilibrium P concentration; and degree of P saturation as assessed by dithionite, oxalate, and Mehlich 3 extraction methods. None of the Mehlich-3 P values of samples in the present study were high enough to suggest a high risk of water impairment due to P. Equilibrium P concentration (EPC) values ranged from 0.01 to 0.23 mg L-1. We found that the EPC values were significantly correlated with Fe extractable by oxalate or the Mehlich-3 extraction as well as with total C and total N. The oxalate and Mehlich-3 extractions provided generally consistent DPS indices, leading us to propose three general risk categories for these soil materials: low, intermediate, and high. We conclude that poorly crystalline Fe oxides and organic matter are likely to exert considerable control over the release of P to stream water from materials eroded from these sites. Moreover, risk rankings based on DPS values may not be consistent with interpretations of P mobility that are derived from EPC and sorption indices.