Refining the Phosphorus Loss Assessment Tool for the Organic Soils of North Carolina
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Date
2006-12-08
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Abstract
Phosphorus (P) runoff and leaching from agricultural fields have been identified as major environmental concerns for the health of aquatic ecosystems. North Carolina has responded by implementing the Phosphorus Loss Assessment Tool (PLAT). The goal of the PLAT is to determine relative P losses from agricultural fields based on several site factors and characteristics, including Mehlich-3 P (M3P) soil test values. Based on previous research, the current version of PLAT is programmed to predict greater soluble P losses from organic soils than from mineral soils with the same M3P values. However, recent research specific to North Carolina's organic soils has indicated decreased soluble P release in the presence of high Al concentrations. Our objectives were to determine (i) the Al content of Typic Haplosaprists and Terric Haplosaprists of North Carolina's Lower Coastal Plain, and (ii) how the Al in these soils affects P retention. We sampled four organic soil series and determined M3P, Mehlich-3 Al (M3Al), Mehlich-3 Fe (M3Fe), water-soluble P (WSP), total P, pH, particle size distribution, and the organic matter content (OM). Water-soluble P and M3P were also measured in a 21-d incubation study in which P was added at a rate equivalent to 150 kg P ha-1. Total CuCl2 extractable Al and inorganic soil P fractions were identified in an Al and inorganic P fractionation study, respectively. According to the results of the incubation, multiple regression, and fractionation studies, Al was the main cation responsible for P retention; the mean topsoil M3Al concentrations (1926 mg kg-1) in these organic soils were much higher than those observed in another study of mostly mineral NC soils. The concentration of M3Fe was low in every series and was not correlated to any P characteristics. Mehlich-3 P was not consistently correlated to P retention and WSP; however, OM, M3Al, and total CuCl2 extractable Al were correlated with P retention. In the incubation study, the percentage of applied P that was adsorbed was greater in soils with lower OM and/or higher M3Al. Increased OM was associated with increased WSP and lower total P, as well as decreased P retention. The opposite effect was observed with increasing M3Al concentrations; however, the ratio of OM to M3Al showed increased correlation to P retention and WSP than when OM and Al were used alone. These results indicate that soils with higher OM and lower Al did not retain P as well as soils with lower OM and higher Al contents. Furthermore, as more Al bound P was extracted by M3P (causing higher M3P⁄Al-P %), WSP increased, and coincided with decreased total CuCl2 Al. The results from this study show that in high OM soils, the concentration of extractable Al controlled the solubility of P. North Carolina's PLAT could be modified to include M3Al concentrations to more accurately predict P losses in the organic soils of the lower coastal plain, thus potentially reducing P runoff and leaching into our aquatic ecosystems.
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histosols, organic soils, Phosphorus, phosphorus loss assessment tool, phosphorus leaching
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Degree
MS
Discipline
Soil Science
