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SOIL CHEMISTRY

Soil Organic Matter Effects on Phosphorus Sorption: A Path Analysis

Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695

^* Corresponding author (jkang{at}ncsu.edu).

While P sorption in mineral soils has been extensively studied, P sorption behavior in organic-rich soils is less known. This study was conducted to determine the relationships between Langmuir P sorption maxima (S_max) and selected physicochemical properties of soils, with particular emphasis on organic matter (OM) content. The S_max values were determined for 72 soil samples from the North Carolina Coastal Plain, along with pH, clay and OM contents, oxalate-extractable P (P_ox), Al (Al_ox), and Fe (Fe_ox), and Mehlich 3 extractable P (P_M3), Al (Al_M3), and Fe (Fe_M3). Path analysis was used to examine direct and indirect effects of soil properties on S_max. In the oxalate path analysis, the direct effects of clay, Al_ox, and Fe_ox on S_max were significant in the order Al_ox > clay > Fe_ox (P < 0.05). The S_max was highly influenced by the indirect effect of Al_ox and Fe_ox through OM content. A two-piece segmented linear relationship existed between S_max and OM and the regression slope in soils with OM 49 g kg^–1 was 10-fold greater than that for soils with OM > 49 g kg^–1. This finding suggested that noncrystalline or organically bound Al and Fe in the soils with OM > 49 g kg^–1 is less effective for P sorption than in the soils with lower OM content. In the Mehlich 3 path analysis, the direct effects of clay, OM, and Al_M3 on S_max were significant in the order Al_M3 > OM > clay (P < 0.05) while the direct effect of Fe_M3 on S_max was not significant. Oxalate may be better suited than Mehlich 3 as an extractant for predicting P sorption capacity in the Coastal Plain soils.

Abbreviations: OM, organic matter • PM3, AlM3, and FeM3, Mehlich-3 extractable phosphorus, aluminum, and iron • Pox, Alox, and Feox, oxalate-extractable phosphorus, aluminum, and iron • S_max, Langmuir phosphorus sorption maxima

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