Soil Organic Matter Effects on Phosphorus Sorption: A Path Analysis

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 Smax were significant in the order Al-M3 > OM > clay (P < 0.05) while the direct effect of Fe-M3 on S-max was nor significant. Oxalate may be better suited than Mehlich 3 as an extractant for predicting P sorption capacity in the Coastal Plain soils.