Soil Organic Matter Effects on Phosphorus Sorption: A Path Analysis

Abstract

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 (Smax) and selected physicochemical properties of soils, with particular emphasis on organic matter (OM) content. The Smax values were determined for 72 soil samples from the North Carolina Coastal Plain, along with pH, clay and OM contents, oxalate‐extractable P (Pox), Al (Alox), and Fe (Feox), and Mehlich 3 extractable P (PM3), Al (AlM3), and Fe (FeM3). Path analysis was used to examine direct and indirect effects of soil properties on Smax In the oxalate path analysis, the direct effects of clay, Alox, and Feox on Smax were significant in the order Alox > clay > Feox (P < 0.05). The Smax was highly influenced by the indirect effect of Alox and Feox through OM content. A two‐piece segmented linear relationship existed between Smax 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 AlM3 on Smax were significant in the order AlM3 > OM > clay (P < 0.05) while the direct effect of FeM3 on Smax was not significant. Oxalate may be better suited than Mehlich 3 as an extractant for predicting P sorption capacity in the Coastal Plain soils.