Soil Organic Matter Effects on Phosphorus Sorption: A Path Analysis.

While phosphorus (P) sorption in mineral soils has been extensively studied, P sorption in organic-rich soils is less known. This study was conducted to examine relationships between Langmuir P sorption maxima (S_max) and selected physicochemical properties in soils varying in OM content. The S_max of 72 North Carolina Coastal Plain soils was determined and same soils were analyzed for pH, clay, OM, oxalate extractable P (P_ox), Al (Al_ox), and Fe (Fe_ox), and Mehlich-3 (M3) extractable P (P_M3), Al (Al_M3), and Fe (Fe_M3). Along with simple correlation analysis, path analysis was used to examine direct and indirect effects of soil properties on S_max based on oxalate and M3 extractions. In the oxalate path analysis, direct effects of clay, Al_ox, and Fe_ox on S_max were significant in the order of Al_ox > clay > Fe_ox while OM showed a unique indirect effect on S_max through metal-OM complexes. A positive relationship between S_max and OM up to 50 g OM kg^-1 shifted to a ten-fold lower linear regression slope for soils with OM ≥ 50 g kg^-1. This finding suggested that non-crystalline or organically bound Al and Fe in soils with OM ≥ 50 g kg^-1 are less accessible for P sorption than in soils with OM < 50 g kg^-1. In the M3 path analysis, direct effects of clay, OM, and Al_M3 on S_max were significant in the order of Al_M3 > OM > clay (p < 0.05) while direct effect of Fe_M3 on S_max was not significant due to a poor Fe extraction efficacy of M3. Oxalate extractable Al and Fe better accounted for the role of OM-metal complexes in estimating P sorption capacity.