Differences in phosphorus biogeochemistry and mediating microorganisms in the matrix and macropores of an agricultural clay loam soil

Abstract Phosphorus (P) derived from the application of fertilizers to agricultural land can often reach surface water bodies through tile drainage systems. Phosphorus fate and transport are dependent on the geochemistry and biological activity of both the soil macropore linings/walls and soil matrix. Macropores can be especially important contaminant transport pathways to groundwater and tile drainage networks. In this study, we investigated P geochemistry and the soil microbiome of a macroporous clay loam soil under corn and soybean cropping practices typical for eastern Ontario, Canada. We used spectroscopic techniques including P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, micro-X-ray fluorescence mapping, and micro-XANES for P speciation. We also used quantitative PCR to investigate the capacity of the soil microbiome to mobilize and transform organic (targeting the genes phoC, phoD, and phnX) and inorganic (targeting the gene pqqC) P pools. Phosphorus was retained in the soil predominantly as β-tricalcium phosphate and P sorbed to calcite. The microbial communities in both the macropore linings/walls and the matrix were functionally capable of transforming P from both organic and inorganic sources, with organic P cycling functions more abundant in surface soils and macropore domains and inorganic cycling P functions equally distributed throughout the soil. As a whole, the diverse biological capacity to cycle the different forms of P found in the soil represents a consistent source of crop-available P. The results of this study augment our understanding of the fate and transport and biogeochemistry of P in the matrix and the more highly transmissive macropores of agricultural soils.