Phosphorus forms and related soil chemistry in preferential flowpaths and the soil matrix of a forested podzolic till soil profile

Preferential flow affects subsurface flow and solute transport processes in forested glacial till soils. Preferential flowpaths (PFPs) have been detected in forest soils, with higher microbial biomass, organic carbon content and increased nitrogen cycling compared with the soil matrix. However, the role of preferential flow in phosphorus (P) cycling in forest soils is still unknown. This study characterised the P forms of a forested podzolic till soil profile in Southern Finnish Lapland using P extraction and ignition procedures, fractionation and solution 31P nuclear magnetic resonance (NMR) spectroscopy. PFPs were identified by introducing the dye tracer Acid Blue 9 at the surface of a 1.25 m2 study plot. The soil profile was vertically sliced and samples were collected from the unstained soil matrix and the dye-stained PFPs of each horizon of the podzol. Amorphous metal sesquioxides, naturally bound P (Pox), total P (Pt), inorganic P (Pin), organic P (Po), different inorganic P fractions (soluble P, Al–P, Fe–P, Ca–P, occluded P) and organic and inorganic P forms (31P-NMR spectroscopy) were analysed in the samples. The concentrations of different forms of P and amorphous metal sesquioxides varied between the different PFPs and the soil matrix within the podzol soil horizons. The PFPs in soils on stone surfaces contained less amorphous metal sesquioxides, Pox and Pt than the other flow regions. The PFPs related to coarse grains and roots contained equivalent or higher Pox and Pt concentrations than the soil matrix. Inorganic P was present as orthophosphates and pyrophosphates, and was mainly bound with Al and Ca oxides in PFPs, but also with Fe oxides in the soil matrix. PFPs contained more occluded P than the soil matrix, especially on stone surfaces. The results indicate increased mobilisation and P losses in PFPs on stone surfaces due to enhanced mineral weathering, root uptake and leaching of P adsorbed by the sesquioxides. The results of the 31P NMR study also showed higher proportions of pyrophosphate, orthophosphate diesters and monoesters other than phytic acid in the PFPs on stone surfaces than in the other flow regions, indicating higher amounts of labile forms of inorganic and organic P on stone surfaces than in other flow regions. To conclude, PFPs seem to have a twofold role in P dynamics: P can leach through PFPs on stone surfaces and accumulate on PFPs related to coarse grains and roots.