『Abstract
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 on 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.
Keywords: Phosphorus; Phosphate; Preferential flow; Speciation;
Podzol; Forest soil』
1. Introduction
2. Materials and methods
2.1. Site description
2.2. Identification of preferential flowpaths and soil analysis
2.3. Analysis of phosphorus forms
3. Results
3.1. Types of preferential flowpaths
3.2. Oxalate-extracted phosphorus and trace element concentrations
3.3. Total, inorganic and organic phosphorus concentrations
3.4. Inorganic phosphorus fractions
3.5. Phosphorus forms by solution 31P NMR spectroscopy
4. Discussion
4.1. Occurrence and role of different preferential flowpaths
in the soil profile
4.2. Role of podzolisation in phosphorus leaching and transfer
4.3. Inorganic phosphorus forms and dynamics
4.4. Organic phosphorus forms and dynamics
5. Conclusions
Acknowledgements
References