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Phosphorus forms of podzolic soils of northern Vancouver Island and their use by western red cedar Cade, Barbara Jean


After clear-cutting and slashburning the hemlock-amabilis fir (HA) forest types of northern Vancouver Island support good growth, but the trees on the cedar-hemlock (CH) forest types suffer a growth check which can be overcome with N and P fertilization. This study focussed on soil phosphorus (P) fractions in CH and HA forests. Extraction methods were evaluated for total, organic and available P. The Parkinson & Allen digest was better than the Saunders & Williams ignition method for total P. Both the Saunders & Williams method and the Bowman & Moir extraction procedure overestimated organic P. The Bray PI and Mehlich 3 procedures were suitable for available P. Extracts of forest floor samples by NaOH, NaOH-EDTA and Chelex in both water and NaOH were analysed by 31P NMR spectroscopy. The NaOH-EDTA extracted the greatest portion of the total P and yielded spectra with a greater diversity of P compounds. However, this extractant also maintained other ions in solution which reduced the quality of the spectra. Evaluation of P status in relation to soil chemistry of mature CH and HA forests revealed that CH forests had higher pH values and C concentrations in the forest floor. The CH forests also exhibited higher loss on ignition, wider C/N and C/P ratios, and increased concentrations of extractable Ca in mineral horizons. The HA forests had higher C concentrations in mineral horizons and higher concentrations of N, extractable Mg, Al and Fe, and more organically complexed Al and Fe. There were no significant differences in P levels between the forest types. P-31 NMR spectroscopy showed a diversity of compounds, and organic forms throughout the profile. The persistence of labile diester phosphates and wide C/N and C/P ratios suggest slow decomposition. Comparison of the P status and soil chemistry of mature CH forests to those after burning revealed increases in pH, available P, inorganic P and extractable N, and decreased organic P postharvest. By 10 years postburn, significant reductions in organic P and organically bound Fe and Al were revealed in mineral horizons. P-31 NMR showed a shift to orthophosphate after burning, but a return to organic forms within 10 years in surface horizons. The results suggest that the burning of organic matter temporarily disrupts illuviation and the P cycle. In a pot study, cedar grown with high (50 mg P/2) or low (10 mg P/1) levels of phytic acid, ATP, glycerophosphate, pyrophosphate or KH2P04 showed adequate growth with all P forms but phytic acid, and grew best with the high rate of the P compounds. The poor growth with phytic acid was attributed to its binding of Ca, Zn and Cu. Utilization of organic P compounds was facilitated by various phosphatases, produced by cedar plants, mycorrhizae and/or rhizosphere microbes. Phosphorus did not appear to play an important role in the growth check problem of CH sites.

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