assessment of changes in the phosphorus status of forest ...

(e) strongly bound occluded Pi forms within sesquioxides by strong HCl; (f) highly
resistant Pi in secondary and primary minerals.
The fractionation methods are very widely used with different levels of success to
assess P status of forest ecosystems. We present here the results of three case studies:
(a) Compton and Cole (1998) compared P cycling between Douglas fir and red alder
stands. Total ecosystem P in the alder stand was 69% of that found in the Douglas-fir
stand (inherit site difference), but the alder stand took up 61% more P annually than
Douglas-fir and also showed higher fluxes of P in litterfall (94%) and resorption
(292%). But all the P extractable fractions, which are meant to represent available soil
P, were lower under alder. They came to the conclusion that all static measures
(pools) of available P do not appear to adequately reflect P supply in these forest
stands.
(b) Binkley et al. (2000) investigated P dynamics in Eucalyptus and Albizia pure
species stands, and also in 1:1 mixture of the two species. A P fractionation technique
with a slight modification (Fe impregnated paper) was used instead of anion exchange
resin to obtain easily desorbed P fraction - solution Pi. Eucalyptus had higher
productivity, but both sites had similar amounts of P uptake at age 15 yrs. For most Pfractions
there was no effect of tree species except HCl-P, which was lower in Albizia
than Eucalyptus soil (HCO3-Po was high in Albizia soil but NaOH was high in
Eucalypts). However, solution Pi was high in Eucalypts.
(c) Richter et al. (2006) attempted to quantify changes in P fractions in an Ultisol
during the growth of an old-field pine forest from 1957-2005. Soil sample were
collected in 1962, 1968, 1977, 1982, 1990, 1997, and 2005 and different P fractions
and available P using the Mehlich III method were determined. The net transfer of P
from mineral soil into tree biomass and O horizons was 82.5 kg ha -1 over a 28 year
period (Table 10). However, this did not diminish the amount of labile soil P (anionexchange
resins, and NaHCO3 and Mehlich-III extractants). Substantial decreases in
slowly cycling Po and Pi associated with Fe and Al oxides and Ca compounds
occurred during 28 years of forest growth, and these accounted for much of the P
supplied to biomass and O horizons, showing the long-term dynamics of P in soils as
discussed above (Fig 11 ). Changes in soil P are small in this aggrading forest (2.9 kg
ha -1 y -1 over 28 yr).
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