Fertilisation of agricultural land causes an
accumulation of nutrients in the top soil layer, among
which phosphorus (P) is particularly persistent. Changing
land use from farmland to forest affects soil properties, but
changes in P pools have rarely been studied despite their
importance to forest ecosystem development. Here, we
describe the redistributions of the P pools in a four-decadal
chronosequence of post-agricultural common oak (Quercus
robur L.) forests in Belgium and Denmark. The aim was to
assess whether forest age causes a repartitioning of P
throughout the various soil P pools (labile P, slowly cycling
P and occluded P); in particular, we addressed the timerelated
alterations in the inorganic versus organic P fractions.
In less than 40 years of oak forest development,
significant redistributions have occurred between different
P fractions. While both the labile and the slowly cycling
inorganic P fractions significantly decreased with forest
age, the organic fractions significantly increased. The labile
P pool (inorganic ? organic), which is considered to be the
pool of P most likely to contribute to plant-available P,
significantly decreased with forest age (from[20 to\10%
of total P), except in the 0–5 cm of topsoil, where labile P
remained persistently high. The shift from inorganic to
organic P and the shifts between the different inorganic P
fractions are driven by biological processes and also by
physicochemical changes related to forest development. It
is concluded that the organic labile P fraction, which is
readily mineralisable, should be taken into account when
studying the bioavailable P pool in forest ecosystems.