Forms of soil phosphorus affected by stand development of mountain beech (Nothofagus) forests in New Zealand

The biogeochemical stability of soil P is often assumed to be related to its solubility but there are few tests of this assumption. We determined differences in forms of soil P among stages of stand development in a replicated (N = 3) age sequence of Nothofagus stands developed after catastrophic disturbance by windthrow in New Zealand. Seedling, sapling, pole, and mature stages used were approximately 10, 25, 120 and > 150 years old, respectively. We hypothesized that insoluble soil P forms would not differ among stages of stand development.Organic forms of soil P depended significantly on stage of stand development. Concentrations of labile (NaHCO3 extractable) and non-labile (NaOH extractable) organic P were significantly greater in the mature stage than in sapling and pole stages. The concentration of occluded organic P (NaOH extractable following extraction with acid) was significantly lower in the seedling and pole stages than in the other stages. The concentration of inorganic P did not depend on stage of stand development except in the case of mineral P (HCl extractable) where the seedling stands had significantly lower concentrations of mineral P than the other stages. This was probably due to different topographical conditions for the seedling stands as compared to the other stands which may have affected long term soil processes such as weathering and leaching. The return of P in litterfall was similar among stages. The storage of P in stemwood was low in the seedling and sapling stages but high in the pole and mature stages whereas the storage of P in coarse woody debris was high in the seedling and sapling stages but low in the pole and mature stages. The storage of P in the forest floor was significantly higher in the pole stage than in the other stages which all had similar amounts of P in the forest floor. The differences in soil organic P forms among stages of stand development could be explained by above ground processes such as accumulation of P in stemwood and release of P from the forest floor and decomposing CWD. Labile and non-labile pools of soil organic P were correlated with the concentration of organic C whereas occluded organic P was unrelated to organic C. Our results suggest that neither labile, non-labile nor occluded organic P is stable during stand development.