Pedogenesis, soil mass balance, phosphorus dynamics and vegetation communities across a Holocene soil chronosequence in a super-humid climate, South Westland, New Zealand

This study quantifies progressive pedogenesis under a super-humid climate on the west coast of South Island, New Zealand. It focuses on soil morphology, pedogenic oxides, soil mass balance, phosphorus transformation and linking pedogenesis trajectories to vegetation communities. The study comprises a set of dune ridges of a coastal sand dune complex covered by unmodified conifer (podocarp)-angiosperm forest. The surface ages of the chronosequence range from 370 y to 6500 y. Rapid podzolisation is characteristic for the study area. Within 1000 y, soils reach the Spodosol stage with typical eluvial–illuvial horizons and mobilisation of Fe and Al. This period is also characterised by the most rapid losses of total phosphorus at a rate of 110 g m− 2 ky− 1, a relative loss of 41%. Beyond 3000 y changes in soil chemistry and losses for all nutrients markedly slow. Soil mass balance shows that the 6500 y soil has lost 75% total P, 62% K, 52% Ca and 54% Na. Soil P fractions substantially change across the gradient. High leaching losses of apatite and non-occluded P in the first hundreds of years coincide with accumulation of organic and occluded inorganic forms in the topsoil and subsoil, which mitigate total P loss. Beyond 1000 y of pedogenesis, all P fractions decline at similar rates to low, more persistent levels with apatite/non-occluded P being the dominant P form after 6500 y of pedogenesis. This incipient steady state is assumed to be sustained by the advection of parent material-derived P through surface lowering and reduced biological cycling. Vegetation communities change from more diverse communities on young and less impoverished soils in the first 1000 y to less diverse and less variable communities beyond 1000 y of ecosystem development. The soil evolution-correlated vegetation changes documented in this study are consistent with general schemes of vegetation succession for the west coast of South Island.

Research highlights
► Key transformations in soils occur within the first 1000 y of pedogenesis.
► Podzolisation can trigger gleying processes by forming drainage impeding horizons.
► Holocene leaching losses of nutrients are severe and not linear with time.
► Phosphorus fractionation data is not fully consistent with the Walker & Syers model.
► Pedogenesis and vegetation communities are strongly correlated.