The effectiveness of actively accumulating dust in rejuvenating soils and ecosystems in a super-humid, high weathering and leaching environment, West Coast, South Island, New Zealand

Abstract

The deposition of mineral dust is an integral component of terrestrial and marine biogeochemical cycles. Dust is recognised as an important rejuvenator by supplying nutrients and unweathered mineral material to ecosystems. While deposition rates have been regularly quantified for terrestrial environments in the past, the rejuvenating effect of dust on soils and terrestrial ecosystems and the accessional processes have been rarely studied. This thesis addresses this gap by combining two environmental gradients, a dust-free chronosequence (170 to 6500 y) and a dust accumulation gradient on a 6500 y old surface (28 g m⁻² y⁻¹ to zero), located on the super-humid West Coast of the South Island, New Zealand. Across both gradients, soils were analysed for indicators of pedogenesis pathways (soil morphology, fractionation of pedogenic oxides, phosphorus (P) and particle sizes), and ecosystem fertility was evaluated by describing vegetation communities and foliar nutrient concentrations. Across the chronosequence, pedogenesis is characterised by rapid podsolisation including formation of distinct eluvial and illuvial horizons, high leaching losses of nutrients (Ca, K, Na, apatite P) and a rapid increase of secondary P fractions. This is followed by declining rates of change after the formation of Spodosols by 1000 y of pedogenesis. This soil fertility pattern is reflected in the vegetation by the presence of fertility-demanding species until the 1000 y old surface followed by a more persistent and less diverse species composition on the older surfaces. Foliar phosphorus concentrations decline rapidly within 2000 y towards levels similar to very old surfaces. The chronosequence is in agreement with progressive pedogenesis and ecosystem evolution driven by declining edaphic fertility. The pedogenesis response to increasing dust flux along the dust gradient is characterised by enhanced podsolisation as a result of upbuilding pedogenesis, which constantly resupplies the acidic topsoil with weatherable materials and forms upwardly growing illuvial horizons. As a result, pedogenic oxides and soil organic carbon increase with dust flux. Chemically, all dust-affected soils along the gradient follow a strongly progressive pedogenesis vector (Spodosols). Soil morphology, however, indicates regressive pedogenesis under the maximal dust flux rate where an Inceptisol has formed, indicating that soils can contain characteristics of both pedogenesis pathways, progressive and regressive, depending on the chosen soil property. As a result of burial by dust increments, the original, unweathered and nutrient-rich parent material becomes increasingly decoupled from the soil processes. This causes a decline of primary, apatite P (~50 to 3 g m⁻² 50 cm⁻¹) as the majority of mineral, dust-derived P is rapidly assimilated by the biota after deposition on the biologically and chemically most active topsoil. This is ultimately returned to the soil as organic P and then stored in the upper, most reactive and leached part of the soil, which raises the P content in these increments when compared to dust-free Spodosols (95 vs. 60 g m⁻² 30 cm⁻¹). The increase of ecosystem fertility in correlation with these processes is reflected by an increase of fertility-demanding species with dust flux, which is, however, not capable of maintaining vegetation suites characteristic of earlier stages of dust-free succession. Also, foliar P responds positively to the dust flux by an increase of up to 100% under the highest flux rate. When compared to the chronosequence, this concentration is equivalent to those of a ~90% younger chronosequence surface. The rejuvenation process by dust deposition is fundamentally different to nutrient accession by subsoil advection in dust-free soils and more effective, as dust is added directly to the part of the soil with the highest nutrient demand. The increase of a more stable, yet plantaccessible organic P pool may have significance in maintaining a higher long-term fertility after dust deposition has ceased. Dust significantly rejuvenates and fertilises soils and ecosystems in the study area but does not act equivalent to a backwards arrow in time. It instead induces characteristic accession processes and soil and ecosystem properties.