The fertilizing effect of actively accumulating dust - fractionation and within-soil distribution of soil phosphorus matters in a super-humid, high leaching environment, West Coast, New Zealand
Abstract
Pedogenesis in humid climates, as found in chronosequence studies, ultimately leads to nutrient depletion and enhanced formation of secondary, less plant-available forms of nutrients with progressing time. Deposition of mineral dust has been shown to be an important process to mitigate such depletion in soils and ecosystems (e.g. Hawaii, Amazon Basin). However, the magnitude of its effect has hardly been quantified and little is known about the accession pathways. This study exploits a unique geomorphic setting of a Holocene sequence of sand dune ridges. This sequence combines an active dust flux gradient along a 6500 y-old dune downwind of a braided riverbed and, distal from the dust source, a chronosequence across the dunes (170-6500 y B.P.). Across the chronosequence, pedogenesis is very rapid with Spodosols developing after 1000 y under a thick organic root mat. Total soil phosphorus (P) across the chronosequence declined by 75% within 6500 y due to strong leaching losses of apatite P (82% loss). This decline was mirrored by a similar rapid decrease of foliar P concentrations across the chronosequence. Along the dust gradient we investigated the potential mitigation of soil P loss through dust deposition. We quantified the dust flux (from soil silt content and dust traps), dust P content, soil P fractions (total P, organic P, apatite P, occluded P, non-occluded P), and foliar P concentrations. Dust deposition rates decreased following a negative logarithmic model (R2=0.90, P< 0.001) from 28.4 to 11.7 g m-2 y-1 within 600 m from the river bed, reaching detection limit beyond 900 m. Dust, approximated by sampling river silts, showed an average P concentration of 1114 mg kg-1 compared to the 410 mg kg-1 P of the unweathered dune sand. Despite supplying up to 205 g m-2 of P to parts of dune ridge over the last 6500 y, no predictable increase of soil total P or mineral P with increasing dust flux was observed for the upper 50 cm of the soils along the dust gradient. However, the fertilising effect and ecological significance of the dust flux was indicated by a positive correlation of dust deposition with a three-fold increase of foliar P concentrations and a two-fold increase of soil organic P while primary apatite P decreased more than 10-fold. In addition, dust deposition mitigated the strong P depletion of topsoils that is typical for dust-unaffected Spodosols in this area. These results point to enhanced cycling of dust P vs. dune sand-derived P presumably because of better accessibility: dust was added directly to the biologically active, densely rooted and strongly acidic topsoil (pH < 4). While such rapid and direct accession appeared to be beneficial for plant fertility, concomitant leaching losses caused the overall failure of dust to raise the net total P content in the upper 50 cm of the soil. The strong leaching of dust, already during transport, was indicated by the low P concentrations of dust collected in the three dust traps along the gradient, which showed P concentrations that are 66% to 92% lower than those of the river silts.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP42D..05E
- Keywords:
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- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions;
- 0439 BIOGEOSCIENCES / Ecosystems;
- structure and dynamics;
- 0486 BIOGEOSCIENCES / Soils/pedology