『Abstract
Results from chronosequences from the arctic to the tropic show
that phosphorus (P) availability, total P, and the fraction of
bedrock-derived P remaining in soil diminishes as soils age. Thus
we predict that ecosystems mantling oil substrates are likely
to have low available P. Yet there are myriad examples in the
biogeochemical literature where the results from chronosequences
are used to argue the reverse, and ecosystems observed to be P
poor are assumed to mantle an old substrate. This premise is difficult
to test, for while the concept of substrate age is useful on uneroded
surfaces that formed at a particular time, it becomes obscured
in denuding landscapes, where substrate ages instead reflect the
rates of rock weathering, denudation and mixing of dust into soil.
Here we explore this premise as it relates to one of the most
ubiquitous assumptions in the biogeochemical literature: that
the differences in nitrogen (N) and P cycling between temperate
and tropical regions are driven by gradients in substrate age.
We build a conceptual framework for quantifying the fraction of
parent material P remaining in soil ([SoilP]/[RockP]), by estimating
P inputs (rock weathering and dust deposition) and outputs (P
leaching). We parameterize our model with spatially explicit (0.5゜)
estimates of global denudation, weathering zone thickness, and
P deposition. To test the assumption that latitudinal gradients
in P status are the result of soil age, we apply a single P loss
rate, derived from a humid tropical system in the Hawaiian Islands,
to our spatially explicit map of soil residence times. Surprisingly,
in this formulation, we find only a modest latitudinal gradient
in soil P depletion, with mean depletion values in the humid tropics
<2× greater than in the previously unglaciated humid temperate
zone. This small latitudinal gradient in P depletion is unlikely
to be sufficient to drive the observed differences in tropical
vs. temperate ecosystem stoichiometry (e.g. trends in foliar and
litter N:P). Thus our results suggest that, to the extent P depletion
is greater in the tropics, the appropriate conceptual model for
attributing causation may not be one of a chronosequence where
time is the primary driver of P loss. We hypothesize that the
covariation of inferred P availability with latitude may be strongly
controlled by latitudinal changes in rates of P leaching and occlusion,
rather than gradients in substrate age.
Keywords: Phosphorus; Soil age; Chronosequence; Tropics; Soil
depletion; Weathering; Erosion; Denudation』
Introduction
Methods
Model formulation
Model parameterization
Caveats
Results
The effect of weathering zone thickness
The effect of dust inputs
The effect of P loss rate
Discussion
Acknowledgements
References