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Abstract:
In old and heavily weathered soils, the availability of P might be so small that the primary production of
plants is limited. However, plants have evolved several mechanisms
to actively take up P from the soil or mine it to
overcome this limitation. These mechanisms involve the active
uptake of P mediated by mycorrhiza, biotic de-occlusion
through root clusters, and the biotic enhancement of weathering
through root exudation. The objective of this paper is
to investigate how and where these processes contribute to
alleviate P limitation on primary productivity. To do so, we
propose a process-based model accounting for the major processes
of the carbon, water, and P cycles including chemical
weathering at the global scale. Implementing P limitation
on biomass synthesis allows the assessment of the efficiencies
of biomass production across different ecosystems.
We use simulation experiments to assess the relative importance
of the different uptake mechanisms to alleviate P limitation
on biomass production. We find that active P uptake
is an essential mechanism for sustaining P availability on
long timescales, whereas biotic de-occlusion might serve as a
buffer on timescales shorter than 10 000 yr. Although active P
uptake is essential for reducing P losses by leaching, humid
lowland soils reach P limitation after around 100 000 yr of
soil evolution. Given the generalized modelling framework,
our model results compare reasonably with observed or independently
estimated patterns and ranges of P concentrations
in soils and vegetation. Furthermore, our simulations suggest
that P limitation might be an important driver of biomass
production efficiency (the fraction of the gross primary productivity
used for biomass growth), and that vegetation on
old soils has a smaller biomass production rate when P becomes
limiting. With this study, we provide a theoretical basis
for investigating the responses of terrestrial ecosystems to P availability linking geological and ecological timescales under different environmental settings.
