Reviews and syntheses: Biological weathering and its consequences at different spatial levels – from nanoscale to global scale
Authors: Finlay Roger D. , Mahmood Shahid , Rosenstock Nicholas , Bolou-Bi Emile B. , Köhler Stephan J. , Fahad Zaenab , Rosling Anna , Wallander Håkan , Belyazid Salim , Bishop Kevin , Lian Bin 
Published: March 2020

Abstract. Plant nutrients can be recycled through microbial
decomposition of organic matter but replacement of base cations and
phosphorus, lost through harvesting of biomass/biofuels or leaching,
requires de novo supply of fresh nutrients released through weathering of soil
parent material (minerals and rocks). Weathering involves physical and
chemical processes that are modified by biological activity of plants,
microorganisms and animals. This article reviews recent progress made in
understanding biological processes contributing to weathering. A perspective
of increasing spatial scale is adopted, examining the consequences of
biological activity for weathering from nanoscale interactions, through in vitro and
in planta microcosm and mesocosm studies, to field experiments, and finally ecosystem
and global level effects. The topics discussed include the physical
alteration of minerals and mineral surfaces; the composition, amounts,
chemical properties, and effects of plant and microbial secretions; and the
role of carbon flow (including stabilisation and sequestration of C in organic
and inorganic forms). Although the predominant focus is on the effects of
fungi in forest ecosystems, the properties of biofilms, including bacterial
interactions, are also discussed. The implications of these biological
processes for modelling are discussed, and we attempt to identify some key
questions and knowledge gaps, as well as experimental approaches and areas
of research in which future studies are likely to yield useful results. A
particular focus of this article is to improve the representation of the
ways in which biological processes complement physical and chemical
processes that mobilise mineral elements, making them available for plant
uptake. This is necessary to produce better estimates of weathering that are
required for sustainable management of forests in a post-fossil-fuel
economy. While there are abundant examples of nanometre- and micrometre-scale
physical interactions between microorganisms and different minerals, opinion
appears to be divided with respect to the quantitative significance of these
observations for overall weathering. Numerous in vitro experiments and microcosm
studies involving plants and their associated microorganisms suggest that
the allocation of plant-derived carbon, mineral dissolution and plant
nutrient status are tightly coupled, but there is still disagreement about
the extent to which these processes contribute to field-scale observations.
Apart from providing dynamically responsive pathways for the allocation of
plant-derived carbon to power dissolution of minerals, mycorrhizal mycelia
provide conduits for the long-distance transportation of weathering products
back to plants that are also quantitatively significant sinks for released
nutrients. These mycelial pathways bridge heterogeneous substrates, reducing
the influence of local variation in C:N ratios. The production of
polysaccharide matrices by biofilms of interacting bacteria and/or fungi at
interfaces with mineral surfaces and roots influences patterns of
production of antibiotics and quorum sensing molecules, with concomitant
effects on microbial community structure, and the qualitative and
quantitative composition of mineral-solubilising compounds and weathering
products. Patterns of carbon allocation and nutrient mobilisation from both
organic and inorganic substrates have been studied at larger spatial and
temporal scales, including both ecosystem and global levels, and there is a
generally wider degree of acceptance of the “systemic” effects of
microorganisms on patterns of nutrient mobilisation. Theories about the
evolutionary development of weathering processes have been advanced but
there is still a lack of information connecting processes at different
spatial scales. Detailed studies of the liquid chemistry of local weathering
sites at the micrometre scale, together with upscaling to soil-scale
dissolution rates, are advocated, as well as new approaches involving stable
isotopes.