A new study shows that long-term drought in biodiverse Amazonian tropical rainforest can fundamentally change the soil, increasing the abundance of an unusual group of fungal species that may help the trees resist drought, and by altering the underlying biogeochemistry.

The activity of micro-organisms that occur in soil helps to supply water and nutrients to growing trees. In the Amazon this helps forests to absorb so much carbon dioxide from the atmosphere that the whole region acts as a globally significant brake on climate warming. These micro-organisms - bacteria and fungi - drive decomposition in soil, and sometimes also interact with plant roots, gaining carbon from the plants and providing them in return with better access to scarce resources, such as nutrients. However, despite their importance, we have almost no understanding of how these complex communities of micro-organisms will respond to climate change.

An inter-disciplinary team including researchers from Australia, Brazil and the UK has combined a new genetic analysis of microbial diversity and soil enzyme activities to test the effects of climate change on tropical rainforest soil. They used a unique long-term experiment designed to exclude half the rainfall from a hectare of Amazonian forest, an area large enough to contain about five hundred trees. The fourteen-year experimental drought treatment increased tree mortality strongly (eg, Rowland et al. 2015, Nature), but until now little was known about changes in the soil. The new measurements show that new species come to dominate the fungal community, some of which may make the trees resistant to drought and may also suppress soil pathogens. The data also suggest a switch in the biogeochemistry of the forest, whereby the main nutrient limiting the metabolism of the micro-organisms changes from phosphorus to nitrogen.

The dramatically named ‘dark-septate fungi’ found in the soil are thought to confer drought resistance to plants by protecting the roots from drought and by helping with the transport of water within the plant. The discovery of these mysterious fungi at the experiment is exciting because they have been largely overlooked in tropical forests globally and exist at very low abundance in the non-droughted part of the experimental forest site. Not only are they potentially associated with helping the trees that have survived the drought to maintain or increase their use of available water, but they may also be suppressing pathogen activity.

The team also measured the activity and abundance of different enzymes in the soil. Enzymes are released by micro-organisms to break down dead plant matter in the soil making nutrients available for plant and microbial growth. In the droughted forest, enzymes known to release nitrogen were more active relative to those releasing phosphorus or carbon, but the reverse was true for the undroughted forest. Many tropical rain forests are thought to have low availability of phosphorus in the soil, so this switch towards investing in acquiring nitrogen suggests that the demand for nitrogen is larger than for phosphorus in the droughted forest. The long-term consequences of this change in biogeochemistry are not fully understood but if widespread this could fundamentally change how strongly the forest grows and absorbs carbon dioxide in a future drier and warmer world.

The study, published in the Nature-group journal, ‘Communications Earth and Environment’ involved lead researchers from Brazil (Universities of Campinas and Brasília, the Emílio Goeldi Museum and the Federal University of Pará), Australia (Australian National University) and the UK (University of Edinburgh).

Post-doctoral scientist, Dr Erika Buscardo (Brasília), who was first author, said ‘This study underlines the fundamental importance of the symbioses between the tiny micro-organisms found in soil and the giant trees of the Amazon rain forest’.

Professor Patrick Meir (Australian National University) who leads the Amazon drought experiment with Professpr AC da Costa (Federal University of Pará), said ‘The long-term effects of climate change on the species diversity and growth of tropical rainforest may be very large, and we are privileged to study such a globally important forest in order to help future management and conservation’.

Dr Laszlo Nagy (Campinas) noted ‘Our study compared forest under normal rainfall and following long-term drought and it is these long-term differences that provide particularly powerful new insights’.

Buscardo, Souza, Meir et al. 2021. Effects of natural and experimental drought on soil fungi and biogeochemistry in an Amazon rain forest. Communications Earth & Environment (Nature group journal). https://doi.org/10.1038/s43247-021-00124-8