Soils store not only vast amounts of carbon, but also contain nutrients that plants need to grow, that are however mostly not accessible if they are bound in too large molecules.
Those molecules need to be broken down by microbial decomposers, by enzymes or be mobilized by other organic compounds excreted by plants or microbes.
Fine roots are small, but microbes are even much smaller, so that such exchange processes happen at very tiny scales (not so tiny if all the root system is taken together though). So, not surprisingly, this fine root - soil - microbial interface is quite a trade market, where carbon from plants to microbes and into the soil is exchanged in return for nutrients plants need to grow, to produce stems, leaves or invest into making fruits. In terms of nutrients the Amazon forest is quite an interesting place. Vast areas grow in rather 'low-fertility soils', so both plants and microbes need to apply strategies to avoid to become 'limited' by nutrients in their functioning. Such strategies are "expensive".
Both plants and microbes need carbon to grow and to maintain their activity, but they also need to invest carbon into acquiring nutrients - so we can hypothesize that a trade-off between those two strategies is needed. See for a synthesis of plant strategies in this paper published by Tatiana Reichert!
In June this year Laynara F. Lugli, Tatiana Reichert (both from TUM) and me made a trip to Manaus, Brazil, to get an idea of the plant investments and collected root organic acid exudation directly from living trees in the Amazon forest. Organic acids can mobilize one of the most important nutrients in this ecosystem - phosphorus - from the soil matrix making it available for plants and microbes. As one can imagine, finding and excavating roots and installing the collection tubes was strenuous work, and also required (A LOT OF) patience and care, in particular in this sometimes a bit challenging climate - not to mention some interesting interactions with tiny and not so tiny wildlife. It was however a real great team effort and a really lot of fun and amazing to work and be back again after more than two years to work in such a wonderful forest. Hope to be back soon to study root associated microbial properties.
Walking through the forest in the rain (Lucia Fuchslueger) Installing the collection tubes to the living roots (photos provided by Tatiana Reichert)
Read more about further exciting projects going on here in this blog:
"In a world with increasing atmospheric CO₂ concentrations, understanding if and how plants can supply their nutrient demand and maintain biomass growth is crucial. The more we learn about the interactions among plants, soils, and climate, the more our deep connection with the forests become clear. Knowing more about the complex and entangled root systems of Amazon forests could tell us about how we, humans, can affect essential Earth climate feedbacks."
and stay tuned for news from the AmazonFACE project investigating the impacts of elevated atmospheric CO₂ on a tropical rainforest, as well as from the AFEX and Imbalance-P projects investigating the role of phosphorus (and other mineral nutrients) on tropical forest functioning.