Lucia Fuchslueger

Supply mechanisms of Phosphorus in the Rhizosphere in tropical soils – interactions of plants and microorganisms

The Amazon rainforest, one of the largest tropical rainforests, stores large amounts of carbon in plant biomass and soils. It is an important sink for atmospheric CO2, and has been projected to offset increased emissions from anthropogenic fossil fuel combustion and land use change. Long term observation data suggests that the forest already is affected by climate change and tree mortality seems to outcompete productivity, yet exact mechanisms that are driving the dynamics are not clear. Similarly, model predictions of the response of the Amazon forest to global change factors, i.e. elevated CO2, warming or more frequent droughts remain highly uncertain. More accurate predictions are needed to fill lingering knowledge gaps on decision making on mitigation or adaptation actions to prevent major socioeconomic damage induced by a causal chain of climate change and forest degradation affecting transportation, food security and health of the entire Amazonian Region.

Apart from an overall high plant net primary productivity, in large parts of the Amazon Basin plants are controlled by soil phosphorus, although this has rarely been accounted for in ecosystem models. A recent comparison of model results revealed that the response of the Amazon forest to elevated CO2, and its capacity to sequester excess carbon from the atmosphere, indeed depended on the type of implemented phosphorus feedback mirroring multiple strategies by plants and their tight competing or cooperation with the soil microbiome to avoid phosphorus limitation. Yet, the scarcity of observations and small scales where plant-microbe interactions and microbe-mediated nutrient cycling are occurring hampers a quantitative upscaling of prevailing strategies. 

In SUP:RHIZE I want to fill these knowledge gaps by combining empirical with theoretical experiments. The overarching objective of SUP:RHIZE is to elucidate mechanisms of plant and microbial interactions facilitating phosphorus supply for plants and microbes and their consequences for ecosystem carbon cycling in a phosphorus impoverished tropical rainforest in Central Amazonia as a model ecosystem. By conducting empirical experiments using state of the art isotope and molecular methods to elucidate plant and soil microbial phosphorus cycling, and simultaneously allowing to explicitly inform model development, I am aiming to provide estimates of carbon costs of phosphorus supply to eventually improve the predictions and reduce the uncertainties of the Amazon forest’s carbon sink capacity in a future climate. 

More specifically, I aim to investigate major hotspots in phosphorus cycling, such as the fate of phosphorus during leaf and root litter decomposition and in the rhizosphere of plants. I want to identify and characterize major phosphorus fluxes and microbial groups being responsible for soil phosphorus mineralization, to investigate competition and cooperation between plants and microbes and consequences for microbial carbon turnover. I propose to investigate the form and distribution of inorganic and organic phosphorus available for plant and microbial uptake, and aim to identify and partition distinct plant and microbial strategies to access different phosphorus forms. SUP:RHIZE will, for the first time apply reverse microdialysis in situ to analyze phosphorus dynamics at the rhizosphere scale in tropical forest soils to trace the fate of labile plant carbon inputs in the soil. By avoiding bulk soil effects, the release of different carbon substrates and simultaneously collecting liberated phosphorus, will allow to differentiate between abiotic and biotic mineralization of phosphorus. With the proposed experiment ensemble in SUP:RIZE and by using a theoretical optimization model scheme, it will not only be possible to quantify major fluxes of phosphorus, but also to estimate the associated carbon costs of both plants and microbes and the consequences for a tropical soil carbon sequestration.