Effects of soil phosphorus on root exudates in central Amazonia

Plants in tropical forests are thought to allocate a substantial portion of their photosynthetically fixed carbon (C) to the rhizosphere as exudates. These exudates serve multiple functions, including the mobilization of soil nutrients such as phosphorus (P), which is crucial for plant growth. In Amazonia, the predominant soils have notably low P concentrations, and plants likely employ a variety of strategies for P acquisition. However, the role of root exudates in P-impoverished Amazonian soils has not been empirically explored so far. To fill this gap, we investigated the largely uncharted territory of root exudation, as part of the Amazon fertilization experiment (AFEX), in a mature tropical forest growing on highly-weathered P-impoverished soils of central Amazonia. Our research examined root exudation in situ, both under natural soil conditions and P addition. We assessed the concentration of total organic carbon (TOC), total nitrogen (TN), and a suite of organic acids in root exudates, as well as additional root physiological and morphological traits of relevance, to potentially explain the variability in root exudation rates.

Our study revealed higher root exudation rates of TOC and organic acids in control, compared to P-addition plots, which suggests that plant C allocation to root exudates is an adaptive response to P availability. We also found that root exudation traits align with various morphological and physiological traits within the root economic space. Our findings provide insights into the hidden dynamics of root-soil interactions and have significant implications for understanding C cycling in tropical forests, shedding light on the complex coordination of root P acquisition strategies under different soil P conditions.