New pathways of plant nutrition and resource partitioning in terrestrial ecosystems: testing the ecological role of DON

Traditionally, the importance of inorganic nitrogen for plant nutrition in terrestrial ecosystems has taken centre stage. Recent studies, however, have started to challenge this, showing that plants can also use dissolved organic nitrogen (DON) in the form of amino acids. This growing awareness of the ability of plants to use DON, and compete with microbes in the soil for this pool of N, has led to radical rethinking about terrestrial N cycling and the processes that control N availability to plants. Another surprising outcome of this new thinking is that it has rekindled debates about the role of resource partitioning for species co-existence in plant communities. Indeed, it has been shown that some plant species differ in their ability to uptake different chemical forms of N, both inorganic and organic, which therefore allows them to avoid competition with their neighbours for soil N. If plants do this in the field, it would provide a mechanism for them to co-exist in natural communities, thereby contributing to the maintenance of plant diversity. Despite this growing awareness of the role of DON in terrestrial ecosystems, recent research has shown that its importance for plant nutrition and resource partitioning varies markedly across different ecosystems. This variability has led to the idea, which remains untested, that plant use of DON is of most importance for plant nutrition and species coexistence (i.e. via resource partitioning based on chemical form) in low productivity ecosystems where the relative availability of DON to inorganic N (DIN) is high, due to low of rates of microbial turnover of the DON pool. This is what we wish to test in this study, our overarching hypothesis being that the availability of DON to plants, and the ability of co-exisiting plant species to partition the soil N pool based on chemical forms of N, is greatest in low productivity ecosystems where DON dominates the soil N pool. We will take a holistic, field based approach to test this hypothesis, using a well characterised and ubiquitous gradient of grassland productivity, coupled with non-destructive stable and radioisotope tracer approaches that allow us to track the turnover and fate of components of DON in these ecosystems. We believe that our data will provide new insights into terrestrial N cycling and the role of DON in plant nutrition and resource partitioning, which is the basis for plant diversity.