Plant nutrition as Earth System Science: understanding the links between plant nutrient gain and soil carbon storage.

The ways in which plants obtain nutrients from the soil, and especially nitrogen (N), are known to be important in determining how quickly soil organic matter (SOM) decomposes and therefore how much C is stored in soils. For example, plants and their associated symbiotic root fungi (mycorrhizas) are known to "prime" SOM decomposition by producing enzymes capable of degrading more resistant organic compounds. Recent evidence also suggests that the form in which plant take up nutrients, whether as inorganic (mineral) ions or as C-containing organic molecules, has an impact on rates of SOM decomposition. The problem is that whilst we know that complex plant-mediated effects on soil C storage exist, we do not understand them well enough to include them in models of the C cycle. Current knowledge suggests that plant nutrition may have a large impact on soil C storage in nutrient-limited environments such as the Arctic. Arctic ecosystems store large amounts of C – approx. 35-45% of total global terrestrial C is found in Arctic and permafrost soils. The future of Arctic soil C under warming conditions is highly uncertain and is a focus of NERC research (e.g. through the recent £5m Arctic Research Programme). There is a particularly pressing need to understand the interactions between plant nutrition and soil C storage in Arctic environments and this is where the fellowship activities will be concentrated. It has long been assumed that inorganic N ions provide most of the N that plants need but there is now growing evidence that organic forms of N also play an important role in sustaining plant growth. In addition to implications for the global C cycle, understanding organic N nutrition therefore has potential ramifications for crop production and food security. The relative importance of organic N to plants has been impossible to measure until now because existing techniques are limited by experimental artefacts. This fellowship proposes new isotopic methods that can quantify the importance of organic N to plants for the first time. These novel techniques include growing plants which are isotopically distinct from the soil, in order to trace the uptake of C-containing organic nutrients from the soil into plant tissues (Continuous Depleted Radiocarbon (CDER) labelling). Experiments will also be carried out at Arctic field sites, in which the isotopic signatures of plants and soils will be used to test the linkages between plant nutrient demand and SOM decomposition under different conditions. The proposed techniques will make it possible to quantitatively test the ways in which plant nutrition impacts decomposition in soils. This new information will then be used to update C cycle models to improve representation of plant nutrient uptake, and test the future impact of environmental change on soil C storage in the Arctic. The outcome of this project will be a significant improvement in our understanding of plant nutrition and of the vulnerability of Arctic C stocks to global change.