Nutrient use efficiency and N:P stoichiometry are highly plastic in herbaceous N-fixing legumes
Global climate change, agriculture, and pollution are changing the distribution and availability of essential plant nutrients, including nitrogen (N). Because of their key role in driving ecosystem processes such as carbon sequestration, understanding how nitrogen-fixing plants will respond to changes in N availability is critical. While it is clear that N fixers generally have higher tissue N concentrations than non-fixers, the degree to which this is flexible based on N availability is not well known. Additionally, some work posits that N fixers have a higher P demand than non-fixers, with unknown flexibility in tissue N:P. Are nutrient ratios of N fixers hardwired, or do they just reflect, in general, their greater access to more N than non-fixers? Here, we examine biomass:N and N:P ratios of eight herbaceous legumes to understand how nutrient use efficiency and nutrient stoichiometry change across a realistic gradient of N supply. We subjected species to nine levels of N fertilization in a greenhouse; half of the individuals per species were inoculated with species-specific rhizobia, the other half remained uninoculated and therefore could not fix nitrogen.
All eight species we examined greatly increased their biomass:N (N use efficiency, NUE) in response to lower N availability, with NUE varying 2-10 fold within species. Uninoculated (non-fixing) conspecifics had significantly higher NUE except at high N availability, when N limitation had been relieved. Additionally, N:P was quite plastic across the N supply gradient, with non-fixing conspecifics showing very low N:P, generally below 5 (g/g), compared with inoculated individuals of the same species, which typically had N:P well over 10 and ranging up to 60. These results suggest that rather than being hard-wired for high N or P demand, the legumes we examined are highly flexible in their nutrient allocation and can adjust N use efficiency and N:P ratios in response to N supply.