Symbiotic nitrogen fixation (SNF) is an important source of nitrogen (N) in many terrestrial ecosystems, and may influence their responses to global change. However, our understanding of what constrains SNF is limited by a lack of experimental data. Theory and greenhouse experiments suggest light, water, and nutrients may influence SNF, nonetheless large-scale experiments exploring these effects are rare. Here we ask whether proliferation of root nodules (where SNF is carried out) in response to patches of nutrients is a proxy for SNF limitation at larger scales. For example, do trees capable of SNF have different responses to patches of soil phosphorus (P) when P is abundant versus limited? We hypothesized plants exposed to abundant P would have similar SNF responses across the root system, but when SNF is P-limited, roots exposed to a patch of P would increase SNF compared with roots not in a P-rich patch. We tested this using a split-root experiment with seedlings of Robinia pseudoacacia. Plants were grown at low and high P levels (n=30, n=29). Within each treatment, half of the root system received no P and the other half received P (at high or low levels depending on the treatment).
Results/Conclusions
We found strong evidence for P limitation on SNF. Plants receiving the high P treatment had significantly greater biomass, nodule biomass and N fixation activity compared with the plants in the low P treatment (p= 0.018, p<0.001, p=0.003). However, we did not observe different localized responses of SNF to patches of P nutrient availability. Neither treatment with roots in soil amended with P had greater nodule biomass or N fixation activity than roots in soil not amended with P (p=>0.25). This suggests that the P required for SNF is readily translocated throughout the plant. For example, roots in the high P treatment but which were in soil not amended with P still had higher nodule biomass than any roots in the low P treatment (p=0.016, p=0.041). Our study suggests that when P is abundant, seedlings translocate nutrients within the root system to allow for higher overall SNF. This suggests that exploration of nutrient limitation to SNF in situ will require manipulation of nutrients at the whole tree or large plot level.