Nitrogen fixation and gas exchange response to evaporative demand in soybean

Atmospheric vapor pressure deficit (VPD) is increasing in many regions and
has a large impact on plant productivity. A VPD increase leads to raising
transpiration rate (TR) and soil-water demand, risking productivity
penalties. Like water, nitrogen is critical to productivity, but the
effect of VPD on legume nitrogen fixation is undocumented. To
address this, we developed a portable system for quantifying nitrogen
fixation noninvasively and at a high temporal resolution by tracking the
rate of hydrogen gas evolution by root nodules. Combining field and
controlled-environment experiments where we measured leaf gas exchange and
H2 production by nodules, we confirmed the ability of the system to track
nitrogen-fixation dynamics. Raising VPD from 0.5 to 3 kPa within
~2.5 hrs under well-watered conditions increased nitrogen fixation by up
to 25% in addition to TR, consistent with the hypothesis that raising VPD
in that range might have alleviated nitrogenase feedback inhibition.
Genotypic differences were found in this response, indicating a potential
for breeding. Our study provides evidence for an important
environmental effect on nitrogen fixation that is not taken into account
in current crop and vegetation models, pointing to untapped avenues for
better understanding climate change effects on legumes and nitrogen
cycling.