Soil nitrogen regulates symbiotic nitrogen fixation in a legume shrub but does not accumulate under it

Legumes in dryland ecosystems face the challenge of maintaining energetically costly symbiosis with N₂-fixing rhizobia, in a water- and nutrient-limited environment. Controlled experiments showed a strong reduction in symbiotic N₂ fixation in response to elevated levels of nitrogen availability, but this regulation of N₂ fixation was not found in dryland field settings. Here, we ask whether regulation of N₂ fixation occurs in the field and what are the possible consequences for dryland soil nitrogen. We measured plant investment in root nodules and rates of bacterial activity, in seedlings and adults of a common N₂-fixing shrub, Calicotome villosa, in five field sites naturally varying in soil nitrogen and phosphorus availabilities. Additionally, we measured nitrogen concentrations and availabilities in the soil under C. villosa and a reference non-fixing shrub. Biomass investment in root nodules was significantly reduced in response to elevated levels of soil nitrate in adult shrubs, but no such effect was found for seedlings. Soil nitrogen concentration under C. villosa shrubs was low compared to the soil under a non-fixing reference shrub. We provide evidence that symbiotic N₂-fixing plants in drylands tightly regulate their investment in symbiotic N₂ fixation, therefore controlling nitrogen inputs into the soil. These findings challenge the long-held notion that the growth of N₂-fixing plants necessarily leads to the accumulation of soil nitrogen in their surroundings over time.