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

Abstract

AbstractLegumes 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 N2fixation in response to elevated levels of nitrogen availability, but this regulation of N2fixation was not found in dryland field settings. Here, we ask whether regulation of N2fixation 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 underC.villosaand 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 underC.villosashrubs 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 N2fixation, 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.