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

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.