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.
Bibliographical noteFunding Information:
We are greatly indebted to Levi Burrows, Chaim Engelen, Amital Haas, Yoni Waitz, and Hen Ron‐Shneiderman for all their help in the field and laboratory work. We also thank Jose Grünzweig and Avi Perevolotsky, for their help with the initial design of the field experiment, and two anonymous reviewers for their comments on the manuscript. This research was supported by the Israeli Science Foundation (ISF) grants no. 508/16 and 1142/19. MA, GD, and ES designed the study and experimental methodology; MA performed all experimental work and data analyses. TM conducted the gas chromatography, elemental, and colorimetric analyses. MA and ES wrote the manuscript, and all authors contributed to the final manuscript and gave final approval for publication.
© 2021 The Authors.
- Calicotome villosa
- facultative strategy