Symbiotic dinitrogen fixation is seasonal and strongly regulated in water‐limited environments

Abstract

Summary Plants, especially perennials, growing in drylands and seasonally dry ecosystems are uniquely adapted to dry conditions. Legume shrubs and trees, capable of symbiotic dinitrogen (N2) fixation, often dominate in drylands. However, the strategies that allow symbiotic fixation in these ecosystems, and their influence on the nitrogen cycle, are largely unresolved. We evaluated the climatic, biogeochemical and ontogenetic factors influencing nitrogen fixation in an abundant Mediterranean legume shrub, Calicotome villosa. We measured nodulation, fixation rate, nitrogen allocation and soil biogeochemistry in three field sites over a full year. A controlled experiment evaluated differences in plant regulation of fixation as a function of soil nutrient availability and seedling and adult developmental stages. We found a strong seasonal pattern, shifting between high fixation rates during the rainy season at flowering and seed‐set times to almost none in the rainless season. Under controlled conditions, plants downregulated fixation in response to soil nitrogen availability, but this response was stronger in seedlings than in adult shrubs. Finally, we did not find elevated soil nitrogen under N2‐fixing shrubs. We conclude that seasonal nitrogen fixation, regulation of fixation, and nitrogen conservation are key adaptations influencing the dominance of dryland legumes in the community, with broader consequences on the ecosystem nitrogen cycle.

The nitrogen economic spectrum of legume stoichiometry and fixation strategy

“…Furthermore, our analysis indicates that the maintenance of high nitrogen concentrations in the foliage of obligate fixers is associated with maintenance of high fixation, potentially at significant energetic and fitness costs (Gutschick, 1981; Hedin et al , 2009; Dovrat & Sheffer, 2019). These costs were not manifested under our experimental conditions, as the experiment did not include competition, or herbivory or scarcity of any resource other than nitrogen.…”

Drought sensitivity of an N2‐fixing tree may slow temperate deciduous forest recovery from disturbance

“…Although it is possible that this reduction in foliar N concentration was due to decreased mass flow or diffusion of N in dry soils, foliar N concentration did not decline for nonfixers along the same gradient, suggesting that a reduction in SNF rate is a more likely explanation. In fact, SNF rate can be correlated with foliar N concentration for R. pseudoacacia seedlings (Minucci et al 2017) and Mediterranean shrub species (Dovrat and Sheffer 2018). Our previous work demonstrates that drought effects on SNF by R. pseudoacacia can be dynamic at the scale of weeks and months, and can depend on drought intensity, duration, and frequency Miniat 2014, Minucci et al 2017).…”

“…In fact, SNF rate can be correlated with foliar N concentration for R. pseudoacacia seedlings (Minucci et al. ) and Mediterranean shrub species (Dovrat and Sheffer ). Our previous work demonstrates that drought effects on SNF by R. pseudoacacia can be dynamic at the scale of weeks and months, and can depend on drought intensity, duration, and frequency (Wurzburger and Miniat , Minucci et al.…”

Global macroecology of nitrogen‐fixing plants

“…Importantly, our results represent the potential for N fixation rather than the actual rate. There are many mechanisms that can inhibit N‐fixing plants from overcoming nitrogen limitation at local scales, including energetic constraints (Dovrat & Sheffer, 2019; Sheffer et al., 2015), co‐limiting nutrients (Yuan & Chen, 2015), ecological constraints (e.g. fire, preferential grazing; Cech et al., 2010), competing and pathogenic microorganisms (van der Heijden et al., 2016), and human impact (Vitousek et al., 2013).…”