Purpose: Legumes form root nodules to gain fixed nitrogen from rhizobia and can also access nitrogen in soil. Data suggest that plants might discriminate among these sources to optimize growth, but recognition of symbiotically fixed nitrogen and its regulation remain poorly understood. Methods: A greenhouse inoculation study manipulated the molecular form and concentration of nitrogen available using two Lotus japonicus genotypes and the nitrogen-fixing symbiont, Mesorhizobium loti. Plants were supplied with sole organic and inorganic nitrogen sources to simulate forms that plants might receive from symbiotic nitrogen fixation or from the soil. Host benefit from and regulation of symbiosis was investigated by quantifying symbiotic trait variation and isotopic analysis of nitrogen fixation. Results: Host growth varied in response to fertilization with alanine, aspartic acid, ammonium, and nitrate, suggesting differences in catabolism efficiency. Net benefits of nodulation were reduced or eliminated under all forms of extrinsic fertilization. However, even when symbiosis imposed significant costs, hosts did not reduce investment into nodulation or nitrogen fixation when exposed to aspartic acid, unlike with other nitrogen sources. Conclusions: L. japonicus can adaptively downregulate investment into symbiosis in the presence of some but not all nitrogen sources. Failure to downregulate any aspect of symbiosis in the presence of aspartic acid suggests that it might be jamming the main signal used by L. japonicus to detect nitrogen fixation.

Purpose: Crops rely on microbes for critical services, but host benefits can be influenced by local makeup of microbiota and the host’s capacity to select optimal strains. We investigated host benefits that cowpeas receive from microbiota depending on plant genotype, their domestication status, and soil source. Methods: We performed a full factorial soil inoculation experiment. Twenty diverse cowpea genotypes, selected from wild and domesticated populations, were exposed to soil rinsates from four agricultural sites across California, all having cowpea cultivation and varied physicochemical features. Cowpea investment in and benefit from microbiota was quantified by measuring host growth response to inoculation, nodulation, and segregating trait variation. Results: Variation in induction of root nodulation and strikingly heterogenous benefits to host growth were observed among soil sites. These effects were restricted to live soil inocula but were absent in autoclaved soil controls that lacked microbiota. Cowpeas expressed heritable variation in nodulation, but there was negligible effect of plant population or domestication status on the net benefit that hosts gained from microbiota. Conclusion: Soils varied substantially and consistently among cultivation sites and were the most prominent driver shaping host growth effects on cowpeas. While growth benefits vary among host cultivars, soil microbiota (and the conditions that maintain them) predominantly shape plant performance in agricultural settings.