Plant adaptation to soil nitrogen: the role of microbes versus the abiotic environment

Local adaptation is widely documented, but contributing selective agents are rarely identified or disentangled. Here, we test whether plants are adapted to their local soil nitrogen conditions and whether microbes mediate plant adaptation to soil nitrogen. By experimentally manipulating both nitrogen and soil microbial communities on 12 Amphicarpaea bracteata populations from across a naturally occurring soil nitrogen gradient, we show that plants that had evolved in high-nitrogen environments shifted resource allocation from mutualism (nitrogen-fixing rhizobia) to reproduction when grown in high nitrogen, while plants from low-nitrogen sites did not, suggesting that mutualism-related traits have diverged across the nitrogen gradient. However, we detected no evidence for plant local adaptation to soil nitrogen or sympatric microbes. We also failed to find evidence for microbe-mediated adaptive plasticity (the phenomenon in which microbes from a particular habitat promote plant fitness in that habitat); even though rhizobia from high-nitrogen sites produced more nodules than rhizobia from low-nitrogen sites in high contemporary nitrogen, this did not affect plant fitness. This study shows that despite the large ecological effects of nitrogen on plants and plant-microbe interactions, soil nitrogen and its effects on microbial communities may not always contribute to local adaptation.