Nitrogen-fixing red alder trees tap rock-derived nutrients.

Symbiotic nitrogen (N)-fixing trees supply significant N inputs to forest ecosystems, leading to increased soil fertility, forest growth, and carbon storage. Rapid growth and stoichiometric constraints of N fixers also create high demands for rock-derived nutrients such as phosphorus (P), while excess fixed N can generate acidity and accelerate leaching of rock-derived nutrients such as calcium (Ca). This ability of N-fixing trees to accelerate cycles of Ca, P, and other rock-derived nutrients has fostered speculation of a direct link between N fixation and mineral weathering in terrestrial ecosystems. However, field evidence that N-fixing trees have enhanced access to rock-derived nutrients is lacking. Here we use strontium (Sr) isotopes as a tracer of nutrient sources in a mixed-species temperate rainforest to show that N-fixing trees access more rock-derived nutrients than nonfixing trees. The N-fixing tree red alder (Alnus rubra), on average, took up 8 to 18% more rock-derived Sr than five co-occurring nonfixing tree species, including two with high requirements for rock-derived nutrients. The increased access to rock-derived nutrients occurred despite spatial variation in community-wide Sr sources across the forest, and only N fixers had foliar Sr isotopes that differed significantly from soil exchangeable pools. We calculate that increased uptake of rock-derived nutrients by N-fixing alder requires a 64% increase in weathering supply of nutrients over nonfixing trees. These findings provide direct evidence that an N-fixing tree species can also accelerate nutrient inputs from rock weathering, thus increasing supplies of multiple nutrients that limit carbon uptake and storage in forest ecosystems.