Red alder fix atmospheric nitrogen through a symbiosis with bacteria that colonize their roots. This study showed that when more nitrogen is produced than is needed by the plant, the resulting excess of nitric acid acts to dissolve bedrock minerals in the soil, making them available to plants.
The substantial increase in mineral weathering by N-fixing [nitrogen-fixing] alder helps explain how this species takes up 65% more P [phosphorus] and 200% more Ca [calcium] than non-fixing Douglas-fir. Enhanced access to P is most likely important to N fixers, and is used to increase photosynthetic tissue mass and N-fixing nodule production to support growth. … Ecosystem supplies of both P and Ca can limit nonfixer tree growth where N is abundant, including in our forests. Alder-enhanced uptake of rock-derived Ca and its subsequent redistribution via litterfall may especially benefit bigleaf maple and western red cedar, two nonfixers with consistently high Ca demands that have limited direct access to rock-derived nutrients [Perakisa & Pett-Ridge 2019: 5012].
This study suggests possibilities for increasing forests’ capacity to absorb carbon and mitigate climate change through the ability of red alder (and potentially other nitrogen-fixing trees) to make otherwise limiting nutrients, including nitrogen, phosphorus and calcium, available within the forest ecosystem.
Our finding that an N-fixing tree species can directly access rock-derived nutrients has implications for nutrient supplies that regulate tree growth and C uptake in forests. Inputs of fixed N can increase tree growth in N-limited forests, and could be further stimulated by access to rock-derived nutrients. Where N is already abundant and other nutrients are limiting, supplies of rock-derived nutrients can be even more important to forest growth and C uptake. It is presently unknown whether high rates of N fixation by trees are geographically widespread, and whether N fixers other than red alder can similarly access rock-derived nutrients [Perakisa & Pett-Ridge 2019: 5013].
Perakisa, Steven S. & Julie C. Pett-Ridge, 2019, Nitrogen-fixing red alder trees tap rock-derived nutrients, PNAS 116:11, https://www.pnas.org/content/116/11/5009.