Nitrogen-fixing trees enhance access to mineral nutrients from bedrock

Excess ecosystem nitrogen (N) supply leading to N enrichment may result from atmospheric deposition, fertilizer loading, or biological N fixation. In particular, some trees that support symbiotic N-fixing bacteria in root nodules are known to support high rates of N-fixation leading to excess N supply. N enrichment, in turn, can shift nutrient limitation away from N toward non-N mineral nutrients such as phosphorus (P) and calcium (Ca). Terrestrial biogeochemists have theorized, therefore, that there may be a link between N fixation and accelerated cycles of Ca and P through mineral weathering.

Here we present the first field evidence that N-fixing tree species access more rock-derived nutrients than non-fixing tree species. We used radiogenic strontium (Sr) isotopes (⁸⁷Sr/⁸⁶Sr) to trace nutrient sources in a mixed species temperate rainforest, and found that the N-fixing tree red alder (Alnus rubra) have more access to rock-derived nutrients compared to five co-occuring non fixing trees, including two non-fixing tree species with high requirements for rock-derived nutrients. We calculate that increased uptake of rock-derived nutrients by N-fixing alder requires a 64% increase in weathering supply of nutrients relative to non-fixing tree species. The increased supply of rock-derived nutrients is likely a direct effect of the nitric acid generation through excess N availability from net nitrification.

Our finding that N-fixing trees can have enhanced access to rock-derived nutrients has implications for the global carbon cycle. By increasing rock weathering, N-fixing tree species can increase supplies of multiple nutrients that limit carbon uptake and storage in forest ecosystems. Nitric acid from excess N fertilizer and microbial nitrification can be a potent agent of mineral weathering in agricultural landscapes, providing a parallel example to forest N-fixation, wherein excess N enrichment may drive mineral weathering more than previously recognized. N-fixers are also speculated to affect the global carbon cycle on geologic timescales, but these effects are highly uncertain. Resolving how N-fixers drive weathering and C uptake more broadly requires new understanding to distinguish species like red alder that routinely fix excess N from other species that adjust N-fixation downward when needed.