Nitrogen-fixing red alder trees tap rock-derived nutrients

Abstract

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.

Keywords: biogeochemistry; forest; mineral weathering; nitrogen fixation; strontium isotopes.

Fig. 1.

Percent rock-derived (A) Sr and (B) Ca in six tree species in a mixed-species temperate rainforest. Values are determined from two end-member mixing calculations that partition atmospheric and rock-derived sources (Sr, Eq. 1; Ca, Eq. 2). Tree species values are means and SEs from six mixed-species plots. Asterisks indicate that rock-derived nutrient sources are significantly greater in ALRU (N-fixing red alder) and significantly lower in TSHE (western hemlock) compared with other tree species (P < 0.001). Shaded areas span the full range of rock-derived Sr and Ca sources measured in the mineral soil exchangeable pool across the forest. See Table 1 for species codes.

Fig. 2.

Percent rock-derived Sr and Sr isotope values for individual trees (A) regressed against the mean % rock-derived Sr of all other species in each plot and (B) compared among species in each plot. In A, solid lines show linear regressions for ACMA, ALRU, PSME, THPL, and TSHE (all P < 0.05), indicating coherent tracking in Sr sources across the forest. In B, lines connect species across mixed-species plots, and the average of all six species is shown, as are individual mineral soil exchangeable Sr isotope values (n = 8) from a transect across the site.