Widespread and ecologically meaningful rock nitrogen contributions to terrestrial ecosystems

Background/Question/Methods

The standard model of the nitrogen (N) cycle suggests new N enters ecosystems solely from the atmosphere, yet roughly 99% of all earth’s fixed N is in the geosphere. Rock weathering has been implicated as a primary N input pathway in some ecosystems, but its broader significance to terrestrial plants and soils remains uncertain. Here we investigate the regional distribution of N in rock reservoirs across the Pacific Southwest and the potential for bedrock to contribute meaningfully to ecosystem N subsidies. Our approach involved quantifying N in diverse rock types from across the region, using plant ¹⁵N/¹⁴N to investigate ecosystem N status, and applying mass balance techniques to quantify N removal from soil minerals.  First, we performed extensive sampling of geologic material - analyzing the N content of over 1000 geologic samples from 531 locations. In addition to rock analysis, we also collected foliage from 34 forests to contrast ¹⁵N/¹⁴N isotope values of rock and foliage at sites with and without substantial rock N reservoirs. Second, at three diverse forested sites in the Klamath Mountains we quantified N removal from minerals by combining classic methods for quantifying fixed-N in soil minerals with standard mass balance methods for quantifying chemical weathering.

Results/Conclusions

Our results indicate that N-rich parent materials are both common and widely distributed across terrestrial ecosystems, with fine-grained clastics (median = 680 mg N kg^-1) containing more N than coarse-grained clastics (240 mg N kg^-1) and carbonates (327 mg N kg^-1) among sedimentary substrates. In contrast, volcanic and plutonic protoliths contained little N (<100 mg kg^-1), indicating only minor contributions of rock N sources to ecosystems developing on igneous substrates. Foliar ¹⁵N/¹⁴N was positively correlated with rock N across our sites, with average foliar ¹⁵N/¹⁴N of -2.1‰ vs. 1.5‰ for sites with low rock N ( <500 mg kg^-1) and high rock N ( >500 mg kg^-1), respectively.  Applying these data to a simple model suggests that more than 30% of regional ecosystems may receive substantial subsidies from rock, especially on sedimentary substrates, where we calculate rock account for 15-70% of total N inputs. Furthermore, our mass balance analysis shows that rock sources can contribute 4.7 - 6.4 Mg N ha^-1 to aggrading forest vegetation and soils, accounting for as much as 80% of ecosystem N accumulation.  We conclude that rock N reservoirs are both widespread and ecologically meaningful in many different types of ecosystems that reside on sedimentary substrates.