Bedrock nitrogen contributes to nitrogen fertility and carbon storage across temperate forest ecosystems

Background/Question/Methods

Our previous work indicated that bedrock nitrogen (N) contributes to N fertility and carbon (C) storage in forests of South Fork Mountain (SFM) - northern California. The regional importance of this new N input is unknown, but given that rocks host large stores of reactive N  (more than 99% of all fixed N on Earth), identifying the extent to which this pathway contributes to N and C cycling is crucial for projecting terrestrial C storage in certain temperate and boreal ecosystems. In an attempt to determine the pervasiveness and importance of rock-derived N to forest fertility and C storage in northwestern California and southern Oregon, we employed two approaches: (i) we performed an analysis of C storage in aboveground tree biomass using data from the US-Forest Service, Forest Inventory and Analysis (FIA) plots (n = 82) for conifer forests of northwestern California on N-rich and N-poor bedrock regions to examine C by N interactions using a multiple linear regression model; and (ii) we measured total N and N isotopes of conifer foliage and bedrock from forests of northwestern California and southern Oregon to elucidate differences in total N and N isotopes of ecosystem pools on sites with and without N-rich parent material.

Results/Conclusions

After accounting for confounding climatic and edaphic factors, analysis of the regional FIA data indicates a substantial effect of bedrock N on C storage in aboveground forest biomass: tree C pools were 40% higher in forests underlain by N-rich rock than in sites with low bedrock N. Field data show that sites on N-rich parent material had elevated δ¹⁵N (0.69‰ ±1.3, mean ± 1σ) vs. sites with N-poor parent material (-2.31‰ ±1.3), and total N was elevated among sites with N-rich parent material (1.19% ±0.16) vs. N-poor parent material (1.04% ±0.15). Rock analysis shows that N-rich bedrock is found in many meta-sedimentary formations of northern California and Southern Oregon, including the Redwood Creek Schist, Condrey Mountain Schist, and Colebrook Schist formations, as well as parts of the extensive Franciscan formation. Taken together, these data provide evidence that rock-N contributes to increased N fertility and C storage across large areas of temperate forest.