Increased forest ecosystem carbon and nitrogen storage from nitrogen rich bedrock

Rock-generated atmospheric nitrogen Bioavailable nitrogen commonly limits plant growth in natural ecosystems. In the absence of humans, this essential nutrient is thought to come mainly from the atmosphere through bacterial metabolism. Morford et al . present measurements of the nitrogen concentration and isotopic composition of rocks, soil and leaves to demonstrate that bedrock is a further, hitherto-overlooked source of bioavailable nitrogen to forests in the western United States. Furthermore, the forests growing above nitrogen-rich geology store appreciably more carbon than their impoverished counterparts. The authors suggest that this novel pathway might be a universal phenomenon. Nitrogen (N) limits the productivity of many ecosystems worldwide, thereby restricting the ability of terrestrial ecosystems to offset the effects of rising atmospheric CO 2 emissions naturally 1 , 2 . Understanding input pathways of bioavailable N is therefore paramount for predicting carbon (C) storage on land, particularly in temperate and boreal forests 3 , 4 . Paradigms of nutrient cycling and limitation posit that new N enters terrestrial ecosystems solely from the atmosphere. Here we show that bedrock comprises a hitherto overlooked source of ecologically available N to forests. We report that the N content of soils and forest foliage on N-rich metasedimentary rocks (350–950 mg N kg −1 ) is elevated by more than 50% compared with similar temperate forest sites underlain by N-poor igneous parent material (30–70 mg N kg −1 ). Natural abundance N isotopes attribute this difference to rock-derived N: 15 N/ 14 N values for rock, soils and plants are indistinguishable in sites underlain by N-rich lithology, in marked contrast to sites on N-poor substrates. Furthermore, forests associated with N-rich parent material contain on average 42% more carbon in above-ground tree biomass and 60% more carbon in the upper 30 cm of the soil than similar sites underlain by N-poor rocks. Our results raise the possibility that bedrock N input may represent an important and overlooked component of ecosystem N and C cycling elsewhere.