While nitrogen within terrestrial soils and vegetation has largely been thought to come from the atmosphere, a new study points to a previously underappreciated, additional source: weathered bedrock. Since nitrogen availability dictates plant growth, these findings have important implications for understanding the carbon cycle, in which plants are chiefly involved, and also for global climate change. In their study, Benjamin Houlton et al. note that the conventional belief is that nitrogen enters Earth's land-surface ecosystems mostly via the atmosphere, yet much nitrogen exists underground and is released as rock is weathered. To explore the extent of this nitrogen (N) release, the authors used three different approaches. First, they look at studies of N exchange among the atmosphere, biosphere, hydrosphere and geosphere at the planetary scale over geological time to construct an overall N budget. In a second approach, they examine a diverse array of geochemical proxies, including organic carbon in fossilized samples, in order to estimate how much carbon - and thus how much N - is released over time, arriving at value that aligns well with their first approach. Lastly, they use modeling to explore nitrogen fluxes. Based on their three analyses, the authors suggest that weathering of rock contributes 6 to 17% of the total terrestrial nitrogen budget, or 11 to 18 teragrams of nitrogen annually, an amount that rivals the level of nitrogen contributed from the atmosphere.
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Journal
Science