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nitrogen × 22atmosphere × 6
climate-change × 3
climate × 3
geochemistry × 2
earth-history × 2
soil × 2
agriculture × 2
oxygen × 2
geophysics × 1
oceanography × 1
planetary-science × 1
environmental-protection × 1
human-influence × 1
rain × 1
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biogeochemistry × 1
carbon × 1
air-quality × 1
soil-science × 1
ecology × 1
air × 1
laboratory × 1
acid-rain × 1
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