Matter Cycles

Objective: Trace evaporation, transpiration, condensation, and runoff.

Water moves among oceans, atmosphere, soils, groundwater, and organisms through evaporation, condensation, precipitation, runoff, infiltration, and biological processes including transpiration. Solar energy drives much evaporation, while gravity moves water through landscapes; water molecules are not created in each cycle step. Plants take up water and release some as vapor, linking water movement to gas exchange and photosynthetic conditions. Human changes to land cover can alter runoff and infiltration. A water-cycle diagram should specify spatial and temporal scale because flows and storage vary greatly among reservoirs.

Objective: Connect photosynthesis, respiration, decomposition, and storage.

Carbon moves among atmospheric and dissolved CO₂, living biomass, soils, sediments, and other reservoirs. Photosynthesis transfers carbon from CO₂ into organic molecules; respiration and decomposition return some carbon as CO₂, while burial and other processes can store carbon for much longer periods. Combustion of carbon-rich material can rapidly transfer stored carbon into the atmosphere. Carbon atoms are conserved across these transformations, but chemical forms and residence times differ. A measured net CO₂ flux reflects several processes at once, so gross photosynthesis should not be inferred directly from net exchange alone.

Objective: Explain fixation, assimilation, nitrification, and denitrification.

Although atmospheric N₂ is abundant, most organisms cannot directly use it as a nitrogen source. Nitrogen-fixing microbes convert N₂ into biologically available forms such as ammonia or ammonium; other inputs also exist. Nitrification by particular microbes converts reduced nitrogen forms through nitrite to nitrate under suitable conditions. Plants and microbes assimilate ammonium or nitrate into organic compounds, and consumers obtain nitrogen through food. Decomposers return organic nitrogen to inorganic forms, while denitrification can return nitrate-derived nitrogen to atmospheric gases. Pathways vary with oxygen availability and ecology; not every organism performs every transformation.