Nutrient Cycles

Nutrient Cycles in Ecosystems

The cycling of nutrients can be studied at different scales from a few millimeters to the global scale. The most common scale at which nutrient cycling is studied is that of a small plot of 100 to 1,000 square meters or of a small catchment of a few hectares to square kilometers, which are considered as representative for a well-defined ecosystem or landscape. Ecosystem functioning is described by input fluxes, output fluxes, state, and transfer functions translating input into change of state and input and change of state into output. Methodologically, nutrient cycling in ecosystems is frequently assessed by budgeting nutrient stocks and fluxes.

Nutrient stocks are usually determined for different ecosystem compartments such as soil (various depth layers) and below- and aboveground biomass (sometimes further subdivided, e.g., into fine and coarse roots or stems, branches, and leaves) in terrestrial ecosystems.

Nutrient fluxes can be gaseous, water bound, or solid-phase associated. Gaseous fluxes include N2 fixation from the atmosphere; deposition of nutrient-containing substances, such as ammonia (NH3), oxides of nitrogen (NOx), sulfurdioxide (SO2), nitric acid (HNO3), sulfuric acid (H2SO4), and hydrochloric acid (HCl); and volatilization of NO, N2O, and N2 via nitrification and denitrifi-cation. Water-bound fluxes include incident precipitation, throughfall (i.e., the part of incident precipitation that falls through the canopy and reaches the soil), stemflow (i.e., the part of incident precipitation that flows via branches and stems to the soil), surface flow (on top of soil), internal soil fluxes (vertical and lateral), stream, and groundwater flow. While water itself can evaporate (via interception, i.e., direct evaporation from the vegetation surface, transpiration through plants, or direct evaporation from soil and water bodies), the nutrients contained in water cannot. Solid-phase-associated fluxes include dry particulate deposition from the atmosphere, litter fall (including fine litter, coarse litter, and root litter), soil erosion, and bedload and suspended matter in streams.

Nutrient Turnover

After biomass dies and returns as dead organic matter to soil, all contained nutrients are released via leaching and mineralization after mechanical degradation of the fresh litter by soil animals. Mineralization is particularly important for the nitrogen cycle (but to a lesser degree also for all other nutrients). Nitrogen mineralization consists of the release of ammonium (NH4+) from organic compounds (“ammonification”) and the subsequent oxidation of ammonium to nitrate (NO3-, “nitrification”) in several steps. Mineralization is a microbiologically mediated process. Mineralization rates depend on climatic conditions (temperature and water availability), chemical milieu (acidity and oxygen availability), and properties of the mineralized organic matter. Mineralization is fastest if the mineralizing microbial community experiences optimum conditions.

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