III.10 Nutrient Cycling and Biogeochemistry
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III.10 Nutrient Cycling and Biogeochemistry Peter M. Vitousek and Pamela A. Matson OUTLINE 1. Element cycles in terrestrial ecosystems 2. Global element cycles 3. Illustration: Nutrient cycling in practice Studies of nutrient cycles involve integrating information from very fine spatial and temporal scales (the dynamics of enzymes in the neighborhood of microbes) to very coarse scales (the global biogeochemical cycles); they involve integrating the dynamics of organisms with those of the environment that they inhabit and help to shape. Some of the finest-scale, most biological of processes (e.g., the growth of microbial populations on chemically recalcitrant plant litter) control important aspects of the Earth system (e.g., the persistence of nitrogen limitation to primary production, as in the example above). Nutrient cycles cannot be studied effectively in isolation, whether that means isolation from a consideration of both biological and geochemical processes or isolation from understanding the substantial and increasing influence of human activity on the Earth system. GLOSSARY biological nitrogen fixation. The enzyme-mediated reduction of atmospheric dinitrogen (N2) to chemical forms that can be used by most organisms. eutrophication. Overenrichment of ecosystems resulting from excessive additions of nutrients; eutrophication may create anaerobic conditions (‘‘dead zones’’) in aquatic ecosystems. mineralization. With reference to phosphorus and nitrogen, mineralization is the microbially mediated conversion of organically bound nutrients to soluble, biologically available inorganic forms. mycorrhizae. Mycorrhizae are a symbiosis between the roots of most higher plants and several groups of fungi, in which the fungal partner typically derives energy from the plant and the plant receives nutrients from the fungus. nitrification. The biologically mediated oxidation of ammonium (NH4) to nitrate (NO3); specialized microorganisms derive their energy from this transformation . nutrient limitation. Nutrient limitation occurs where the rate of a biological process like productivity or decomposition is constrained by a low supply of one or more biologically essential elements. weathering. The breakdown of rocks and minerals, at least partly into soluble and biologically available components. within-system cycle. Transfers of nutrients among plants, animals, microorganisms, and soil and/or solution, within the boundaries of an ecosystem. We define a ‘‘nutrient’’ as an element that is required for the growth of some or all organisms—and one that plants typically acquire from soil or solution (as opposed to the uptake of carbon from gaseous forms). The cycles of nutrients are interesting to ecologists for many reasons, including the following:  A low supply of a nutrient can constrain the growth and populations of organisms and the productivity, biomass, diversity, and dynamics of entire ecosystems.  Losses of nutrients from terrestrial ecosystems represent inputs to aquatic systems and to the atmosphere. In the atmosphere, reactive nitrogen gases influence atmospheric chemistry and climate ; in freshwater and marine systems, inputs of N and P can drive eutrophication (overenrichment ). Element losses thus represent a useful currency for evaluating land–water and land– atmosphere interactions.  The cycles of multiple elements are altered on regional and global scales by human activity. Much research in this area has focused on the global cycle of carbon, in part because of the importance of CO2 in the climate system, but humanity has altered the cycles of nitrogen, phosphorus, and sulfur to a much greater extent than that of carbon.  Substantial differences in the biology, geology, and chemistry of the cycles of different elements make attempts to integrate cycles on local, regional , and global scales both challenging and rewarding.  Just as element supply can shape the growth and distribution of organisms, organisms can affect the supply of elements by affecting inputs, outputs , or rates of cycling of nutrients. Ecological research in nutrient cycling and biogeochemistry has focused strongly on nitrogen and phosphorus for the good reason that of the many elements that plants and animals require, these two most often control plant growth, community diversity, and ecosystem-level processes such as productivity. They are not the only such controls; the supply of iron controls the growth and biomass of algae in large areas of the ocean, a low supply of potassium or sulfur can constrain the growth of plants (especially after nitrogen and phosphorus requirements are met), silica supply often regulates the growth of diatoms in lakes and ocean, and calcium availability has long been recognized as an excellent predictor of the distribution of plants and plant communities in many regions. Nevertheless, nitrogen and phosphorus control organisms and ecosystems across a very broad range of sites and conditions, and we focus on them here. 1. ELEMENT CYCLES IN TERRESTRIAL...