Which nutrient is NOT primarily cycled in ecosystem ecology?
Sulfur is actively and importantly cycled in ecosystems, so the keyed claim that it is not primarily cycled is scientifically incorrect. Along with carbon, nitrogen, and phosphorus, sulfur participates in a major biogeochemical cycle linking organisms, soil, rocks, water, and the atmosphere. Plants take up sulfate and incorporate it into amino acids such as cysteine and methionine; food webs transfer organic sulfur, and decomposition mineralizes it. Microbes oxidize reduced sulfur compounds or reduce sulfate under anaerobic conditions. Weathering, sea spray, volcanic emissions, decomposition, and fossil-fuel combustion move sulfur among reservoirs. Carbon cycles through photosynthesis, respiration, and decomposition; nitrogen through fixation, nitrification, assimilation, and denitrification; phosphorus mainly through weathering, uptake, decomposition, sedimentation, and uplift. Every listed nutrient is therefore substantially cycled in ecosystem ecology. Some introductory treatments emphasize carbon, nitrogen, and phosphorus more often, but that curricular emphasis does not make sulfur non-cycling. As written, the item has no scientifically valid unique answer, and the key should be flagged rather than rationalized.
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 17