Environmental Problems, Their Causes, and Sustainability 1

miningFertilizerexcretionuptake byautotrophsGuanoweatheringuptake byautotrophsagricultureMarineFoodWebsDissolvedin OceanWaterleaching, runoffDissolvedin Soil Water,Lakes, RiversLandFoodWebsdeath,decompositiondeath,decompositionsedimentationsettling outweatheringMarine Sedimentsuplifting overgeologic timeRocksFigure 4-33 Natural capital: simplified model of the phosphorus cycle. Phosphorus reservoirs are shown asboxes; processes that change one form of phosphorus to another are shown in unboxed print. This cyclehelps keep you alive. (From Cecie Starr and Ralph Taggart, The Unity and Diversity of Life, 9th ed.,Wadsworth © 2001)in Figure 4-33. Trace the flows and paths in this diagram.With the exception of small particles of phosphatein dust, very little phosphorus circulates in theatmosphere because soil conditions do not allow bacteriato convert chemical forms of phosphorus to gaseousforms of phosphorus. The phosphorus cycle is slow,and on a short human time scale much phosphorusflows one way from the land to the oceans.Phosphorus is typically found as phosphate saltscontaining phosphate ions (PO 43)interrestrial rockformations and ocean bottom sediments. The weatheringand erosion of phosphorus-containing rocks releasesphosphorus into soil water, lakes, and rivers asphosphate ions, which are taken up by plant roots.Animals get most of the phosphorus they need fromthe food they eat. Decomposers break down organicphosphorus compounds in dead organisms into thesoil where it can be reused by plants.Phosphate can be lost from the cycle for long periodswhen it washes from the land into streams andrivers and is carried to the ocean. There it can bedeposited as sediment on the sea floor and remainfor millions of years. Someday geological uplift processesmay expose these seafloor deposits from whichphosphate can be eroded to start the cyclical processagain.Because most soils contain little phosphate, it is oftenthe limiting factor for plant growth on land unlessphosphorus (as phosphate salts mined from the earth)is applied to the soil as a fertilizer. Phosphorus alsolimits the growth of producer populations in manyfreshwater streams and lakes because phosphate saltsare only slightly soluble in water.How Are Human Activities Affectingthe Phosphorus Cycle? More Messingwith NatureWe remove large amounts of phosphate fromthe earth to make fertilizer, reduce phosphorusin tropical soils by clearing forests, and addexcess phosphates to aquatic systems.We intervene in the earth’s phosphorus cycle in threeways. First, we mine large quantities of phosphate rockto make commercial inorganic fertilizers. Second, wereduce the available phosphate in tropical soils whenwe cut down areas of tropical forests. Third, we disruptaquatic systems with phosphates from runoff of ani-82 CHAPTER 4 Ecosystems: What Are They and How Do They Work?