Issues in Ecology Number 2 Spring 1997 required by plants.Hydroponic systems supply water recycling of nutrients-the fourth service soils provide- and nutrients to plants without need of soil,but the mar are two aspects of the same process.The fertility of error is much smaller of -that is.their ability to su ply nutrients to plant is largely the u of the activities of diverse spec of bacteria.fungl.algae,crustacea.mites,termites.spring ent concentrations,pH.and salinity of the nutrient solu tails,millipedes,and worms,all of which,as groups,play tion in hydroponic systems,as well as the air and solu- important roles.Some bacteria are responsible for "fix tion temperature,humidity,light,pests,and plant dis. ing"nitrogen,a key element in proteins,by drawing it eases.Worldwide,the area under hydroponic culture is out of the atmosphere and converting it to forms usable only a few thousand hecta and is unlikely to plants and. atol hu be ngs and other the Certain type of fungi play e about roles in supplying nutrients to many kinds of trees. 1993) worms and ants act as “mechani Third.soil plays a central cal blenders.breaking up and mix role in the decomposition of dead ing plant and microbial material and organic matter and wastes,and this other matter (enny 1980).For example,as much as 10 metric harmles y potential humar onne erial may pass pathogens.People generate a tre through the bo mendous amount of waste,includ on a hectare of land each year.re ing household garbage,industrial sulting in nutrient rich“casts”thal waste,crop and forestry residues enhance soil stability,aeration,and and sewage from their own popula- drainage (Lee 1985) tions and their billions st cated a key fac h the Earth's tion of the amount of dead organi lement cycle of carbon matter and waste (mostly agricul- nitrogen,and sulfur.The amount tural residues)processed each year of carbon and nitrogen stored in is 130 billion metric tons,about 30 soils dwarfs that in vegetation,for percent of which is associated with example.Carbon in soils is nearly human activities (derived fron double(1.8 times)that in plan Vitousek et al.6).Fort ure 9-Bacteria (Bradurhizobium jap and nit abou there is a wide array of decompos d3.550 times greater (Schles ing organisms-ranging from vul 1991).Alterations in the carbor tures to tiny bacteria- -that extract gen into a form that can be utilized by plants. and nitrogen cycles may be costly energy from the large,complex organic molecules found over the long term.and in many cases.irreversible on a in many types of waste.Like assembly-line workers,di- time scale of interest to society.Increased fluxes of car verse microbial species process the particular compounds bon to the atmosphere, such as occur when land is cor whose che mical bonds the ong to ed to a tonMany ndustrl wastes incun or when wet othe are dra co buildup of ke y greenhous gase namely carbon dioxide and methane,in the atmosphere pesticides.oil.acids.and paper.are detoxified and de (Schlesinger 1991).Changes in nitrogen fluxes caused composed by oroanisms in natural ecosystems if the con by production and use of fertilizer,burning of wood and centration of waste does not exceed the system's capac other biomass fuels and clearing of tronical land lead to ity to transform it.Some modern wastes.ho increasing atmospheric concentrations of nitrous oxide such as stics and the her nhouse ga that is also produc ctof the pesticide shield.Thes The simple inorganic chemicals that result from and other changes in the nitrogen cycle also result in natural decomposition are eventually returned to plants acid rain and excess nutrient inputs to freshwater sys. as nutrients.Thus,the decomposition of wastes and the tems,estuaries,and coastal marine waters.This nutrient 9 Issues in Ecology Number 2 Spring 1997 Figure 9-Bacteria (Bradyrhizobium japonicum) in a soybean root nodule cell, magnified 3,550 times. These bacteria fix atmospheric nitro￾gen into a form that can be utilized by plants.Photo by L. Evans Roth/Biological Photo Service required by plants. Hydroponic systems supply water and nutrients to plants without need of soil, but the mar￾gin for error is much smallereven small excesses of nutrients applied hydroponically can be lethal to plants. Indeed, it is a complex undertaking to regulate the nutri￾ent concentrations, pH, and salinity of the nutrient solu￾tion in hydroponic systems, as well as the air and solu￾tion temperature, humidity, light, pests, and plant dis￾eases. Worldwide, the area under hydroponic culture is only a few thousand hectares and is unlikely to grow significantly in the foreseeable future; by contrast, glo￾bal cropped area is about 1.4 billion hectares (USDA 1993). Third, soil plays a central role in the decomposition of dead organic matter and wastes, and this decomposition process also renders harmless many potential human pathogens. People generate a tre￾mendous amount of waste, includ￾ing household garbage, industrial waste, crop and forestry residues, and sewage from their own popula￾tions and their billions of domesti￾cated animals. A rough approxima￾tion of the amount of dead organic matter and waste (mostly agricul￾tural residues) processed each year is 130 billion metric tons, about 30 percent of which is associated with human activities (derived from Vitousek et al. 1986). Fortunately, there is a wide array of decompos￾ing organismsranging from vul￾tures to tiny bacteriathat extract energy from the large, complex organic molecules found in many types of waste. Like assembly-line workers, di￾verse microbial species process the particular compounds whose chemical bonds they can cleave and pass along to other species the end products of their specialized reac￾tions. Many industrial wastes, including soaps, detergents, pesticides, oil, acids, and paper, are detoxified and de￾composed by organisms in natural ecosystems if the con￾centration of waste does not exceed the system’s capac￾ity to transform it. Some modern wastes, however, are virtually indestructible, such as some plastics and the breakdown products of the pesticide DDT. The simple inorganic chemicals that result from natural decomposition are eventually returned to plants as nutrients. Thus, the decomposition of wastes and the recycling of nutrientsthe fourth service soils provide are two aspects of the same process. The fertility of soilsthat is, their ability to supply nutrients to plants is largely the result of the activities of diverse species of bacteria, fungi, algae, crustacea, mites, termites, spring￾tails, millipedes, and worms, all of which, as groups, play important roles. Some bacteria are responsible for fix￾ing nitrogen, a key element in proteins, by drawing it out of the atmosphere and converting it to forms usable by plants and, ultimately, human beings and other ani￾mals. Certain types of fungi play extremely important roles in supplying nutrients to many kinds of trees. Earth￾worms and ants act as mechani￾cal blenders, breaking up and mix￾ing plant and microbial material and other matter (Jenny 1980). For example, as much as 10 metric tonnes of material may pass through the bodies of earthworms on a hectare of land each year, re￾sulting in nutrient rich casts that enhance soil stability, aeration, and drainage (Lee 1985). Finally, soils are a key fac￾tor in regulating the Earth’s major element cyclesthose of carbon, nitrogen, and sulfur. The amount of carbon and nitrogen stored in soils dwarfs that in vegetation, for example. Carbon in soils is nearly double (1.8 times) that in plant matter, and nitrogen in soils is about 18 times greater (Schlesinger 1991). Alterations in the carbon and nitrogen cycles may be costly over the long term, and in many cases, irreversible on a time scale of interest to society. Increased fluxes of car￾bon to the atmosphere, such as occur when land is con￾verted to agriculture or when wetlands are drained, con￾tribute to the buildup of key greenhouse gases, namely carbon dioxide and methane, in the atmosphere (Schlesinger 1991). Changes in nitrogen fluxes caused by production and use of fertilizer, burning of wood and other biomass fuels, and clearing of tropical land lead to increasing atmospheric concentrations of nitrous oxide, another potent greenhouse gas that is also involved in the destruction of the stratospheric ozone shield. These and other changes in the nitrogen cycle also result in acid rain and excess nutrient inputs to freshwater sys￾tems, estuaries, and coastal marine waters. This nutrient