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Figure1.The global carbon cycle.All pools are expresd in units of1015gC and allfluxes in units of1015gC/yr,averaged for the1980s.Modified from Schlesinger(1997).
the estimates are roughly compatible with estimates of net primary produc-tion(NPP;Field et al.1998)and litterfall(Matthews1997)in the range of 50to60×1015gC/yr.Soil respiration is higher than NPP becau of the respiration of plant roots and mycorrhizae.Reprentatives from government and industry should realize that while thefluxes are large,the uptake and loss of carbon by land plants an
d soils were cloly balanced before human intervention.It is changes in theflux of CO2from human activities,including the disruption of soils,that play a role in the ri of atmospheric CO2and the potential for global warming.
It is not surprising that theflux of CO2from soils is cloly tied to plant growth,which supplies organic residues to decompors.Across major world biomes,Raich and Schlesinger(1992)show a direct relationship between soil respiration and NPP,with an r2=0.87.When organic carbon is added to soils, the rate of soil respiration increas(Gallardo&Schlesinger1994;Hogberg &Ekblad1996).The greatest rates of soil respiration are found in the tropics, where plant growth is luxuriant and the conditions are ideal for decompors.
所得税滞纳金怎么算Adjusting for the contributions from live root respiration,theflux of CO2 from soils indicates an overall mean residence time(mass/flux)of32years for carbon in soil organic matter(Raich&Schlesinger1992).However,this differs strongly between regions,and most models of soil carbon dynamics find it convenient to view the mass of soil organic matter as consisting of veral pools with different turnover times.Typically a small amount of detritus,consisting of fresh residues,is found near the soil surface,where
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9 the input from litterfall andfine root turnover is greatest.Larger pools of humic substances are found disperd throughout the soil profile,where they are often complexed with soil minerals.
Trumbore(1993)developed a4-compartment model of soil organic matter,with turnover times ranging from10to10000years,bad on the radiocarbon content of individual fractions.In postulating changes in theflux of carbon from soils,one must focus on changes in the labile pools near the surface.A sink for carbon in soil organic matter will appear most rapidly in the small pools with rapid turnover,while the immediate sink for carbon in humus is very small(Schlesinger1990).Similarly,an increadflux of CO2 from soils as a result of disturbance or global warming will largely derive from labile pools with the fastest turnover times.
Elevated atmospheric CO2
All other variables held constant,one would expect that the ri in atmo-spheric CO2,to the extent that it increas plant growth,should result in a greater delivery of plant debris to the soil,where a small fraction will remain undecompod and contribute to a sink for atmospheric CO2(Van Veen et al.1991).This process is perhaps aided by the fact that the largest increas in plant growth are often en underground,as a result of plant allocations to roots and root activities(Rogers et al.1994).Usin
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g a model with donor-compartment control,Harrison et al.(1993)suggested that the CO2stimulation of plant growth might explain about half of the“missing sink”for atmospheric CO2as a result of a greater storage of carbon in soils.
A fewfield experiments suggest that soil organic matter increas when plants are grown at high CO2(Wood et al.1994;Hungate et al.1997).We believe,however,that many recent estimates of the global sink for carbon in soils are overly optimistic,becau the microbial community in most soils is limited by the availability of organic substrates(Zak et al.1994).Give them more carbon,and the microbes will happily decompo it!The exception,of cour,is in the boreal forest,where cold temperatures inhibit decomposition and large quantities of organic debris accumulate in soils(Schlesinger1977).
Incread activity of the belowground microbial community was en when a grassland community in California was expod to elevated CO2for 3years(Hungate et al.1997).Theflux of CO2from the soil surface incread from323gC/m2/yr to440gC/m2/yr.Similar respons were en in a 15-year-old stand of loblolly pine,maintained under Free Air CO2Enrich-ment(FACE)in North Carolina;the concentration of CO2in the soil pore space and theflux of CO2from the soil surface both incread approximately 30%over values en in ambient conditions(Figure2).About30to50%
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Figure2.Concentration of CO2at30-cm depth in the soil of15-year old loblolly pine(Pinus taeda)plantations expod to550ppm atmospheric CO2,compared to that measured in reference stands at ambient CO2.Unpublished data of the authors.
of the soil respiration of CO2is derived from root activity and the remainder from soil microbes(Bowden et al.1993;Andrews et al.,in press).Thus,plant growth at high CO2may add additional carbon to soils,but most of it is likely to return to the atmosphere as CO2.
Elevated global temperature
If the Earth’s temperature ris due to the greenhou effect,we can expect soils,globally,to be warmer,especially at high latitudes.Except in some derts,soil respiration increas with increasing temperature–en both in compilations of literature values(Raich&Schlesinger1992)and in studies with impod soil warming(Peterjohn et al.1994;Christenn et al.1997; Rustad&Fernandez1998).The increa in soil respiration per10◦C ri in temperature–the Q10of the relationship–is about2.0(Kirschbaum1995; Palmer-Winkler et al.1996;Kätterer et al.1998).The greatest respon is found in samples of surface detritus and in soils from cold climates(Lloyd& Taylor1994;Nikli´n ska et al.1999).Root respiration is particularly responsive
11 to increas in temperature,showing a Q10as high as4.6(Boone et al.1998). Nearly all models of global climate change predict a loss of carbon from soils as a result of global warming(Schimel et al.1994;McGuire et al.1995).
Trumbore et al.(1996)suggested that the greatest loss of soil carbon would be en in tropical regions,where their measurements of radiocarbon content show a large pool of soil organic matter with a relatively rapid turnover time(cf.McGuire et al.1995).By themlves,however,radiocarbon measurements of turnover belie the situation in boreal forest and tundra habitats.As a result of cold,water-logged conditions,organic matter accu-mulates in the soils(Harden et al.1997;Trumbore&Harden1997). Radiocarbon measurements indicate limited turnover,but nearly all the organic matter is found in labile fractions that will be easily decompod should the climate warm(Chapman&Thurlow1998;Lindroth et al.1998). Indeed,Oechel et al.(1993,1995)found evidence of a large loss of soil organic matter in tundra habitats as a result of recent climatic warming in Alaska,and Goulden et al.(1998)found a significant loss of carbon from soils during veral warm years,which caud an early spring thaw in a boreal forest of Manitoba.In the tundra,melting of permafrost and concomitant lowering of the water table may lead to a large increa in decomposition (Billings et al.1983;Moore&Knowles1989).We believe that in respon to global warm
ing,the loss of carbon from soils will be greatest in regions of boreal forest and tundra,which have the largest store of labile organic matter and the greatest predicted ri in temperature.Large loss of CO2from the soils could reinforce the greenhou-warming of Earth’s atmosphere (Woodwell1995).
Elevated CO2and temperature
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Of cour,the most important scenario to understand–and unfortunately the scenario we know least about–is one with simultaneous increas in CO2and global temperature.Will soils be a net source or sink for carbon in the future conditions on our planet?Publicly,one of us has argued1that nature has performed this experiment for us:tropical rainforests have high NPP(as with higher CO2)and warm,wet conditions(as with most models of global warming),yet the carbon content of tropical soils is much smaller than that of the boreal region(Schlesinger1977;Batjes1996).Cebrian and Duarte(1995)find only weak correlations between the pool of soil organic matter and NPP across world biomes(Figure3).Apparently,large accumu-lations of soil organic matter do not derive from large inputs,but rather, soil organic matter accumulates where other ,temperature)limit decompors.As the planet warms,the area of temperature-limited decom-采取措施英语
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