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« في: فبراير 09, 2007, 01:20:28 صباحاً »
Tropical Rainforest Nutrients Linked To Global Carbon Dioxide Levels
Heba M. Naser
Department of Biology, University of Jordan.
.
Abstract. Tropical forests contain up to 40% of global terrestrial biomass carbon ©,
And they account for at least one-third of annual biosphere–atmosphere carbon dioxide (CO2) exchange and global soil organic C storage Because of their dominant role in the terrestrial C cycle, even small changes in tropical CO2 fluxes can modify the global C budget, climate, and atmospheric composition. Many studies showed that Extra amounts of key nutrients in tropical rain forest soils cause them to release more carbon dioxide into the atmosphere." when phosphorus (P) or nitrogen (N) -- which occur naturally in rain forest soils -- were added to forest plots in Costa Rica, they caused an increase in carbon dioxide emissions to the atmosphere by about 20 percent annually." This study will show the factors that affect these nutrients availability, and the way some Processes in the tropics affect what is happening around the globe knowing that human activities are changing the amount of phosphorus and nitrogen in ecosystems all over the globe, including the tropics.
Introduction
The tropical rain forest is a forest of tall trees in a region of year-round warmth. An average of 50 to 260 inches (125 to 660 cm.) of rain falls yearly. Rain forests belong to the tropical wet climate group. The temperature in a rain forest rarely gets higher than 93 °F (34 °C) or drops below 68 °F (20 °C); average humidity is between 77 and 88%; rainfall is often more than 100 inches a year. There is usually a brief season of less rain. In monsoonal areas, there is a real dry season. Almost all rain forests lie near the equator.( figure 1)
Nutrient Cycling in Tropical Forests
How can the high measured productivity of tropical forests be supported by soils of low fertility? The answer to this question was not firmly established until the 1960s and 1970s, when scientists began to measure the chemical content of tropical vegetation and compare it to that of the underlying soils. It was then discovered that the nutrients of tropical ecosystems are mainly to be found in living and recently dead organic matter—in the plants themselves and in the litter of decomposing plant parts that carpets the soil. See ( figure2).
That more nutrients are often contained in -plant matter than in the soil suggests that plants are recapturing dissolved minerals as they are released during decomposition. There are many organisms that are players in this decomposition process: termites, bacteria, fungi, various invertebrates. Of particular importance are micorrhizal fungi which invade the roots of trees to obtain nourishment.
Figure2: Nutrient budget in a tropical forest
In some circumstances tree roots even grow upward toward the soil surface, permeating the litter layer, for further nutrient uptake from litter and also for aeration. See (figure3).
(Figure3)
Tropical forests show some variability due to variation in rainfall , which will affect soil nutrients content , see ( table 1) after studying some of these forests scientists found that :-
*Soils of seasonally dry tropics are often more fertile than those of wetter regions .
* Poorest site has the highest root biomass .
*Richest site has the lowest root biomass.
Table 1
shows 7 tropical forests, arranged roughly from least to most fertile soils Ecosystem Characteristics in Tropical Moist Forests and Rain Forests
Oxisol Evergreen Dipterocarp rain
Amazon forest rain forest forest forest forest MOIST
San Carlos, San Carlos EL Verde, Banco, Pasoh, La selva. forest
Parameter Venezuela Venezuela Puerto Rico Ivory Coast Malavst Costa Rica Panama
1. Total calcium in soil 195 7 176 — 115 6530 22.166
(kilograms per hectare)
2. Total nitrogen in soil 785 1697 — 6500 6752 20.000 —
(kilograms per hectare)
3. Total phosphorus
in soil 36 243 — 600 44 7000 23
(kilograms in soil)
4. Root biomass 132 56 72.3 49 20. 5 14.4 11.2
(tons per hectare)
Carbon cycle
Tropical forests contain up to 40% of global terrestrial biomass carbon ©, And they account for at least one-third of annual biosphere–atmosphere carbon dioxide (CO2) exchange and global soil organic C storage Because of their dominant role in the terrestrial C cycle, even small changes in tropical CO2 fluxes can modify the global C budget, climate, and atmospheric composition. Soil carbon content depend on vegetation type, as forests have long life they will have larger carbon storage than cultivated areas and grasslands which have short term carbon storage.
Nitrogen (N) and Phosphorus cycles
Hypotheses Regarding N/P were collected from global data set including 5,087 observations of leaf nitrogen (N) and phosphorus (P) for 1,280 plant species at 452 sites and of associated mean climate indices demonstrates broad biogeographic patterns. In general:-
(i) Leaf N and P decline toward the equator as average temperature and growing season length increase.
(ii) The N/P ratio increases with mean temperature and toward the equator, because P is a major limiting nutrient in older tropical soils and N is the major limiting nutrient in younger temperate and high-latitude soils.
Effects of human activities on the nutrient cycle
Human activities are changing the amount of phosphorus and nitrogen in ecosystems all over the globe, including the tropics.
· Human activities responsible for Phosphorus additions to the system:-
v Slash-and-burn agriculture.
v crop fertilization activities
· Nitrogen pollution also is increasing around the world, including in tropical forests, as a result of :-
v Fossil-fuel combustion
v Crop fertilization activities
Many studies showed that when phosphorus or nitrogen -- which occur naturally in rain forest soils -- were added to forest plots they caused an increase in carbon dioxide emissions to the atmosphere by about 18 percent annually.
Terrestrial biosphere–atmosphere carbon dioxide (CO2) exchange is dominated by tropical forests, where photosynthetic carbon © uptake is thought to be phosphorus (P)-limited. In P-poor tropical forests, P may also limit organic matter decomposition and soil C losses. a field-fertilization experiment were conducted to show that P fertilization stimulates soil respiration in a lowland tropical rain forest in Costa Rica.(Cleveland and Townsend study)
In the early wet season, when soluble organic matter inputs to soil are high, P fertilization drove large increases in soil respiration. Although the P-stimulated increase in soil respiration was largely confined to the dry-to-wet season transition, the seasonal increase was sufficient to drive an 18% annual increase in CO2 efflux from the P-fertilized plots.
Nitrogen (N) fertilization caused similar responses, and the net increases in soil respiration in response to the additions of N and P approached annual soil C fluxes in mid-latitude forests.
After 3 yr of fertilization, the additions of P , N and +NP together the additions of P stimulated in situ soil respiration in the fertilization plots (Fig. 4a). In April, when soil respiration reached a seasonal maximum, CO2 fluxes were 37% higher in +P plots than in control plots .The most dramatic nutrient-stimulated increases in soil respiration occurred during this dry-to-wet season transition, but the seasonal, P-stimulated increase in soil respiration was still sufficient to drive an 18% increase in total annual CO2 flux in the +P plots relative to the controls; over the course of 2004, CO2 efflux from control plots was 1,880 g of C·m–2·yr–1, whereas +P plots respired 2,227 g of C·m–2·yr–1. These results corroborate those obtained in the laboratory assays; in both cases, P fertilization significantly enhanced soil respiration rates Together, these data provide strong evidence that P availability strongly limits soil respiration in this ecosystem and that P input to the soil may stimulate significant CO2 losses to the atmosphere.
time
(Figure. 4)
Another study conducted by Cleveland Townsend and Schmidt by Manipulations of carbon © and P supply, to test the effects of P availability on the decomposition of multiple forms of C including dissolved organic carbon (DOC) and soil organic carbon (SOC).in different soil types( Mollisol soil and Oxisol soil) by knowing that P availability in Mollisol soil is higher than Oxisol soil.
The results of these studies were as the following:
l Oxisol respond rapidly with ( Glu + P) addition.
l Mollisol respond the same with (Glu + P) addition.
So it’s the phosphorus availability that strongly limits soil respiration( figure 5)
In the substrate addition incubations, the two forest Sites displayed strikingly different responses to P Fertilization (Figure2aandb).Respiration rates in Soil from the P-poor (OF) site increased linearly(r0.96) following additions of Glu alone(Fig- ure2a). Typically, dividing populations during microbial growth result in exponential CO response curves (Coloresandothers1996); thus, the linear increase in CO following Glu addition suggests that microbial growth in this site is not solely limited by Carbon, as is most common in the microbial com-Mutiny (Wardle1992).In contrast, OF soil amended with both Glu and P
responded much More rapidly (Figure 5a), and CO evolution rates Increased exponentially (r0.99).FluxesofCO2 In the Glu-only additions reached a maximum and Occurred 32 h after the substrate addition; whereas OF soil fertilized with Glu P reached maximum CO production rates after20 h after the experiment P over the same time period. In the P-rich site (MF), substrate induced growth Respiration following additions of Glu alone and of Glu P was nearly identical in samples from each Treatment (Figure2b).Although the C P addition Caused a significantly higher maximum flux of CO 14 h after the incubation began,(Figure 5b) Exponential responses were elicited in both the Glu –only treatment (r0.99) And the Glu P treatment(r0.97).
Other factors increasing global warming
Cultivation of land causes large releases of carbon. The biomass carried by
Agricultural lands are generally much less than that of the previously growing natural Vegetation. Conversions from natural forest to cropland could reduce the soil carbon By up to 50% (GuoandGifford, 2002).Land use change will most likely add a considerable carbon source term to the results reported ,though the future Magnitude of this effect is difficult to estimate. It depends on many factors including Population growth, agricultural productivity, and consumption patterns..
Results of increased future CO2 concentrations
Regional analysis suggests that increased future CO2 concentrations alone will
Tend to increase NPP and enhance carbon storage in many areas, particularly in
The North. Increasing temperature leads to longer growing seasons, increased NPP And vegetation growth and hence enhanced carbon storage .A combination of the Two acts towards increasing NPP, growth and north ward plant migration, i. e .carbon storage in arctic zone and north ward migration of the tree line. Increased Temperature also leads to increased rates of heterotrophic respiration, as shown in field experiments (Fangetal.,2005),which supports the assumption that the Resistant soil carbon pools areas temperature responsive as the labile soil carbon pools.Knorretal.(2005)show that results of soil warming experiments can be well reproduced by an Arrhenius model(LloydandTaylor1994),supporting the
temperature –dependence of soil repiration used in the LPJ-DGVM. In a warming climate, the resulting effect will potentially dominate the carbon balance where soil stocks are large, particularly in the boreal zone.
References
1- Schaphoff S, Lucht W, Gerten D, Sitch S, Cramer W, Prentice C. 2006.
Terrestrial Bisphere Carbon Storage Under Alternative Climate Projections.
ClimaticChange DOI:10.1007/s10584-005-9002-5.
2-. Cleveland CC, Townsend AR, Schmidt SK. 2002. Phosphorus Limitation of Microbial Processes in Moist Tropical Forests Evidence from Short-term Laboratory Incubations and Field Studies. Ecosystems 5:680–691
3- Reich PB, Oleksyn J. 2004. Global patterns of plant leaf N and P in relation to temperature and latitude. The National Academy of Sciences
PNAS 101(30): 11001– 11006
4- DE Moraes RM, DelIitti WB, DE Vuono YS. 1999. Litterfall and litter nutrient
Content in two Brazilian Tropical Forests. Revista Brasileira de Botânica Print ISSN 0100-8404
5- Cleveland CC, Townsend AR. 2006. Nutrient additions to a tropical rain forest drive substantial soil carbon dioxide losses to the atmosphere. PNAS (The National Academy of Sciences) 10316-10321.
6- Townsend AR, Baswell BH, Holland EA, Penner JE. 1996. Spatial and temporal patterns in terrestrial carbon storage due to deposition of fossil fuel nitrogen. Ecological Applications. 6(3). 1996. pp 806 -814.
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