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size of the live biomass in broad-leaved humid forest, which amounts to 212 petagrams (Pg = 1015 grams) versus warm grasslands that have only 24 Pg live biomass. When comparing the amounts of soil carbon stored with carbon in live biomass, there is relatively less storage in the humid broad-leaved forest (156Pg), giving a ratio of 212:156, or 1.36, of live biomass to soil organic matter (SOM) (Fig. 8.1) (Anderson, 1992). Warm grasslands, with 213 Pg in soil organic matter, have a ratio of 24:213, or 0.11, in biomass versus that in the SOM. Tundra, with only 9Pg in live biomass versus 200 in the SOM, has a ratio of only 0.05 in living biomass versus SOM (Anderson, 1992). What are the climatological versus plant physiological and microbiological implications of such differences in these widely different biomes? Research in this area requires considerable effort in soil science and also microbial ecology, because we are faced with problems of measuring substrate quality, covered earlier in Chapter 5, and its feedback effects on future primary production and nutrient dynamics. Of course the modes of growth of grasses versus trees are also influential, because more of the total growth effort is invested belowground in both grassland and tundra soils.

Recent reviews have addressed key aspects of the terrestrial carbon cycle: carbon fixation by primary production, and then mechanisms for either sequestering the carbon during organic matter decomposition and transformation processes, or mechanisms for mineralization via human-induced or natural processes (e.g., Lal, 2002; Houghton, 2003). A central concern for ecologists and soil scientists is that soil organic carbon is the second largest pool in the terrestrial organic carbon cycle, with about 1550 Pg involved.

Concerns about imbalances in the global carbon cycle are not new; rapidly increasing amounts of CO2 entering the atmosphere from human activities, including burning of fossil fuels, were first noted a century ago (Arrhenius, 1896). Since then, interest in the rates of flow of carbon, and amounts sequestered in various pools in the biosphere, has waxed and waned. For example, Plass (1956) expressed concern about the amounts of CO2 being released by the burning of fossil fuels worldwide. An additional contribution to increased global CO2 is the relatively large amounts of soil organic matter being "mined" by extensive cultivation throughout the major "breadbaskets" of the world. In several regions, for example, the North American Great Plains, the former Soviet Union, and Canada, the loss is quite large, perhaps up to 40% of the surface layers (Haas etal, 1957; Wilson, 1978; Coleman etal., 1984). Mann (1986) concluded in a survey of 625 soils studied pairwise, cultivated versus noncultivated on the same soil type, that 20% or more of carbon was lost over decadal time spans from soils with high amounts of carbon (ranging from 6 to 16 kilogram per square meter). Interestingly, she noted that modest gains occur in soils that are initially very low in soil organic carbon, such as very sandy-textured ones, if they are put into cultivation. A significant amount of carbon fixation, and subsequent movement into the SOM, in the surface-to-30-centimeter (cm) depth, will occur over several years' time span. If extensive application of fertilizers is required to achieve these gains, then overall the global carbon balance is still toward the positive side, in terms of carbon costs for fossil fuel-derived nitrogen, for example (Vitousek et al., 2002).

In an extensive review of SOM models and global estimates of changes in soil organic carbon under a 2 x CO2 climate, Post et al. (1996) ran simulations with the Rothamsted model over a 100-year time period. They found that global soil organic matter change was only about one-third of the way toward an eventual equilibrium under the enriched CO2 regime. The change predicted with the United Kingdom Meteorological Office (UKMO) shows a small net sequestration of carbon in soil early in the climate transition period resulting from increases in tropical ecosystem soil carbon. However, this is followed by large carbon releases from arctic and boreal soils later in the century-long climate transition period. The largest net release of carbon from soil occurred at the end of the 100-year climate transition, after which the net releases decreased gradually as the soil carbon pools approached equilibrium under the double CO2 regime (Post et al., 1996).

Houghton (2003) noted that the carbon balance of the world's terrestrial ecosystems is uncertain. Several top-down (atmospheric) and bottom-up (forest inventory and land-use change) approaches are in use and difficult to compare, because they contain incomplete accounting inherent in their methods. After a brief discussion of the methods and their inherent limitations, we consider the possible resolution of the uncertainties arising from use of these methods. Of the top-down estimates, the first uses concentrations of oxygen (O2) and CO2 to partition atmospheric sinks of carbon between land and ocean. Using this assessment, terrestrial ecosystems were globally a net sink for carbon, averaging 0.2 (±0.7)PgCyr-1 and 1.4 (±0.7)PgCyr-1 in the 1980s and 1990s, respectively. The reason for the large increase between decades is unknown. A second top-down method is inverse modeling, which uses atmospheric transport models, together with spatial and temporal variations in atmospheric concentrations of CO2 obtained through a network of flask air samples, to infer surface sources and sinks of carbon. The budget will not reflect accurately any changes in the amount of carbon on land or in the sea if some of the carbon fixed by terrestrial plants or used in weathering minerals is transported by rivers to the ocean and respired or released to the atmosphere there. The two top-down methods based on atmospheric measurements yield similar global estimates of a net terrestrial sink of about 0.7 (±0.8)PgCyr-1 for the 1990s

(Houghton, 2003). Two bottom-up estimates have been used to estimate terrestrial sources and sinks over large regions: analyses of forest inventories and analyses of land-use change. One recent synthesis of forest inventories, which included converting wood volumes to total biomass and accounting for the fate of harvested products and changes in pools of woody debris, forest floor, and soils, found a net northern midlatitude terrestrial sink of from 0.6 to 0.7PgCyr-1 for the years around 1990 (Goodale et al., 2002, cited in Houghton, 2003). The estimate is only one-third of that calculated from atmospheric data corrected for river transport. Houghton (2003) noted that accumulation of carbon belowground, not directly measured in forest inventories, was underestimated and might account for the difference in estimates. Because the few studies that have measured the accumulation of carbon in forest soils have consistently found soils account for only a small proportion (5-15%) of measured ecosystem sinks, Houghton (2003) concluded that, despite the fact that soils worldwide hold from two to three times more carbon than does biomass, there is no evidence as yet that they account for a significant terrestrial carbon sink. The second sort of bottom-up estimate, analyses of land-use change, calculated that globally, all factors of land-use change averaged 2.0 and 2.2PgCyr-1 respectively, in the 1980s and 1990s (Houghton, 2003). In contrast to the unknown biases of atmospheric methods, analyses based on land-use change have deliberate biases built into them. These latter analyses consider only the changes in terrestrial carbon resulting directly from human activity. In other words, there may be other sources and sinks of carbon not related to land-use change, such as those caused by CO2 fertilization or changes in climate, that are considered by other methods but ignored in analyses of land-use change. The terrestrial sources and sinks of carbon in peta-grams of carbon per year as estimated by different methods are given in Table 8.2. (Houghton, 2003).

A major concern noted by Houghton (2003) was the unknown rate of turnover of carbon belowground. One of the major sources of carbon inputs in all terrestrial ecosystems has been attributed to fine roots. Uncertainties in estimates of root longevity have markedly hampered proper quantification of net primary production (NPP) and belowground carbon allocation, particularly in forests. In a comparison of fine root carbon inputs in two field sites, a hardwood forest (sweetgum Liquidambar styraciflua L.) in Tennessee and a loblolly pine (Pinus taeda) forest in North Carolina, Matamala et al. (2003) measured the carbon-13 (13C) isotopic signatures of live roots before and after carbon enrichment was applied in Free Air Carbon Enrichment (FACE) experiments. There was a marked difference in tree species, with mean residence time (MRT) in roots between 1 and 2 millimeters (mm) being 5.7 years for pine, and only 3 years for sweetgum roots of the same diame-

TABLE 8.2. Terrestrial Sources (+) and Sinks (-) of Carbon (PgCyr 1) Estimated by Different Methods

Region

Inversions based on atmospheric data and models

Analysis of land-use change

Forest inventories

Globe

-1.4 (±0.8)

2.2 (±0.8)

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