Hawaiian chronosequence

Brimhall et al. (1991) used lead isotopes in zircons within a bauxite profile from Western Australia to differentiate between zircons derived from the underlying bedrock and zircons of eolian origin. Borg and Banner (1996) applied both neodymium and strontium isotopes to constrain the sources of soil developed on carbonate bedrock. Using these isotopes and Sm/Nd ratios, they were able to delineate the importance of atmospheric versus bedrock contributions in controlling the composition of the soil. Kurtz et al. (2001) used neodymium and strontium isotopes to determine the amount of Asian dust in a Hawaiian soil chronosequence. They found that the basaltic bedrock isotope signatures in soils had, in many cases, been completely overprinted by dust additions, demonstrating the profound effect of Asian dust on soil nutrient supplies. 

Figure 10 The Sr/ Sr ratio of the soil-exchangeable pool and foliage for a chronosequence of soils developed on Hawaiian basaltic lava flows. Also plotted is the result of a calculation of the percent weathering contribution needed to explain the Sr/ Sr ratio— assuming that all Sr is derived from basaltic weathering and marine atmospheric deposition. Sr isotopes clearly demonstrate the transition of the forest ecosystems from weathering dominated Sr to atmospherically dominated Sr as soils mature and easily weathered Sr is removed from soils (source Kennedy et al., 1998).

Across a chronosequence of soils on the Hawaiian islands (Crews et al., 1995), Tom et al. (1997) found that both the quantity of stored carbon and its turnover time correlated with the noncrystalline (allophane, imogolite, and ferrihy-drite) mineral content of the soil (Figure 30). These amorphous minerals possess a unique geometry with a very high surface area (Table 13) which facilitates the formation of highly stable bonds with SOM (Oades, 1988). 

Figure 30 Content of (a) organic C, (b) A C, (c) noncrystalline minerals, and (d) crystalline minerals versus age along a chronosequence of soils on the Hawaiian Islands. Filled circles represent total soil profile and filled triangles represent surface (O and A) horizons only (Torn et al., 1997) (reproduced by permission of Nature Publishing Group from Nature, 1997, 389, 170-173).

Fig. 15.6. CENTURY model-simulated results for soil phosphorus loss (organic and inorganic phosphorus) total nitrogen loss (nitrate, gaseous nitrogen and dissolved organic nitrogen) and nitrate loss (a) and change in live leaf carbon-to-nitrogen and carbon-to-phosphorus ratios (b) for the Hawaiian 4.1 million year soil chronosequence.

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