| This dissertation presents a combination of field, laboratory, and modeling work on the role of soil organic matter (SOM) in the global carbon cycle. Chapter 1 describes a simple model of the terrestrial carbon cycle, which showed that the high rates of carbon turnover in tropical ecosystems may cause the equatorial regions of the earth to dominate the short-term carbon cycle feedbacks to a warmer climate. In contrast to the conventional argument, the model suggests that the large carbon stocks of boreal and tundra biomes will not necessarily lead to a rapid loss of soil C to the atmosphere; instead the relatively greater response of net primary productivity to temperature in these regions may cause some carbon storage over a limited range of warming.; Chapters 2 and 3 presents a series of studies which took advantage of a naturally occurring temperature gradient in parallel C{dollar}sb3{dollar} forests and C{dollar}sb4{dollar} pastures on the northeast flank of Mauna Kea Volcano, island of Hawaii. I used measurements of soil carbon and respiration, along with the isotopes {dollar}sp{lcub}13{rcub}{dollar}C and {dollar}sp{lcub}14{rcub}{dollar}C, to partition SOM into pools of fast, intermediate and very-slow turnover, and to estimate turnover times for the large intermediate pool. This work suggested that estimates of the responsiveness of soil carbon to environmental changes must account for its multi-pool structure; turnover times calculated by three different multi-pool models were three times slower than those from a single pool model. The work also showed that active and intermediate SOM pools do not appear to have different sensitivities to temperature, so that SOM pool structure is likely to affect the rate but not the ultimate pattern of change. |
