Soil carbon formation and persistence

Soil organic and inorganic carbon fractions are significant and dynamic components of the global carbon cycle. Most soil carbon pools are highly influenced by changes in net primary production (NPP) inputs and climate. The global carbon cycle is comprised of a short cycle dominated by biological processes and a long cycle dominated by geological outcomes including rock weathering, volcanism, etc. The long-term carbon cycle is considered Earth’s thermostat, while the short cycle introduces climate variability through changes in pools such as soil carbon. The history of soil organic carbon (SOC) research dates back several 100 years, with formal chemistry investigations beginning in the 1700s. A historical summary of major discoveries will be presented in this chapter. A discussion of the sources of above- and belowground carbon inputs follow. In the 1970s, conceptual models were introduced that described soil carbon turnover and maintenance. Chemical structural models were introduced in the 1990s and are a major area of ongoing research. Debate on the molecular diversity of soil carbon inputs and protection mechanisms dominates current research agendas. Other factors, such as soil priming, the influence of new carbon inputs on the stability of old carbon, and nitrogen turnover from mineral-organic matter fractions are summarized. Land use change impacts on different biomes are discussed along with factors affecting SOC persistence in climate sensitive tundra, wetland, and peatland ecosystems. This chapter concludes with a brief synthesis and an outlook for future SOC research directions.