Soil Organic Carbon Maintenance

Declining amounts of arable land, increasing world populations, and increasing costs of fertilizer and food and energy needs will make it increasingly difficult to maintain our soil resources. A key component for sustaining soil productivity is the maintenance of soil organic carbon (SOC). SOC maintenance requires the amount of carbon added to the system to equal the amount of relic carbon mineralized

(Ortega et al. 2002; Prakash et al. 2002). The carbon cycle is driven by photosynthesis which produces organic biomass that is respired by microorganisms. Biomass mineralization rates are influenced by many factors including water, temperature, stability of the carbon, and management (Lloyd and Taylor 1994; Mikha et al. 2006). SOC content has been directly linked to productivity, temperature, cation exchange capacity, plant available water, bulk density, available nutrients, erosion, management, and native vegetation (Morachan et al. 1972). Several excellent reviews on soil organic matter turnover are available (Kuzyakov and Domanski 2000; West and Post 2002; Wilhelm et al. 2004; Amos and Walters 2006; Causarano et al. 2006; Johnson et al. 2006). These and other papers identified the need to develop complete carbon budgets when estimating SOC maintenance requirements (Arrouays and Pelissier 1994; Collins et al. 1999; Zach et al. 2006; Huggins et al. 2007).

Even though only a small proportion of the non-harvested biomass returned to soil ends up in soil humus, SOC is needed to improve water infiltration rates and reduce erosion. SOC contents can be increased by adding more non-harvested biomass to soil or by slowing the mineralization rate of fresh biomass.

To calculate carbon sequestration potentials, accurate measures of carbon inputs and outputs are needed. Long-term management studies may provide information needed in SOC maintenance calculations (VandenBygaart et al. 2003; McVay et al. 2006; Richter et al. 2007). One of the oldest management studies conducted in the world is the Rothamsted long-term study. Information on this study is available at http://www.rothamsted.bbsrc.ac.uk/resources/LongTermExperiments.html. Links to other long-term studies are available at http://ltse.env.duke.edu/resources/links.

Most historical studies do not contain the detailed information needed to develop carbon budgets. They are also confounded by erosional losses, changes in the chemical methods to measure SOC, management-induced differences in bulk density, and different methods to calculate turnover kinetics. The consequences of these problems are that it is difficult to compare studies and calculate carbon turnover rates. To overcome these problems simplifying assumptions are often used (Clay et al. 2006; Johnson et al. 2006; Bolinder et al. 2007). Assumptions can reduce the usefulness of the findings. This chapter reviews non-isotopic and 13C isotopic approaches for determining SOC maintenance and implications of simplifying assumptions on SOC turnover calculations.

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