Fractal Kinetics Parameters Regulating Carbon Decomposition Rate under Contrasting Soil Management Systems

Abstract

Agricultural soils can sequester and release large amounts of carbon. Accessibility of soil carbon to microbial attacks depends on biological, chemical, and physical protection mechanisms such as organic matter composition and particle size, soil aggregation, and chemical protection through the silt-clayorganic matter complex. While soil and organic matter are fractal objects controlling exposure of reactive surfaces to the environment, soil aggregation and biomass production and quality are regulated by agricultural practices. Organic matter decomposition in soil is generally described by the classical first-order kinetics equations fitted to define distinct carbon pools. By comparison, fractal kinetics assigns a coefficient to adjust time-dependent decomposition rate of total soil carbon to protection mechanisms. Our objective was to relate fractal parameters of organic matter decomposition to soil management systems. Retrieving published data, the decomposition of organic matter was modeled in a silt loam soil maintained under pasture, annual cropping or bare fallow during 11 years. The classical first-order kinetics model returned quadratic relationships indicating that reactive carbon decreased with time. Fractal kinetics rectified the relationships successfully. Initial decomposition rate (k 1 at t = 1) was 7 × 10 -4 for pasture, 1 × 10 -4 for annual cropping, and 0.5 × 10 -4 for bare-soil fallow. Fractal coefficients h were 0.71, 0.45, and 0.25 for pasture, annual cropping and fallow, respectively. Due to aggregation, physical protection against microbial attacks was highest under pasture management, leading to higher carbon sequestration despite higher biomass production and “priming” effects. Parameters k 1 and h proved to be useful indicators for soil quality classification integrating the opposite effects of labile carbon decomposition and carbon protection mechanisms that regulate the decomposition rate of organic matter with time as driven by soil management practices.