Soil Carbon Sequestration under Agroforestry Systems

Abstract

Carbon sequestration is essentially the process of transforming carbon in the air (carbon dioxide or CO2) into stored soil carbon. Carbon dioxide is taken up by plants through the process of photosynthesis and incorporated into living plant matter. As the plants die, the carbon-based leaves, stems and roots decay in the soil and become soil organic matter. This is the basic process called carbon sequestration. Although most carbon enters ecosystems via leaves, and carbon accumulation is most obvious when it occurs in aboveground biomass, more than half of the assimilated carbon is eventually transported below ground via root growth and turnover, exudation of organic substances from roots, and incorporation of fallen dead leaves and wood (litter) into soil. Soils contain the major proportion of the total ecosystem carbon stock in all ecosystems. As with total ecosystem carbon stocks, soils tend toward "equilibrium" carbon levels. With a change in carbon input and/or decomposition rates, soil carbon stocks change. This change is most rapid for the active fraction, including structural carbon (i.e., cellulose and hemicellulose) and metabolic carbon components (i.e., proteins, lipids, starches, nucleic acids); the slow fraction (i.e., microbial walls and metabolic components protected by soil clays and aggregates); and the passive soil carbon (i.e., clay-protected humics). Carbon inputs to soil are determined by the amount and distribution of primary production, the life cycle of the vegetation, and exogenous organic matter additions (e.g., composts, manure). Thus, practices that increase net primary production (NPP) and/or return a greater portion of plant materials to the soil have the potential to increase soil carbon stocks. Organic matter decomposition is influenced by numerous physical, chemical, and biological factors that control the activity of microorganisms and soil fauna (Swift et al., 1979). These factors include the abiotic environment (temperature, water, aeration, pH, mineral nutrients), plant residue quality (i.e., C:N ratio and lignin content), soil texture and mineralogy, and soil disturbance (tillage, traffic, logging, grazing, etc.). The root system, depth distribution, and chemical characteristics of the root biomass also play significant roles in SOC dynamics (Gale and Cambardella, 2000). Practices that reduce the decomposition rate by altering these physical, chemical, or biological controls also lead to carbon storage.