Litter mass decreases exponentially with time in the course of the decomposition process. In general, leaf litter typically loses 30-70% of its mass in the first year and another 20-30% in the following 5-10 years. Only a few of the important chemical changes occurring during decomposition have been completely understood, while most of these reactions still need to be investigated. The organic compounds in plant tissues can be grouped into several broad classes on the basis of their different rates of decomposition. These organic compounds are typically grouped (from fast to slow decomposition) into (1) sugars, starches, and simple proteins, (2) crude proteins, (3) hemi-cellulose, (4) cellulose, (5) fats and waxes, and (6) lignin and phenolic compounds. A three-stage model is often used to describe the chemical changes and rate-regulating factors contributing to decomposition. These stages include an early, a late, and a near-humus stage. At the early stage, the decomposition of water-soluble substances and unshielded hemicellulose/cellulose is stimulated by high levels of nutrients; at the late stage, the degradation rate of lignin determines the rate of litter decomposition, while higher N levels decrease the decomposition rates; in the near-humus stage, the rate of litter decomposition tends to zero, while the total amount and the lignin content of the residual soil organic material remain constant. An alternative, commonly accepted three-phase model of litter decomposition, considers the following phases: (1) leaching of cell solubles; (2) fragmentation and chemical alteration; this second phase occurs slowly and includes most of the fragmentation and alteration of the litter structure; (3) chemical alteration of litter detritus mixed with mineral soils. In this final phase, decomposition occurs at a slower pace.
Climate determines regional patterns of decomposition. In general, regions with higher temperature and soil moisture tend to experience higher decomposition rates. However, decomposition is limited under water logging conditions typical of wetland environments. At a given location, most decomposition occurs near the ground surface, where soils are substrate-rich, due to the presence of relatively high amounts of leaf and root litter.
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