Modeling microbial respiration

Microorganisms are major agents by which C and energy move through the soil. Soils provide a major source and sink for greenhouse gases particularly CO2, CH4 and N2O.The global C cycle (Pidwimy, 2005; RCEP, 1996; Houghton and Hackler., 1995). The world's soils represent a very large reservoir of C. The amount of C in decaying plant litter and soil organic matter (SOM) may exceed the amount in living biomass by a factor of 2 or 3 (RCEP, 1996; EE, 1993). A pool of 9,322 Mt C is present in British soils, peat and litter and 114 Mt C in British vegetation (with 80% of this value in forests and woodlands). The release of CO2 from soils is a natural process, occurring through the oxidation of SOM and plant litter by microbial populations. The rate at which CO2 is released depends on land use and soil management. Accelerated loss can be triggered by the changes in land use for example when forests or grasslands are converted to arable cropping (RCEP, 1996).C can accumulate in soil when arable land is converted to grassland or forest but it takes about ten times longer to build up soil C following conversion to pasture than it takes to deplete C stocks after pasture land has been ploughed (RCEP, 1996). It has been observed that C accumulates much more slowly following a change from arable to pasture; 49 t C ha-1 might be added over 275 years half of it in the first 38 years (RCEP, 1996; Cannel et al., 1994). Human influence on the natural cycle has resulted in the accelerated release as atmospheric CO2, contained in the chalk, limestone, and fossil fuels formed over a very long time from oceanic sediments (RCEP, 1996). Living cells need a constant supply of energy, which for heterotrophic microflora is derived from the transformation of organic matter such as cellulose, proteins, nucleotides and humifieds compounds. Energy supplying reactions in the cell are redox reactions based on the transfer of electrons from a donor to an acceptor. Through aerobic respiration that is the oxidation of organic matter by aerobic micro-organisms oxygen functions as the terminal acceptor of the electrons. The metabolic activities of soil micro-organisms can therefore be quantified by measuring the CO2 production or O2 consumption (Alef, 1995; Nannipieri et al., 1990). Soil respiration is one of the oldest and still most frequently used parameters for quantifying microbial activities in soils (Kieft and Rosacker, 1991). Basal respiration is defined as respiration without the addition of organic substance to soil. Substrate-induced respiration (SIR) is the soil respiration measured in the presence of an added substrate such as glucose. Respiration is not only restricted to micro-organisms but it is also carried out by other organisms inhabiting the soils. Like other metabolic activities it depends on the physiological state of the cells and is influenced by different factors in the soil (Alef, 1995). Respiration is influenced by soil moisture, temperature, the availability of nutrients, soil structure and tillage. Air drying significantly reduces soil respiration. Re-moistened soils however show very high initial activities; probably as a result of the high concentrations of easily degradable organic compounds such as amino and organic acids caused by chemical and physical processes at the moistening of dry soils (Clark and Kemper, 1967; Anderson, 1975; Wilson and Griffin, 1975a, 1975b; Kowalenko et al., 1978; Krockel and Stolp, 1986; Kieft et al., 1987). The re-moistening of air dry soils containing carbonate also causes the release of abiotic CO2. In this case it is recommended that the O2 consumption is measured (Anderson, 1982; Kieft et al., 1987). Soil respiration decreases with the depth of soil and correlates significantly with SOM (Corg), and most microbial parameters. This book describes additional benefits of using CENTURY 4.0 model to simulate microbial respiration.