Soil Organic Matter Decomposition And Respiration

Heterotrophic microbial communities oxidize naturally occurring organic material such as carbohydrates according to the generalized equation

CH2O + O2 ^ CO2 + H2O + intermediates + cellular material + energy.

Under anaerobic conditions, the most common heterotrophic metabolic pathway is that of fermentation, which in its simplest form is described as

C6H12O6 ^ 2 CH3CH2OH + 2 CO2 + intermediates + cellular material + energy.

Measuring microbial activity is complex under anaerobic conditions. Fermentation products and CH4 produced within anaerobic microsites can diffuse to aerobic areas, where oxidation to CO2 and H2O can occur. Microbial respiration is determined by measuring either the release of CO2 or the uptake of O2. Because the atmospheric CO2 concentration is only 0.036%, versus 20% for O2, measurements of CO2 production are more sensitive than those for O2. One method of CO2 measurement involves aeration trains; here, NaOH is used to trap evolved CO2 in an airstream from which CO2 is removed before the air is exposed to the soil sample. The reaction occurs as follows:

Before titration, BaCl2 or SrCl2 is added to precipitate the CO32~ as BaCO3 or SrCO3, and excess NaOH is backtitrated with acid. The use of carbonic anhydrase and a double endpoint titration provides greater accuracy when CO2 concentrations are low. In the laboratory, NaOH containers placed in sealed jars are convenient and effective for CO2 absorption. The jar must be opened at intervals so that the O2 concentration does not drop below 10%. After incubation, the NaOH trap is titrated as described above or the electric conductivity of the alkali trap is measured to calculate CO2 evolved. Gas chromatography can also be used to measure soil respiration by placing soil samples in airtight, sealed containers and periodically sampling the headspace gas to measure CO2 evolved. A gas chromatograph, with a thermal conductivity detector, is used to measure the CO2 concentration after CO2 is separated from other constituents on column materials such as Poropak Q. Computer-operated valves in conjunction with GC allow time-sequence studies to be conducted automatically. Infrared gas analyzers are sensitive to CO2 and can be used for both static and flow systems in the laboratory and in the field after H2O, which adsorbs in the same general wavelength, has been removed. The results are expressed either per unit of soil dry weight (^g CO2-C soil h_1) or per unit of microbial biomass (mg CO2-C g_1Cmich_1). The ratio of respiration to microbial biomass is termed the metabolic quotient (qCO2) and is in the range of 0.5-3 mg CO2-C g "1 Cmic h~The metabolic quotient is particularly useful in differentiating the response of soil biota to sustainable soil management practices. For example, stress, heavy metal pollution, and nutrient deficiency increase qCO2 because microbial biomass decreases and respiration increases.

The measurement of CO2 can also be augmented by incorporating 14C or 13C into chosen substrates. The tracer may be known molecules such as glucose, cellulose, amino acids, or herbicides or complex materials such as microbial cells or plant residues. The CO2 respired is trapped in alkali as described above. The measurement of 14CO2 or 13CO2 allows the calculation of the decomposition rate of soil organic matter as well as establishing a balance of the C used in growth relative to substrate decomposition and microbial by-products. A mass spectrometer capable of directly analyzing a gaseous sample for 13CO2 is preferable to the precipitation procedure.

Field studies can be performed by placing an airtight chamber on the soil surface to measure CO2 in situ. This procedure does not alter the soil structure and, therefore, field respiration rates of the indigenous microbial population are more reliably measured. Gas samples are taken from the field chamber using a gastight syringe and then injected into a gas chromatograph or infrared analyzer. Measurement of N2O and CH4 from the same samples is possible. Recently, a new sampling technique was developed to monitor trace gases (CO2, CH4, and N2O) below the soil surface at well-defined depths (Kammann et al., 2001). Probes are constructed from silicone tubing closed with silicone septa on both ends, thereby separating an inner air space from the outer soil atmosphere without direct contact (Fig. 3.2). Gas exchange between the inner and the outer atmosphere takes place by diffusion through the walls of the silicone tube. The advantage of this method is that the silicone probe enables trace gas sampling in wet and waterlogged soils. In general, respiration measurements in the field show much higher analytical variability than those in the laboratory. This is due to higher spatial variability of chemical and physical soil properties and to more variable environmental conditions. Since both soil microorganisms and plant roots contribute to the overall CO2 production in fields, CO2 release in the field has been viewed as a measure of the gross soil metabolic activity.

The net ecosystem exchange (NEE) rate of CO2 represents the balance of gross primary productivity and respiration in an ecosystem. The eddy covariance technique is used to measure the NEE. The covariance between fluctuations in vertical wind velocity and CO2 mixing ratio across the interface between the atmosphere and a plant canopy is measured at a flux tower (Baldocchi, 2003). Although flux tower data represent point measurements with a footprint of typically 1 X 1 km they can be used to validate models and to spatialize biospheric fluxes at the regional scale (Papale and Valentini, 2003).

Soil organic matter decomposition in the field can be examined by following the decay process (i.e., weight loss) of added litter. Site-specific litter or standard litter (e.g., wheat straw) is placed in nylon mesh bags, which are placed on or just below the soil surface. Organic matter decomposition can also be followed easily by using a minicontainer-system (Fig. 3.3). The system consists of polyvinylchloride bars as carriers and minicontainers enclosing the straw material, which can be exposed horizontally or vertically in topsoils as well as on the soil surface. The minicontainers are filled with 150 to 300 mg of organic substrate and closed by nylon mesh of variable sizes (20 ^m, 250 ^m, 500 ^m, or 2 mm) to exclude or include the faunal contribution to organic matter decomposition. After an exposure time of several weeks to months, organic matter decomposition is calculated based on the weight loss of the oven-dried material, taking into consideration the ash content of the substrate. An analysis of the time series allows the dynamics of decomposition processes to be investigated.

Steel tool with knife blade

Horizontal cavity

Soil air probe Probe in place and soil partly restored

FIGURE 3.2 (A) Sampling technique to monitor concentrations of CH4, NO2, and CO2 in air at well-defined depths using a silicone soil air probe fitted with one stainless steel tube connection. The flat silicone coil is fixed with wire mesh (not shown) to maintain the flat "snail" form. The silicone septum used for the steel tube connection has flexible side walls that can be rolled up and pulled over the silicone tubing to ensure better fixing. (B) Insertion of silicone probe into the soil. After a hole of adequate size is dug (1), the silicone probe is inserted into the hole. After insertion the pit is filled with previously removed soil. Silicone probes can be installed at different soil depths and can also be used in wet or even waterlogged soils (with permission from Kammann et al., 2001).

FIGURE 3.2 (A) Sampling technique to monitor concentrations of CH4, NO2, and CO2 in air at well-defined depths using a silicone soil air probe fitted with one stainless steel tube connection. The flat silicone coil is fixed with wire mesh (not shown) to maintain the flat "snail" form. The silicone septum used for the steel tube connection has flexible side walls that can be rolled up and pulled over the silicone tubing to ensure better fixing. (B) Insertion of silicone probe into the soil. After a hole of adequate size is dug (1), the silicone probe is inserted into the hole. After insertion the pit is filled with previously removed soil. Silicone probes can be installed at different soil depths and can also be used in wet or even waterlogged soils (with permission from Kammann et al., 2001).

20 mm

360 mm or 255 mm

360 mm or 255 mm

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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