Biomass turnover

Physical protection is exerted by occlusion of particulate organic matter (POM) inside aggregates. It is responsible for the physical separation of organisms active in decomposition and substrates, reduced oxygen availability in the substrate compartment, and reduced biomass turnover through protection from microbial grazers (Mamilov and Dilly, 2002). 

From considerations of biomass turnover in natural ecosystems. h Production by termites. 

Temporal fluctuations in microbial phosphorus (Fig. 7.2) have been used to estimate the annual phosphorus flux through the soil microbial biomass and the turnover time of microbial phosphorus under field conditions (Chen et al, 2003). This approach assumes that the sum of the fluctuations representing a decrease in microbial phosphorus is equivalent to the mean annual biomass turnover (McGill et al., 1986). Results depend on the frequency and time of sampling, but allow a qualitative compari-... 

Terrestrial biomass is divided into a number of subreservoirs with different turnover times. Forests contain approximately 90% of all carbon in living matter on land but their NPP is only 60% of the total. About half of the primary production in forests yields twigs, leaves, shrubs, and herbs that only make up 10% of the biomass. Carbon in wood has a turnover time of the order of 50 years, whereas turnover times of carbon in leaves, flowers, fruits, and rootlets are less than a few years. When plant material becomes detached from the living, plant carbon is moved from the phytomass reservoir to litter. "Litter" can either refer to a layer of dead plant material on the soil or all plant materials not attached to a living plant. A litter layer can be a... 

The subsequent fate of the assimilated carbon depends on which biomass constituent the atom enters. Leaves, twigs, and the like enter litterfall, and decompose and recycle the carbon to the atmosphere within a few years, whereas carbon in stemwood has a turnover time counted in decades. In a steady-state ecosystem the net primary production is balanced by the total heterotrophic respiration plus other outputs. Non-respiratory outputs to be considered are fires and transport of organic material to the oceans. Fires mobilize about 5 Pg C/yr (Baes et ai, 1976 Crutzen and Andreae, 1990), most of which is converted to CO2. Since bacterial het-erotrophs are unable to oxidize elemental carbon, the production rate of pyroligneous graphite, a product of incomplete combustion (like forest fires), is an interesting quantity to assess. The inability of the biota to degrade elemental carbon puts carbon into a reservoir that is effectively isolated from the atmosphere and oceans. Seiler and Crutzen (1980) estimate the production rate of graphite to be 1 Pg C/yr. 

Under stable conditions of extremely low productivity imposed by mineral nutrient stress (position 7 in Fig. lb) there is little seasonal change in biomass. Leaves and roots often have a functional life of several years and there is usually an uncoupling of resource capture from growth (Grime, 1977 Chapin, 1980). Because of the slow turnover of plant parts, differentiating cells occupy a small proportion of the biomass and morphogenetic... 

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

An alternative approach is to combine models with field measurements to assist in developing carbon budgets (Huggins et al. 1998). Clay et al. (2005) used first-order models to calculate the amount of residue returned to the soil from C3 and C4 plants over an 8-year period. Based on the mineralization rates and when the C3 and C4 residues were returned, the 813C signature of non-harvested biomass was determined. Based on the rates, carbon turnover, the amount of SOC mineralized, and the amount of fresh biomass incorporated into the SOC over an 8-year period were determined. 

Carbon turnover in production fields can be determined, using non-isotopic techniques, by combining historical soil samples, current soil samples, and whole field yield monitor data. Sensitivity analysis of such data shows that the amount of above-ground biomass that could be harvested decreases with root to shoot ratio (Table 8.1). For example, if root biomass is ignored, analysis suggests that only 20-30% of the above-ground biomass can be harvested, whereas if the root to shoot ratio is 1.0, then between 40% and 70% of the residue could be harvested. 

The ash content of Arid Steppe and Desert ecosystem vegetation is about 2 times higher than that of forest species. Accordingly, the biogeochemical fluxes of elements are similar to those in the forest ecosystems, in spite the smaller biomass (see above). The compartments of biogeochemical turnover in Steppe and Desert ecosystems are shown in Table 1. 

The characteristic biogeochemical feature inherent in all Steppe and Desert ecosystems is the most intensive cycling of different chemical species in comparison with forest ecosystems. For a Steppe ecosystem the biogeochemical cycle is 2-3 years and this means that the complete renewal of all ecosystems biomass takes place over this period. Remember that in Forest ecosystems the biogeochemical cycling is about 3->25 years and even about 50 years in Forest Swamp ecosystems. The turnover is the highest in Ephemeral Desert and gradually decreases to the north. 

The average sum of total ash elements in the biomass of Tropical Rain Forest ecosystems is about 8,000 kg/ha. The annual ash element turnover and heavy metal exposure rates are shown in Table 8. 

The parameters for characterising soil microbial activity used in the reviewed research results are total microbial biomass, diverse enzymatic parameters, carbon turnover parameters andffiycorrhization, ... 

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