Estimation of microbial biomass carbon and Nitrogen

1. fumigation-incubation method
2. the fumigation-extraction method
3. the substrate-induced respiration method
4. the ATP method 

The fumigation-incubation method is the basic technique which is also used for calibration of the three other methods. It is characterized by simple performance without the need of expensive equipment. Its application is limited to soils with a pH above 5 and to soils that do not contain easily degradable C sources. If these limitations are not considered, too low or even negative biomass values will be obtained. These restrictions are largely overcome by the fumigation-extraction method. 

The substrate-induced respiration requires expensive equipment for the hourly measurement of soil respiration. This method is also susceptible to amendment of soils with C sources, leading to an overestimate of biomass

The microbial biomass accounts for only 1-3 % of soil organic C but it is the eye of the needle through which all organic material that enters the soil must pass. During this process these materials are converted by microorganisms in order to generate energy and to produce new cellular metabolites to support their maintenance and growth. In the C-limited soil system available C in organic materials entering the soil is the driving force behind these processes but other essential nutrient elements (particularly N, P, K) are also involved. Under suitable environmental conditions the extent of the turnover will mainly be controlled by the size and activity of the microbial biomass. In order to elucidate the intricate interrelationships and controlling mechanisms of the input/output fluxes of nutrients and energy in the soil ecosystem a reliable quantification of the microbial biomass is required. Valuable information on biomass growth, turnover time, death rates, and the efficiency of C use can be derived from reliable biomass C data. The microbial biomass itself may represent a labile pool of C and nutrient elements. In agricultural soils 200-1000 ~tg biomass C g-~ soil is often found. This cell mass fixes 100-600 kg N and 50-300 kg P per hectare in the upper 30 cm of soil. These amounts often exceed the annual application of nutrients supplied as fertilizer to soils in agricultural practice. The liberation or fixation of these nutrients depends on the life dynamics of the microorganisms. Growth of biomass and fixation of nutrients is promoted by rhizodeposits and plant debris and the liberation of nutrients is the consequence of microbial death. These processes provide the inc centive for a reliable quantification of the microbial biomass as a whole and 88 for the inclusion of its life dynamics in considerations about nutrient cycling in soil. 

The fumigation-incubation method

As early as 1908, under the title "Uber die Wirkungen des Schwefelkohlenstoffs und tihnlicher Stoffe auf den Boden" (Effects of carbon disulfide and related compounds on soil) K. StOrmer described and interpreted the effects of biocidal fumigants on soils. He postulated that (1) the observed effect of improved plant growth after a transient treatment of soils with toxic fumigants is caused by a liberation of additional N; (2) this N originates from the bodies of the organisms killed by the toxicant; and (3) after treatment of the soils an increased proliferation of bacteria can be observed, which degrade the killed organisms and liberate the N fixed in the cell mass. This explanation for the observed phenomena, now accepted as correct, did not find the general acceptance it deserved and was overlain by other explanations. Jenkinson (1966) summarized the most important early theories. One hypothesis assumed that microbial activity and development is restrained in unsterilized soil by unknown toxic compounds, reduced microbial vigour, or by inhibiting, antagonistic effects between different sections of the microbial populations. Partial sterilization suspends these effects for a transient time. A second theory postulated a physical or chemical protection of otherwise unavailable substrates in unsterilized soil. This protecting barrier may consist of waxes which are dissolved by exposure to lipophilic solvents, such as CHC13, CS 2, or CC14. The third possible explanation was based on the observation that most ways of partial sterilization (heating, air drying, irradiation) increase the amount of water-soluble organic matter. This can be explained chemical alteration of the non-living parts of the soil organic matter but also by killing of microorganisms and the ensuing lysis. In order to examine these different theories, Jenkinson (1966) investigated the CO2 and 14CO2 liberation from soil samples subjected to different treatments. These