Mineralization Methods

The buried bag technique is a common approach to measuring N mineralization. It consists of (1) collecting soil core samples and measuring the initial concentration of inorganic N; (2) reburing subsamples of this core in polyethylene bags for specific period of time; and (3) measuring the inorganic N after incubation. The net rate of N mineralization is calculated by the difference in N concentrations between the two measurements. The buried bag method is relatively simple, cost effective, and provides results that can be compared with other studies, due to its widespread use in many different ecosystems around the world. The major problem ofthis method arises from the disturbance ofthe soil sample. For example, the method eliminates plant uptake, with a consequent increase in soil inorganic N. In N limited systems, this increase leads to higher microbial N immobilization, which, in turn, results in the underestimation of plant uptake. The mineralization rates of other nutrients essential to plants, such as P and S, can also be measured using buried bags with appropriate adjustments. Several other methods can be used to determine the rates of N mineralization, including the N budget analysis (ideally for large temporal scales), the 'super sinks' analysis (e.g., ion exchange resin), the use of substrate analogs, as well as the use of isotope tracer and dilution measurements. Over the last decade, the general notion of N mineralization has evolved; the concept of N mineralization as the driving process in the N cycle has been replaced by the idea that exoenzyme-driven depolymerization is the rate-limiting step in the generation ofbioavailable N. This new N cycling paradigm does not invalidate the traditional methods of net mineralization measurement as fundamental tools in the study of N cycling, but caution needs to be used in the interpretation of the results. Net mineralization is an indirect indicator of N availability and not the key step of N cycling.

See also: Biodegradation.

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