SOIL STRUCTURE - CARBON AND NITROGEN-METABOLISM

Physical factors confer biological stability on organic matter in soils through the constraints they place on the opportunities for reactions between substrate and enzyme and/or decomposer organism; and, in the context of C and nutrient turnover through decomposer biomass, on the limitations they impose on the activities of predators on prey. The concept of protection afforded to substrates because of their location in micropores of sizes which deny access to decomposer cells, or to microflora in pores which deny access to microfaunal predators, has heightened interest in soil microstructure as a determinant of the availability of organic substrates for decomposition and of the rates of survival of decomposer cells. Widely different methodologies have been used in studies which have attested to the importance of physical protection as mechanisms for stabilization of organic matter in soils. In this review we appraise some of the established methodologies and offer suggestions for extending their usefulness; and we briefly consider some new approaches where methodologies have not as yet been fully developed and whose potential for successful application to soils is unknown. Studies considered include those which seek to characterize enzymes, substrates and organic residues, and their location in situ, in soil aggregates. Characterization of enzymes in situ derives from the use of electron microscopy in conjunction with cytochemical techniques. Characterization of substrates and residues in situ derives from the application of techniques of electron microscopy (SEM, TEM), and spectroscopy (NMR, IR), to soil aggregates or to fractions thereof. The successful development of methodologies combining electron microscopy and autoradiography could demonstrate directly within soil aggregates, sites of activity of specific microorganisms or enzymes in response to selected conditions of substrate addition. Other studies seek to define more closely the relationships between decomposer activities and processes of C and N turnover and soil properties, especially those properties pertaining to soil microstructure. Water retention characteristics of a soil have been utilized to introduce substrates to different pore size locations within aggregates, in order to determine the effects of substrate location on C turnover. Physical fractionation techniques have been utilized to determine the location within aggregates of decomposer organisms, in order to assess the stability of fraction C and N in microbial biomass and organic residues resulting from the metabolism of introduced substrates.