http://andrewsforest.oregonstate.edu/pubs/pdf/pub1374.pdf

Stabilization and destabilization of soil organic matter: mechanisms and controls

We have developed a model to simulate the dynamics of C, N, P, and S in cultivated and uncultivated grassland soils. The model uses a monthly time step and can simulate the dynamics of soil organic matter over long time periods (100 to 10,000 years). It was used to simulate the impact of cultivation (100 years) on soil organic matter dynamics, nutrient mineralization, and plant production and to simulate soil formation during a 10,000 year run. The model was validated by comparing the simulated impact of cultivation on soil organic matter C, N, P, and S dynamics with observed data from sites in the northern Great Plains. The model correctly predicted that N and P are the primary limiting nutrients for plant production and simulated the response of the system to inorganic N, P, and S fertilizer. Simulation results indicate that controlling the C:P and C:S ratios of soil organic matter fractions as functions of the labile P and S levels respectively, allows the model to correctly simulate the observed changes in C:P and C:S ratios in the soil and to simulate the impact of varying the labile P and S levels on soil P and S net mineralization rates.

Dynamics of C, N, P and S in grassland soils: a model

Comparative aspects of cycling of organic C, N, S and P through soil organic matter

Soil architecture is the dominant control over microbially mediated decomposition processes in terrestrial ecosystems. Organic matter is physically protected in soil so that large amounts of well-decomposable compounds can be found in the vicinity of largely starving microbial populations. Among the mechanisms proposed to explain the phenomena of physical protection in soil are adsorption of organics on inorganic clay surfaces and entrapment of materials in aggregates or in places inaccessible to microbes. Indirect evidence for the existence of physical protection in soil is provided by the occurrence of a burst of microbial activity and related increased decomposition rates following disruption of soil structures, either by natural processes such as the remoistening of a dried soil or by human activities such as ploughing. In contrast, soil compaction has only little effect on the transformation of 14C-glucose. Another mechanism of control by soil structure and texture on decomposition in terrestrial ecosystems is through their impact on microbial turnover processes. The microbial population is not only the main biological agent of decomposition in soil, it is also an important, albeit small, pool through which most of the organic matter in soil passes. Estimates on the relative importance of different mechanisms controlling decomposition in soil could be derived from results of combined tracer and modelling studies. However, suitable methodology to quantify the relation between soil structure and biological processes as a function of different types and conditions of soils is still lacking.

Soil structural aspects of decomposition of organic matter by micro-organisms

Publisher Summary This chapter presents a detailed review of biotic interactions, principally microbe–plant, microbe–microbe, and microbe–fauna. The ecological concepts of stability and resilience in relation to food-web complexity and nutrient cycling are also discussed. The nutritional characteristics of wild plants grown under low-fertility conditions (by agronomic standards) are quite different from crop plants in fertile habitats. In most cases, plants have not adapted to nutrient stress by evolving an enhanced capacity to extract nutrients—absorption per unit weight of root—from the soil. The energy cost for producing absorbing surfaces by ectomycorrhizae (ECM) fungi, as well as vesicular-arbuscular mycorrhizal (VAM) fungi, is much less than that required for a comparable amount of absorbing surface as root tissue. Nutrient turnover is a valuable diagnostic feature of soil systems. The bulk of nutrients are present in large inactive pools versus small pools of biologically active components. Small pools, of rapidly cycling components, may account for a major proportion of the total available N and P taken up by plants in terrestrial ecosystems.

Biological Strategies of Nutrient Cycling in Soil Systems

Summary

A study of the A horizons of cultivated soils from Saskatchewan, Canada showed that the organic carbon contents were in the sequence semi-arid Dark Brown soils (1.7 per cent), < Black soils (2.55–2.8 per cent), < sub-humid Dark Gray soils (4.5 per cent). The relative proportions of humic acids and the ratios of humic acid: fulvic acid increased in a similar sequence. There were accompanying increases in extinction values at 280 nm, and in levels of nonhydrolyzable nitrogen suggesting more extensive polycondensation of humic materials. These observations are discussed in terms of a working hypothesis for humus formation and transformations, that gives particular emphasis to the soil environment and to interactions between organic and inorganic components.

D. W. Anderson

Papers

Processes of humus formation and transformation in soils of the canadian great plains