The effects of fumigation on organic C extractable by 0.5 M K2SO4 were examined in a contrasting range of soils. EC (the difference between organic C extracted by 0.5 M K2SO4 from fumigated and non-fumigated soil) was about 70% of FC (the flush of CO2-C caused by fumigation during a 10 day incubation), meaned for ten soils. There was a close relationship between microbial biomass C, measured by fumigation-incubation (from the relationship Biomass C = FC/0.45) and EC given by the equation: Biomass C = (2.64 ± 0.060) EC that accounted for 99.2% of the variance in the data. This relationship held over a wide range of soil pH (3.9–8.0).

ATP and microbial biomass N concentrations were measured in four of the soils. The (ATP)(EC) ratios were very similar in the four soils, suggesting that both ATP and the organic C rendered decomposable by CHCl3 came from the soil microbial biomass. The C:N ratio of the biomass in a strongly acid (pH 4.2) soil was greater (9.4) than in the three less-acid soils (mean C:N ratio 5.1).

We propose that the organic C rendered extractable to 0.5 m K2SO4 after a 24 h CHCl3-fumigation (EC) comes from the cells of the microbial biomass and can be used to estimate soil microbial biomass C in both neutral and acid soils.

An extraction method for measuring soil microbial biomass c

Summary
The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient, mainly polysaccharides, (b), temporary, roots and fungal hyphae, and (c) persistent, resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macro-aggregates, defined as > 250 μm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water-stability of micro-aggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.

Organic matter and water-stable aggregates in soils

Significantly more carbon is stored in the world's soils--including peatlands, wetlands and permafrost--than is present in the atmosphere. Disagreement exists, however, regarding the effects of climate change on global soil carbon stocks. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Unravelling the feedback effect is particularly difficult, because the diverse soil organic compounds exhibit a wide range of kinetic properties, which determine the intrinsic temperature sensitivity of their decomposition. Moreover, several environmental constraints obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed 'apparent' temperature sensitivity, and these constraints may, themselves, be sensitive to climate.

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Temperature sensitivity of soil carbon decomposition and feedbacks to climate change

As the largest pool of terrestrial organic carbon, soils interact strongly with atmospheric composition, climate, and land cover change. Our capacity to predict and ameliorate the consequences of global change depends in part on a better understanding of the distributions and controls of soil organic carbon (SOC) and how vegetation change may affect SOC distributions with depth. The goals of this paper are (1) to examine the association of SOC content with climate and soil texture at different soil depths; (2) to test the hypothesis that vegetation type, through patterns of allocation, is a dominant control on the vertical distribution of SOC; and (3) to estimate global SOC storage to 3 m, including an analysis of the potential effects of vegetation change on soil carbon storage. We based our analysis on .2700 soil profiles in three global databases supplemented with data for climate, vegetation, and land use. The analysis focused on mineral soil layers. Plant functional types significantly affected the vertical distribution of SOC. The per- centage of SOC in the top 20 cm (relative to the first meter) averaged 33%, 42%, and 50% for shrublands, grasslands, and forests, respectively. In shrublands, the amount of SOC in the second and third meters was 77% of that in the first meter; in forests and grasslands, the totals were 56% and 43%, respectively. Globally, the relative distribution of SOC with depth had a slightly stronger association with vegetation than with climate, but the opposite was true for the absolute amount of SOC. Total SOC content increased with precipitation and clay content and decreased with temperature. The importance of these controls switched with depth, climate dominating in shallow layers and clay content dominating in deeper layers, possibly due to increasing percentages of slowly cycling SOC fractions at depth. To control for the effects of climate on vegetation, we grouped soils within climatic ranges and compared distributions for vegetation types within each range. The percentage of SOC in the top 20 cm relative to the first meter varied from 29% in cold arid shrublands to 57% in cold humid forests and, for a given climate, was always deepest in shrublands, inter- mediate in grasslands, and shallowest in forests ( P , 0.05 in all cases). The effect of vegetation type was more important than the direct effect of precipitation in this analysis. These data suggest that shoot/root allocations combined with vertical root distributions, affect the distribution of SOC with depth. Global SOC storage in the to p3mo fsoil was 2344 Pg C, or 56% more than the 1502 Pg estimated for the first meter (which is similar to the total SOC estimates of 1500-1600 Pg made by other researchers). Global totals for the second and third meters were 491 and 351 Pg C, and the biomes with the most SOC at 1-3 m depth were tropical evergreen forests (158 Pg C) and tropical grasslands/savannas (146 Pg C). Our work suggests that plant functional types, through differences in allocation, help to control SOC distributions with depth in the soil. Our analysis also highlights the potential importance of vegetation change and SOC pools for carbon sequestration strategies.

The vertical distribution of soil organic carbon and its relation to climate and vegetation

Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.

/pdf/persistence-of-soil-organic-matter-as-an-ecosystem-property-ago0khy192.pdf

Persistence of soil organic matter as an ecosystem property

The stability of pores and particles is essential for optimum growth of plants. Two categories of aggregates macro- (> 250 μm) and micro- (< 250 μm) depend on organic matter for stability against disruptive forces caused by rapid wetting. Dispersion of clay particles from microaggregates is promoted by adsorption of complexing organic acids which increase the negative charge on clays. The acids are produced by plants, bacteria and fungi. However, the dispersibility of clay in microaggregates is offset by the binding action of polysaccharides, mainly mucilages produced by bacteria, but also by plant roots and fungal hyphae. The stability of microaggregates is also enhanced by multivalent cations which act as bridges between organic colloids and clays. Macroaggregates are enmeshed by plant roots, both living and decomposing, and are thus sensitive to management, and increase in number when grasses are grown and the soil is not disturbed. Lack of root growth, i.e. fallow, has the opposite effect. Various implications for management of soil structure are discussed.

Soil organic matter and structural stability: mechanisms and implications for management

The turnover of C in soils is controlled mainly by water regimes and temperature, but is modified by factors such as size and physicochemical properties of C additions in litter or root systems, distribution of C throughout the soil as root systems, or addition as litter, distribution of C within the soil matrix and its interaction with clay surfaces. Soil factors which retard mineralization of C in soils are identified from correlations of C contents of soils with other properties such as clay content and base status. The rate and extent of C mineralization depends on the chemistry of the added organic matter and interaction with clays of the microbial biomass and metabolites. The organomineral interactions are shown to depend on cation bridges involving mainly Ca in neutral to alkaline soils, Al in acid soils and adsorption of organic materials on iron oxide surfaces. The various organomineral interactions lead to aggregations of clay particles and organic materials, which stabilizes both soil structure and the carbon compounds within the aggregates.

The retention of organic matter in soils

An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 µm diameter breaking down to microaggregates 20-250 µm diameter before particles <20 µm were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 µm diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 µm but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.

J.M. Oades

Papers

Organic matter and water-stable aggregates in soils

Soil organic matter and structural stability: mechanisms and implications for management

The retention of organic matter in soils

Aggregate hierarchy in soils

The role of biology in the formation, stabilization and degradation of soil structure