Structure Of Soil Organic Matter

During the 1960s and 1970s, Schnitzer (1978) undertook exhaustive chemical oxidation and reduction to degrade humic substances into specific compounds. These procedures showed a significant aliphatic content of SOM (Fig. 12.17). The results also suggested that aromatic structures were cross-linked by longer chain aliphatic compounds. Schnitzer (1978) concluded that aliphatic compounds were more important structural features than had previously been assumed. Schulten et al. (1997), using pyrolysis-methylation gas chromatography/mass spectrometry, confirmed the results of chemical degradation studies. These studies found that the major structural components of SOM consisted of benzenes and n-alkyl benzenes. The alkyl-aryl compounds consisted of aromatic rings covalently bonded to aliphatic chains. They also showed that the aromatic fraction contained significant heterocyclic N (pyrroles and pyridines), N derivatives of benzene, and long-chain nitriles. Studies using 13C NMR verified that SOM was not dominated by aromatic structures (Preston, 1996). 13C NMR analysis has been useful for determining the chemical bond configurations of SOM that are often lost with extraction and chemical degradation methods. This research has provided insight into the chemical composition and structure of the SOM matrix and revealed possible mechanisms of humate interactions with clay minerals, metals, and anthropogenic compounds. However, this research fails to identify the active components of SOM that directly influence nutrient cycling processes.

quantity and distribution of organic matter in soils

Soil organic matter plays a major role in sustaining ecosystems by regulating nutrient cycling and impacting physical properties as described earlier. The quantity of SOM is dependent on the balance between primary productivity and the rate of decomposition. The presence of silt and clay generally preserves some C from primary production and increases SOM under certain environments. The effects of climate, specifically moisture and temperature, primarily control the accumulation of soil C from its effect on both NPP and decomposition. The highest accumulation of C is found in swamps and marshes (723 t ha-1) where NPP is high but decomposition is inhibited by a lack of O2 (Table 12.4). Decomposition also is inhibited by cold in tundra soils and C tends to accumulate there primarily as litter and POM.

Humid tropical forests and boreal and temperate forests as well as temperate grasslands all accumulate approximately 200tha-1 with turnover times of from 29 to 91 years as calculated by dividing SOM content by plant residue input. In contrast to temperate grassland, where C accumulates with an overall turnover time of 61 years, the low levels of SOM in tropical grassland have total turnover rates of only 10 years. As will be shown in the next chapter, SOM comprises a

336 Chapter 12 Carbon Cycling and Formation of Soil Organic Matter TABLE 12.4 The Area, Stock of C, NPP, and C Turnover of Various Biomesa

Global C stock

336 Chapter 12 Carbon Cycling and Formation of Soil Organic Matter TABLE 12.4 The Area, Stock of C, NPP, and C Turnover of Various Biomesa

Global C stock

Biome

Area (109 ha)

Plant

Soil

Total

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