Organic-matter quality and C/N ratio as controlling factors of RSOM turnover

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Berg, Björn:
Organic-matter quality and C/N ratio as controlling factors of RSOM turnover.
In: Mitteilungen der Deutschen Bodenkundlichen Gesellschaft. Bd. 87 (1998) . - S. 79-91.
ISSN 0343-107x

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Abstract

The decomposition rate of fresh plant litter may decrease from about 0.1% day-1 in fresh litter to 0.00001 day-1 or lower in more far-decomposed material. This is due to changes in its organic-matter quality as the recalcitrant chemical components become enriched. The decrease in decomposability (substrate quality) is complex, involving both direct chemical changes in the substrate itself and the succession in microorganisms able to compete for the substrate with a given chemical composition. The concept 'substrate quality' varies among litter species, though.In newly shed litter when e.g. celluloses are decomposed there may be a lack of macronutrients, such as N, P and S and the rate may be positively related to e.g. the concentration of N. With the disap-pearance of celluloses, the concentration increases of the more recalcitrant compound lignin and the effects of N concentration on decomposition rates change completely. The degradation rate of lignin determines the decomposition rate of the whole piece of former litter, now in reality soil organic matter. At this stage high N concentrations will have a rate-retarding effect on lignin degradation and thus on the litter. It appears that this total retarding effect of N may be ascribed to two different mechanisms. First, low-molecular N reacts with lignin remains creating more recalcitrant aromatic compounds and, further low-molecular N may repress the synthesis of lignin-degrading enzymes in white-rot fungi.The retardation of the decomposition rate may be so strong that the decomposition of the litter can be estimated to reach a limit value for total mass loss. At such a stage the litter would be close to soil organic matter. These limit values range from about 45 to 100 % decomposition indicating that between 0 and 55 % should be sta-bilized or decompose extremely slowly. We found that N concentra-tion had an overall effect on this limit value in no less than 130 cases investigated, meaning that the higher the N concentration in the newly shed litter (the lower the C/N ratio) the more organic matter was left. The relation could be described by a highly signi-ficant and negative linear relation. Other nutrients were correl-ated with other effects. Thus Mn and Ca had a generally opposite effect to N and high concentrations appeared to stimulate further decomposition in all studies investigated.This limit value may mean that at higher initial N concentrations this stage with stabilized OM or a very low decomposition rate was reached earlier, viz. at a lower mass loss. Such an effect would mean that there may be a faster humus accumulation in stands with N-rich litter.