Litter decomposition and organic matter turnover in northern forest soils

The decomposition rate of fresh plant litter may decrease from ca. 0.1% per day in fresh litter to 0.00001 per day or lower in more completely decompo sed material. This is due to changes in its organic-matter quality as the r ecalcitrant chemical components become enriched in the material. The decrea se in decomposability (substrate quality) is complex, involving both direct chemical changes in the substrate itself and the succession in micro-organ isms able to compete for the substrate with a given chemical composition. T he concept 'substrate quality' varies among litter species, though. In fresh litter, there may be a lack of macronutrients, such as N, P, and S thus limiting the decomposition rates of, for example, the celluloses, and the rates may be positively related to, for example, the concentration of N. With the disappearance of celluloses, the concentration of the more reca lcitrant compound, lignin, increases and the effects of N concentration on decomposition rates change completely. In partly decomposed litter the degr adation rate of lignin determines the decomposition rate of the whole piece of litter, which now in reality is turning into soil organic matter (SOM). At this stage high N concentrations will have a rate-retarding effect on l ignin degradation and thus on the litter. It appears that this total retard ing effect of N may be ascribed to two different mechanisms. First, low-mol ecular N reacts with lignin remains creating more recalcitrant aromatic com pounds, and, further, low-molecular N may repress the synthesis of lignin-d egrading enzymes in white-lot fungi. The retardation of the decomposition rate may be so strong that the decompo sition 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 more stabilized SOM. Th e limit values estimated to date range from about 45 to 100% decomposition indicating that between 0 and 55% of the litter mass should either stabiliz e or decompose extremely slowly. We found that N concentration had an overa ll effect on this limit value in no less than 130 cases investigated, meani ng that the higher the N concentration in the fresh litter (the lower the C /N ratio) the more organic matter was left. The relationship could be descr ibed by a highly significant and negative linear relation. Other nutrients were also correlated to the limit value. Thus, Mn and Ca had a generally op posite effect to N, meaning that high concentrations of these nutrients wer e correlated to further decomposition in all studies investigated. The 'limit-value' concept may mean that at higher initial N concentrations, the stage with either stabilized SOM or a very low decomposition rate was reached earlier, i.e. at a lower mass loss. Such an effect would mean that in stands with N-rich litter there may be a faster humus accumulation. (C) 2000 Elsevier Science B.V. All rights reserved.