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 decomposed material. This is due to changes in its organic-matter quality as the recalcitrant chemical components become enriched in the material. The decrease in decomposability (substrate quality) is complex, involving both direct chemical changes in the substrate itself and the succession in micro-organisms able to compete for the substrate with a given chemical composition. The 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 recalcitrant compound, lignin, increases and the effects of N concentration on decomposition rates change completely. In partly decomposed litter the degradation 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 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 more stabilized SOM. The limit values estimated to date range from about 45 to 100% decomposition indicating that between 0 and 55% of the litter mass should either stabilize or decompose extremely slowly. We found that N concentration had an overall effect on this limit value in no less than 130 cases investigated, meaning 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 described by a highly significant and negative linear relation. Other nutrients were also correlated to the limit value. Thus, Mn and Ca had a generally opposite effect to N, meaning that high concentrations of these nutrients were 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.