Björn Berg

Litter decomposition, climate and liter quality

Litter decomposition is controlled by three main factors: climate, litter quality and the nature and abundance of the decomposing organisms. Climate is the dominant factor in areas subjected to unfavourable weather conditions, whereas litter quality largely prevails as the regulator under favourable conditions. Litter quality remains important until the late decomposition stages through its effects on humus formation. Interest in the role of litter decomposition in the global carbon cycle has increased recently since (1) increased atmospheric carbon dioxide will probably affect the chemical quality of litter (especially nitrogen content), and (2) global warming may enhance decomposition rates.

Litter decomposition and organic matter turnover in northern forest soils

Abstract 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.

Nutrient release from litter and humus in coniferous forest soils—a mini review

In northern coniferous forests nutrient release from litter occurs primarily in late stages of decomposition when mainly extensively lignified parts remain. An important regulating factor in these ...

Decomposition of root litter and some factors regulating the process: Long-term root litter decomposition in a scots pine forest

Abstract Decomposition of root litter was studied using Scots pine roots (six diameter classes) and rhizomes from heather (three diameter classes) and cowberry (one diameter class). For Scots pine roots, root diameter was correlated with initial concentrations of N, P, S and Mg but not with organic-chemical composition. The highest nutrient concentrations were found in Scots pine roots and the lowest in heather rhizomes, with cowberry rhizomes intermediate. The highest lignin concentrations were in heather and cowberry rhizomes. In the early decomposition stages diameter and nutrient concentration correlated with mass loss rate in Scots pine roots: in a comparison Scots pine roots were degraded faster than cowberry rhizomes which, in turn, were degraded faster than heather rhizomes. Root diameter, however, may not be important in decomposition of cowberry and heather rhizomes but nutrient and lignin concentrations appear important in all three species. In the late decomposition stages only Scots pine roots could be compared and it appeared that there was a negative correlation with lignin concentration and mass loss, and no correlation with any nutrient.

Fungal biomass and nitrogen in decomposing scots pine needle litter

The development of fungal biomass and increase of amounts of N was studied in decomposing pine needle litter for about 3 yr. After a relatively rapid increase of the amount of mycelium the fungal biomass became rather constant after about 2 yr. The absolute amount of N in the needles increased between the 4th and the 16th months and this increase was correlated to the increase of fungal biomass in the needles.

Nitrogen level and decomposition in Scots pine needle litter

The decomposition of Scots pine green needles and brown needle litter was followed. The nitrogen level was initially 1.2% in green and 0.40% in brown needles. The decomposition rate of the nitrogen-rich litter was initially higher than for the nutrient-poor but after three years the accumulated weight loss was similar in both cases. It was found that the decomposition rate of the lignin of the nitrogen-rich litter was significantly lower than that of the nitrogen-poor in the incubation period after 500 d.

Reduction of decomposition rates of Scots pine needle litter due to heavy-metal pollution

Decomposition of unpolluted Scots pine needle litter was studied in two heavy-metal-pollution gradients in Sweden; one near a brass mill and the other around a primary smelter. In the latter area locally collected polluted Scots pine needle litter was also incubated. Decomposition rates were strongly influenced by the metal pollution and a decrease in the rate of mass-loss occurred. In the brass-mill gradient this occurred until about 1 km from the pollution source which corresponded to about 500 µg Cu and 1 000 µg Zn g−1 soil. Data are presented to indicate that lignin decomposition was more sensitive to pollution than decomposition of whole litter and affected further away from the pollution sources. At the smelter sites, the metal-polluted needle litter decomposed more slowly than the unpolluted needle litter, and this difference was enhanced close to the smelter. The results indicate that heavy metals accumulated in needles prior to shedding have a long-term impact on the subsequent decomposition of the litter. Both litter quality and soil factors thus contribute to the reduced litter decomposition rate in metal-polluted forests. A new non-linear model with decreasing decay rate was used in the statistical evaluation. The model can be used to characterize the effects of pollution on decomposition rate.

Decomposition and nutrient release in needle litter from nitrogen‐fertilized scots pine (pinus sylvestris) stands

Decomposition of Scots pine needle litter originating from five stands treated with different amounts of nitrogen fertilizer was measured over a 4‐year period in a mature Scots pine forest. The litter types, which differed in initial concentrations of nitrogen, phosphorus, potassium, and sulfur, but not in gross organic composition, were studied with respect to mass loss, ingrowth of total fungal mycelium, and net release of nutrients. During the first year of decomposition, rates of mass loss and ingrowth of fungal mycelium were highest in the nutrient‐rich litter. Phosphorus concentration was found to be the main factor affecting mass‐loss rate, and the rate of fungal ingrowth was positively correlated with initial nitrogen concentration. After this initial period, decomposition rates decreased, and after 4 years, accumulated mass loss and amounts of fungal mycelium were similar in all five litter types. These findings may be due, in part, to a lower rate of lignin decomposition in nitrogen‐enriched lit...

Regional variation in rate of mass loss of Pinus sylvestris needle litter in Swedish pine forests as influenced by climate and litter quality

The climatic influence on plant litter decomposition has been successfully correlated on a regional level by using estimated actual evapotranspiration (AET) and annual mass loss. This approach was applied to decomposition studies carried out in a transect along Sweden with litter incubated in four different forest types. A unified needle litter was used and among 14 Scots pine sites about 80% of the mass‐loss rate could be explained. A simple model was made on the influence of both climate and nutrient concentrations (nitrogen and phosphorus) on mass‐loss rate. About 90% of the first‐year mass loss could be explained by this approach. As early decomposition stages were studied (<40%) no influence of lignin was observed.

Decomposition and nutrient dynamics of litter in long‐term optimum nutrition experiments

The chemical composition of the falling litter was investigated over the different fertilization regimes. The nitrogen concentration in the needle litter increased with dosage of nitrogen fertilizer. Nitrogen concentration in needle litter was positively correlated to those of lignin, sulfur and calcium, but negatively to those of zinc, aluminum, and manganese. Phosphorus concentrations were negatively correlated to those of lignin, manganese, zinc, calcium and magnesium. Concentrations of sulfur were negatively correlated to concentrations of zinc, calcium, aluminum, magnesium and boron. The decomposition patterns of Norway spruce needle litter were followed in an optimum nutrition field experiment. Control plots and plots given high dosages of nitrogen and phosphorus fertilizer were used. As decomposition proceeded soluble substances disappeared quickly and the concentration of soluble substances consequently decreased fast. Lignin, being a recalcitrant compound, increased in concentration and the conce...