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Soils are dynamic, complex systems of inorganic, organic and biotic components that have the capacity to support plant life. Soils are classified on the basis of their chemical and physical properties. These properties include texture, structure, colour and the nature and properties of soil horizons. Soils are an important component of natural capital, providing for food and fibre production and physical support of cultural infrastructure, but soils are degrading. Soils are the major site for plant nutrient regeneration through the process of decomposition. Decomposition processes also generate long-term soil organic matter and play an important role in the global carbon cycle. Decomposition of organic matter in soils is accomplished largely by microorganisms, often in association with animals. The rate and course of decomposition is influenced by climate, organic matter composition and nutrient availability from the environment. Key Concepts: Soils are dynamic systems composed of organic, inorganic and living components. Soils are classified according to measurable and observable properties, but there are many different classification schemes around the world. Key physical characteristics of soils are texture, bulk density and porosity all of which influence nutrient and water dynamics in the soil. Key chemical properties of soils include acidity (pH), cation exchange capacity (CEC), base saturation, organic matter content and nutrient availability. Soils are a slowly renewable natural resource that is being degraded worldwide due to erosion, desertification, salinisation and overuse. Decomposition of organic matter is regulated by climate and the nature and chemical composition of the organic matter. Soil organisms, particularly microorganisms (fungi and bacteria) and animals, play a major role in the decomposition process. Decomposition is important in regulating soil nutrient availability to plants. Soils are an important reservoir for carbon in the global carbon cycle and sequester significant amounts of atmospheric carbon dioxide. Keywords: soil taxonomy; paedogenesis; soil formation; soil degradation; organic matter; biogeochemistry; organic matter decomposition; humus; carbon sequestration; global carbon cycle

The patterns of some chemical changes and litter mass-loss rates were investigated for a variety of types of decomposing litter in pine forests under different climatic conditions and at sites with different nutrient status. A mixed deciduous forest was also compared. In initially chemically identical Scots pine needle litter incubated under different climatic conditions, the lignin concentration increased faster as a function of accumulated mass loss when the climatic conditions promoted a higher initial mass-loss rate. Also under artificially created conditions, e.g., after fertilization and irrigation, the same phenomenon occurred. Litter mass-loss rates decreased during decomposition as lignin concentrations increased. The relative decrease was significantly larger at sites with a climate that promoted an initially higher mass-loss rate. At the same lignin concentration, however, the mass-loss rate was significantly lower in drier and colder conditions, viz. climatic conditions that promote a lower initial mass-loss rate. Nevertheless, at very high lignin concentrations that lignin clearly dominated over climate as a rate-regulating factor. A possible consequence of this observation could be a higher rate of organic matter accumulation at sites that initially promote a high initial mass-loss rate for litter than at sites with conditions that give lower initial rates, at least for a given species of litter. Key words: litter, decomposition, lignin, chemical changes, climatic transect, effect of climate change.

By extrapolation of measured mass‐loss values, maximum values for accumulated litter‐mass loss were estimated by using a nonlinear statistical model. This model predicted that the proportion of mass lost through decomposition should be on the average about 85% for Scots pine needle litter and for lodgepole pine needles the corresponding value was almost 100%. The single incubations of Scots pine and lodgepole pine needle litter were made simultaneously and in parallel plots and could thus be compared pairwise. When thus compared, the limit values (asymptotic values) were significantly different in five cases out of six—the exception being a short measurement series. Lodgepole pine is an introduced species in this system.

International audience; Forest ecosystems exert an important influence on global biogeochemical cycles. A global dataset of nitrogen (N) and phosphorus (P) concentrations in leaf-litter of woody plants was compiled from the literature. Among the 677 data sets, 482 included P concentrations and the N:P ratio. * At a global scale, the mean leaf-litter N and P and N:P ratio were 10.9 mg g-1, 0.85 mg g-1 and 18.3, respectively. Leaf-litter N and P were significantly correlated. When the data was grouped by continents, the highest mean N was found in Africa (19.5 mg g-1), and the lowest in North America (8.18 mg g-1). P was significantly smaller in the Asian Islands (Japan and Malaysia, 0.44 mg g-1) than on the Asian mainland. For the global dataset, leaf-litter N increased linearly with mean annual temperature and annual precipitation and decreased with latitude. Although leaf-litter P showed no significant relationship with temperature, it declined linearly with precipitation and there was a convex quadratic relationship with latitude. For the global dataset and also for different functional groups (e.g. shrubs, evergreen broadleaf, deciduous broadleaf, and conifers) the leaf-litter N:P ratio generally followed a positive linear relationship with temperature and precipitation, and showed a concave quadratic response with latitude. * The differences in leaf-litter N:P ratio among functional groups and among continents should be taken into account when modeling biogeochemical cycles in different regions as well as on a global scale.

We synthesized available data for decomposition of pine (Pinus) needle litter in pine forests to determine the litter chemical characteristics and climate factors that explained variation in the limit value, i. e. the level of accumulated mass loss at which the decomposition process either continues at a very low rate or possibly stops. Our data base included 56 separate studies on decomposition of pine needle litter, spanning Scots pine, lodgepole pine, Aleppo pine, stone pine and white pine, mainly incubated at the site of collection. Studies had 5 to 19 samplings, on average 10, and the decomposition was followed to a mass loss ranging from 47 to 83%, on average 67%. The periods from 3.0 to 5.4 years, on average 3.9 years, were of sufficient duration to allow estimates of limit values of decomposition. We used a linear mixed model with regression effects to relate limit values to potential explanatory variables, namely the sites' long-term mean annual temperature (MAT) and mean annual precipitation (MAP) and to substrate-chemistry factors. Regarding the latter, we explored two models; one that included initial concentrations of water solubles, lignin, N, P, K, Ca, Mg, and Mn and one that included only lignin, N, Ca, and Mn to focus on those nutrients known to influence lignin degradation. Using backward elimination significant explanatory variables were determined. For litter decomposed in its site of origin we found the limit value to depend mainly on the initial concentration of Mn, with higher Mn concentrations resulting in higher accumulated mass loss. Thus, litter with higher Mn reached a higher limit value and left a smaller stable fraction. This is likely due to the fact that Mn is an essential component of ligninolytic enzymes important for degrading litter in the later stages of decomposition. Manganese has received little attention in decomposition studies to date. Given its significance in this synthesis, the role of Mn in influencing variation in the late stages of decomposition among ecosystems and among litters of other genera besides Pinus deserves further attention. © 2010 Springer Science+Business Media B.V.

With a data set of litter decomposition collected by means of literature survey, our objectives are 1) to determine the differences in the variation in the first-year mass loss (%) of leaf litter with regard to climate and litter quality among different functional groups of tree species in Eurasian forests, and 2) to determine the difference in effect of mean annual temperature (°C), annual precipitation (dm), as well as concentration of nitrogen (%), and lignin (%) on first-year mass loss over a wide range in climate and litter quality. The main results are as follows. 1) The significant differences between litter types in the relationships between first-year mass loss and climatic factors plus litter quality revealed clearly different decomposition patterns over the continent. Thus, differences were found between coniferous and broadleaf litter, between deciduous broadleaf and evergreen broadleaf as well as between genera and even within a genus, viz. between deciduous and evergreen Quercus. 2) With a change in a relative unit of climate and litter quality variables, there were clear differences in effects of mean annual temperature, annual precipitation, and nitrogen on first-year mass loss for different functional groups of trees. 3) We identified some broadleaf litter species that decomposed to 100% in one year and thus did not contribute to carbon sequestration in a humus layer. Thus, the variation in pattern of foliar litter decomposition with climate and litter quality across functional groups in Eurasian forests showed different decomposition strategies for litter of different groups and genera.

We studied late-stages decomposition of four types of coniferous needle and three types of deciduous leaf litter at two sites, one nutrient-poor boreal and one nutrient-rich temperate. The late stage was identified by that reached by litters at the onset of net loss of lignin mass, i.e. at about 1 year after the incubation when the highest amount of lignin had been detected; the study extended over the following 2 year period. Decomposition rates were significantly lower at the boreal than at the temperate site and did not differ between needle litter and leaf litter. In the boreal forest: (1) mass-loss was positively correlated with N and Mn release, (2) Mn concentration at the start of the late stage was positively correlated with lignin decay, (3) Ca concentration was negatively correlated to litter mass loss and lignin decay. In the temperate forest neither lignin, N, Mn, and Ca concentration at the start of the late stage, nor their dynamics were related to litter decomposition rates and lignin decay. In leaf litter mass-loss and lignin decay were positively correlated with N and Ca release and with Ca concentration. In needle litter mass-loss was positively correlated to Mn release and N concentration negatively with lignin decay. We concluded that Ca, N and Mn have different roles in controlling lignin decay depending on type of litter and site conditions. © 2008 Springer Science+Business Media B.V.

The decomposition dynamics of four types of needle litter and three types of leaf litter were followed for 4 years. Mass losses and certain chemical changes were studied. Most of the nutrient-rich litters appeared to decompose relatively quickly during the first 12–18 months. After 3–4 years, however, their accumulated mass losses were lower compared with litter types that intially had lower rates. Thus the more nutrient-rich litters had considerably lower mass-loss rates in the later stages. This pattern was even more pronouced for extract-free lignocellulose: its mass-loss rate was negatively related to the lignin concentration, which increased progressively as litter decomposition proceeded. During late stages in litter with a high nitrogen content, there was also a clear negative relation between nitrogen concentration and lignin mass-loss rate, as well as between nitrogen concentration and litter mass-loss rate. By extrapolation of measured mass-loss values, maximum values for accumulated litter–mass loss were estimated. A nonlinear statistical model predicted that the proportion of mass lost through decomposition should be 50% for grey alder leaves, 54% for green leaves of white birch, and 57% for brown leaves of white birch. For Scots pine the predicted maximums for accumulated mass loss were 68% for green needles and 89% for brown needles, whereas corresponding values for lodgepole pine needles were 81% (green) and 100% (brown). Lodgepole pine is an introduced species in this system. Key words: litter, decomposition, lignin, nitrogen, maxium mass loss.

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...

The concentration of live fungal biomass (FDA mycelium) was investigated in seven chemically very different litter types, i.e. grey alder leaves, green and brown leaves of white birch, green and brown needles of both Scots pine and lodgepole pine. Concentration of FDA mycelium in the litters changed with the season, being low during the dry late summer and relatively higher during the more rainy, wet autumn. During autumns, when moisture was not limiting, lignin concentration and live fungal biomass were negatively related. It was concluded that the concentration of live fungal biomass in litter decreased linearly as the lignin concentration increased and thus also decreased linearly to decomposition level of the litter.

Publisher Summary There are numerous definitions of the concept ‘‘humus'’ and the one adopted is not uncontroversial. This definition is widely accepted and appears to fit well with the concepts of litter decomposition and humus formation. The stabilized part of the soil organic matter or stabilized humus is organic matter that has lost not only the original fiber and tissue structure but also has a modified chemical structure, is relatively resistant to decomposition, and is present either in an organic layer on top of the mineral soil or mixed with mineral soil. This chapter describes and discusses soil organic matter (SOM) using the stated definitions. It first presents a traditional separation of SOM into the subgroups fulvic acids, humic acids, and humins, then it discusses secondary organic matter, and later humus buildup, quantified as sequestration of SOM (C), and finally, comments on the stability and decomposition of humus. SOM has been described and discussed using the already stated definition. The chapter uses the terminology and definitions for humus and soil organic matter suggested by Waksman et al.

Publisher Summary This chapter treats selected heavy metals as nutrients in unpolluted systems and discusses their dynamics in that context. It describes detailed chemical changes for Scots pine needle litter as a case study and presents data from other boreal and temperate species. The patterns discussed have been found mainly in boreal systems but probably have higher generality and even such different systems as decomposing chaparral shrubs show similar decomposition patterns as litter from boreal tree species. A possible explanation to that differing decomposition pattern is that spruce trees produce more heterogeneous foliar litter which, in addition, is in a late decomposition stage already when shed, as a consequence of an advanced decomposition of the needles while still attached dead to the twigs. Most litter species leach carbon compounds to differing extents. Such leaching may start in the early phase and continue throughout the following decomposition stages. Recent findings have indicated that raised N concentrations in foliar litter may support the leaching process of carbon compounds. The chapter intends to demonstrate and systemize decomposition patterns as well as the effects of several chemical components and the chemically changing litter substrate on decomposition rates.

Publisher Summary This chapter emphasizes the functional roles of microorganisms rather than their taxonomy. The concepts of White-rot, brown-rot, and soft-rot and what they functionally stand for in terms of degradation processes have been presented. These functional concepts have been used as a basis to discuss the degradation of litter tissues. Although the terms originally referred to visually different types of lignin degradation, it now appears that the degradation of not only lignin but also cellulose and hemicellulose is different among the taxonomic groups of microorganisms. The terms relate to the type of rot rather than to the group of organisms—namely, rots giving the wood a white or brown color. The common use of the terms has been adopted and refers to fungi when using the terms white-rot, brown-rot, and soft-rot. As the focuses have been on boreal and temperate systems, with an evident dominance of microorganisms in the decomposition process, special attention have been given to microbial communities and the enzymatic degradation mechanisms for the polymer carbohydrates and lignin. The chapter thus presents basic properties of microorganisms.

Publisher Summary This chapter presents an overview of a range of field and laboratory methods to study decomposition. Some methods of data analysis and presentation have been discussed. They all represent more general techniques and conventions used in data handling and advantages and disadvantages of using particular models and methods have been discussed rather than giving detailed formulas for calculating statistics. As decomposition of organic matter is a set of most complex biological, physical, and chemical processes, a broad range of research techniques and tools is required to study it. Depending on the research problem, techniques are needed to expose plant litter in the field or to apply atomic absorption spectrometry (AAS), nuclear magnetic resonance (NMR), chromatography, or isotopic analyses. The chapter presents those methods that are used solely in decomposition studies and mentions some more general techniques with other possibilities and what to look for when more detailed studies are required. It pinpoints the pitfalls and indicates some solutions that pertain especially to the studies of organic matter decomposition. Thus, the chapter can be used as a reference for specific litter decomposition techniques. It presents a brief overview of mathematical decomposition models and some useful statistical methods.

Publisher Summary Studies of decomposition dynamics have been performed using different litter types, at sites in different climatic regimes and in different forest types. With the increasing emphasis on understanding the impact of climate change, and the broad-scale patterns of biological processes, the issue of geographic scale versus decomposition patterns versus litter chemical composition becomes critical. This chapter focuses on litter decomposition in stands with monocultures and uses results from five main transects with either only Scots pine or different pine species and one with Norway spruce in which foliar litter decomposition was studied. The results are possibly contrasting enough to illustrate that different patterns should be expected among species under varying climates. To illustrate this, the effect of climate on different decomposition stages have been described—that is, early stage and late stage separately. The chapter also presents results from a transect in which root litter decomposition was studied. The chapter describes respiration from humus at seven sites from one of the transects.

Publisher Summary This chapter focuses on nitrogen as there appears to be more knowledge generated on N dynamics in litter and humus than on other nutrients, making it possible to create a conceptual model for its dynamics. The details of the dynamics and the release mechanism are still not well explained, though, and are often related to litter species, giving the observations an empirical character. Thus it focuses on a common pattern for foliar litter. The chapter attempts to create a system for describing N dynamics in decomposing litter and to do this, several case studies we have been used that are consider to be representative, at least for litter in boreal and temperate ecosystems. It presents a system for N dynamics in decomposing litter, describing different phases of the dynamics as well as a suggested release mechanism. Starting with newly shed litter, the N concentration in humus has been calculated.

Publisher Summary This chapter describes the fate of pollutants such as; heavy metals, organic compounds, and acidic precipitation, on litter and soil and give an overview of the present knowledge about their effects on decomposition processes. It discusses the possible effects of global warming and changes in water regimen on litter decomposition. The term anthropogenic impacts cover a broad range of human activities leading to various effects on soil processes. Intensive agriculture and forestry frequently cause massive losses of the most fertile, surface soil layer, but, on the other hand, reasonable management can turn infertile soils into arable acreage. These problems are mostly the domain of intentional activities and have been extensively studied. The chapter concentrates on anthropogenic impacts of specific importance for organic matter decay in forest ecosystems—impacts that usually are unintentional and undesired.

The quantification of terrestrial sources and sinks for carbon dioxide and nitrogen-based greenhouse gases is one of the most important tasks facing environmental scientists today. Central to this is the determination of mechanisms for carbon (C) and nitrogen (N) sequestration in the soil organic matter (SOM). The buildup rate of an SOM layer depends not only on the amount of litter fall and the quality of the decomposing plant litter but also on the completeness of its decomposition. A clear problem is that the accumulation of SOM is a slow process that normally spans generations of scientists, thereby causing continuity problems in studying its buildup as well as the mechanisms controlling it.

Litterfall is the largest natural inflow of organic material and nutrients to the forest floor and in most European forests is dominated by that from the trees. The chemical composition of this material and the temperature and moisture content of the upper soil layers are considered to be the main factors controlling the turnover rates of the shed litter and the release of nutrients. Thus they also determine the quantity of nutrients released and the accumulation of humus and nutrients. Few larger studies have been made on litterfall on a European basis. For needles of Norway spruce and different pine species, Berg and Meentemeyer (2001) made a study covering the main part of western Europe, whereas for European beech, for example, few values have been published.

Although litter chemical composition and climate appear to dominate as regulating factors in decomposition over a regional scale (Chaps. 4–7), there are numerous factors that are important in regulating decomposition at the local or even micro scale. These are related to soil characteristics, nutrient availability and cycling, plant community composition and structure, soil fauna, and topography. Some of these factors exert their influence by modifying the microclimate. Other factors operate primarily through biochemical or nutritional influences on microbial metabolism. Not only can these site-specific factors influence microbial metabolism, they can also alter the composition of the microbial community, as was discussed in Chapter 3.

The microbial decomposition of plant litter is a basic process in the functioning of ecosystems, not only for the general release of nutrients to plants, but also for the buildup of a stable humus and the accompanying storage of nutrients.

Human activities impact the entire planet. Global climate change and thinning of the stratospheric ozone layer are two anthropogenic factors with worldwide impact that may also influence decomposition of plant litter. On a more continental scale, acid precipitation, including both wet and dry deposition of S and N, is diminishing, but remains important regionally. Ozone is often elevated in urban and industrial areas with possible impacts on decomposition and other ecosystem processes. Also heavy metals that are released into the environment due to human activity, may be detrimental to microorganisms. Some human activities are intended to increase productivity of agricultural and forest ecosystems, and the associated practices can have an impact on decomposition. There is an immense variety of activities that are undertaken in the pursuit of agriculture and forestry, ranging from pesticide application to prescribed burning, activities that to different extents may influence the decomposition of plant litter.

The graphs for accumulated litter mass loss vs time form different patterns, which may be described using a few simple mathematical functions. Generally we may state that litter may decompose completely or leave a stabilized residue, which may decompose very slowly. Complete decomposition may be described using the single exponential, with a constant rate, the same function as for radioactive decay. In another approach the complete decomposition may be described in two steps using a two-factorial ‘double exponential’ with part of the litter decomposing at a high and part at a lower rate. A flexible, decreasing rate and a calculable stabilized residue are described using an asymptotic function. The decreasing decomposition rate allows the calculation of a limit value, often between 50 and 100% accumulated mass loss, using the decomposition rate of zero. Although this estimated rate gives an extremely slowly decomposing ‘stable’ fraction there are few studies on the stability of this residue. Possibly a raised nitrogen concentration may be stabilizing.

Decomposition of plant litter involves a complex set of processes including chemical, physical and biological agents acting upon a wide variety of organic substrates that are themselves constantly changing. Due to the immense diversity of possible factors and interactions, decomposition in a natural setting can be described in general terms only. In spite of this complexity, several major processes are involved and general trends can be outlined.

Chapter 5 reviewed the effects of initial litter chemical composition on the pattern of changes that occur during decay. This chapter focuses attention on the influence of changing litter quality on the decay processes, and will show that the influences of selected litter chemical components change dramatically during the process, sometimes even reversing the direction of their effect. For this purpose, the three-phase model introduced in Chapter 2 is applied to explain the effects of chemical variation, changes in mass-loss rates and decomposition patterns. Litter types that have been found to deviate from the general pattern will also be discussed.

The decomposition dynamics of four types of needle litter and three types of leaf litter were followed for 3 years at two very contrasting coniferous forest systems, a nutrient-rich silver fir (Abies alba Mill.) forest in south Italy (Monte Taburno) and a nutrient-poor Scots pine (Pinus sylvestris L.) forest in central Sweden (Jädraås). Decomposition of the same litter type at the two sites did not differ in the early stages but proceeded further at the nutrient-rich forest site than at the nutrient-poor one. Limit values for decomposition were calculated and the differences for the same litter type between the two contrasting coniferous systems were investigated. At both sites six of the seven litter types gave significant (asymptotic) limit values for decomposition, which varied with litter type. For four litter types out of six the limit values differed significantly between the two sites and were always higher at the nutrient-rich site (Monte Taburno). Using all available data for litters incubated at the two sites revealed that at the nutrient-poor site (Jädraås) there was a significant negative relationship between litter N levels and limit values and there was also a significant negative relationship between initial concentrations of heavy metals (e.g. Zn, Cd, Cu) and limit values. In contrast, at the site Monte Taburno, rich in nutrients and in heavy metals, there was no such relationship. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

The dynamics of Mn, Zn, Fe, Cu, Pb, and Cd during litter decomposition was studied to understand how litter and soil metal concentrations influence the accumulation and/or the release of metals. Three types of leaf litters (green and brown leaves of Populus tremula L., green leaves of Betula pubescens Ehrh.) and four types of needle litters (green and brown needles of Pinus sylvestris L., brown needles of Pinus contorta L. and Pinus pinea L.), differing in heavy metal concentrations were incubated at two unpolluted coniferous forest sites: a temperate silver fir (Abies alba Mill.) forest at site Monte Taburno, southern Italy, and a boreal Scots pine (Pinus sylvestris L.) forest at site Jadraas, Sweden. Compared to the Scots pine forest, the humic surface horizon of the silver fir forest had a higher pH, higher concentrations of available Cu and Cd, and lower Fe concentration. After a period of about 900 days, all litter types were more decomposed at M. Taburno than at Jadraas. Zn, Fe, Pb and Cd concentrations in leaf and needle litters increased at both sites; Cu concentrations increased only in the silver fir forest; Mn concentration decreased in the litter richest in Mn and increased in the litters poorest in Mn. Litter decomposition in an early phase (0 up to 528/565 days) was significantly and positively correlated to initial Cu and Cd concentrations in litter. In a late phase (528/565 up to 918/929 days), decomposition was correlated significantly and negatively with litter Zn and Cu, and significantly and positively with litter Mn concentration at the start (528/565 days) of the period. At both sites, increases in absolute amounts of Fe, Cd, and Pb were observed in all litters; however, P. pinea, which showed the highest initial concentration of Pb, released Pb during decomposition. All litters released Cu at Jadraas and accumulated Cu at M. Taburno. Zn was released at both sites from all leaf litters and from the needle litter of P. contorta. Mn was released at both sites from all litters, except the litter of P. tremula, which accumulated Mn at Jadraas. The results indicate that heavy metal accumulation or release may depend on the gradient of metal concentration between litter and soil, on the pH of the soil, and on the capacity of litter to bind metal; atmospheric deposition could account at least partly for the increase of absolute amounts of Pb, Fe, and Cu at M. Taburno.

The fungal mycelium ingrowth and the rates of mass loss and respiration of needle litter of Pinus pinea, Pinus laricio, Pinus sylvestris, and Abies alba were investigated, in three coniferous forests, over a 3-year period by means of a composite set of incubations. In the early stages, the fungal flora of the decomposing needles was dominated by dematiaceous hyphomycetes and coelomycetes. Basidiomycetes reached a peak after 6 months on pine needles, but were absent from the N-rich needles of A. alba. Soil fungi (Penicillium, Trichoderma, Absidia, Mucor sp. pl.) became most frequent in later stages. At the end of the study period, the total mycelium amount showed the lowest values in all pine needles incubated in the P. laricio forest and the highest ones in P. pinea needles incubated in the P. pinea forest. In all data sets, as in data for boreal forests examined for comparison, the concentration of litter fungal mycelium versus litter mass loss followed a common exponential model. However, in later stages, the amount of litter fungal mycelium was very close to that of the humus at the incubation site, thus supporting the hypothesis of a logistic growth pattern. Respiration rates of decomposing litters varied with season and decreased with litter age to values close to those of the humus at the incubation site. Respiration of water-saturated litter was negatively correlated with the total mycelium concentration, and this was consistent with the observation that in far-decomposed litter only a minor fraction of the total mycelium is alive. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

International audience; Standard C-13-labelled plant material was exposed over 2-3 yr at 8 sites in a north-south climatic gradient of coniferous forest soils, developed on acid and calcareous parent materials in Western Europe. In addition to soils exposed in their sites of origin, replicate units containing labelled material were translocated in a cascade sequence southwards along the transect, to simulate the effects of climate warming on decomposition processes. The current Atlantic climate represented the most favourable soil temperature and moisture conditions for decomposition. Northward this climatic zone, where the soil processes are essentially temperature-limited, the prediction for a temperature increase of 3 degrees C estimated a probable increase of C mineralisation by 20-25% for the boreal zone and 10% for the cool temperate zone. Southward the cool Atlantic climate zone, (the Mediterranean climate), where the processes are seasonally moisture-limited, the predicted increase of temperature by 1-2 degrees C little affected the soil organic matter dynamics, because of the higher water deficit. A significant decrease of C mineralisation rates was observed only in the superficial layers recognised in Mediterranean forest soils as 'xeromoder' and subject to frequent dry conditions. In the deeper Mediterranean soil organic horizons (the mull humus types), representing the major C storage in this zone, C mineralisation was not affected by a simulated 2 degrees C temperature increase. The temperature effect is probably counteracted by a higher water deficit.

Les donnees de chute de litiere, essentiellement des chutes d'aiguilles, etaient disponibles pour 64 sites le long d'un transect depuis le cercle polaire en Scandinavie (41 sites) jusqu'au Sud de l'Espagne (22 sites supplementaires en Europe continentale) et un site dans le mid-ouest americain. Les donnees proviennent de huit especes de Pin. Des regressions ont ete calculees principalement pour la chute d'aiguilles et dans certains cas pour la chute totale de litiere. Il existe une relation lineaire hautement significative entre la chute des aiguilles et la latitude (R 2 adj = 0.285; n = 58; p < 0,001) lorsque l'ensemble des donnees pour toutes les especes sont utilisees. La combinaison des sites en climat boreal et atlantique donne un R 2 adj de 0.732 pour n = 45 (p < 0,001). Une relation lineaire multiple, utilisant l'âge du peuplement, la latitude et la surface terriere est hautement significative et donne un R 2 adj de 0.412 (n = 54; p < 0.001). La meilleure relation lineaire, pour les retours d'aiguilles chez le Pin sylvestre en Scandinavie a ete obtenue en utilisant comme variables predictives l'indice de fertilite stationnelle « H 100 » (R 2 adj = 0.349), la latitude (R 2 adj = 0.331), et la surface terriere (R 2 adj = 0.324) alors que la regression sur les variables altitudes et âge des peuplements etait significative seulement a p < 0,01. Une fonction X 2 pour l'âge du peuplement ameliore la relation avec l'âge, R 2 adj = 0,243. Les relations multiples entre la chute des aiguilles et la latitude associee a la surface terriere et celle associee a la surface terriere plus l'âge, pour les sites Scandinaves, sont hautement significatives (R 2 adj = 0,605 et R 2 adj = 0,661, respectivement, avec n = 41). La procedure de regression progressive sur les donnees des memes sites, combinaisons des variables latitude, indice de fertilite, surface terriere et âge du peuplement permet d'expliquer 78 % de la variation de la chute des aiguilles. Des relations hautement significatives ont ete calculees sur les donnees des retours totaux de litiere, utilisant la meme methodologie sur 32 des sites. Elles demontrent que la quantite des chutes des seules aiguilles peut etre utilisee comme indice pour la chute totale de la litiere.