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During the initial stages of leaf and needle litter decomposition, microorganisms face nitrogen (N) and phosphorus (P) scarcity since plant litter is very N- and P-poor compared to microbial biomass. The processes that microorganisms use to cope with the unfavorable stoichiometry, such as transport of nutrients into decomposing litter, are still not fully understood.The aim of the study was to explore the import and release of nutrients (N, P, K, Mn, Ca, and Mg) into and from decomposing Norway spruce (Picea abies Karst) and Scots pine (Pinus silvestris L.) needle litter. For this purpose, we conducted a paired-stand litterbag study at eight temperate and boreal forest sites in Sweden that each have a spruce and a pine stand, over a period of five years. The mass of N in decomposing spruce and pine needle litter increased during the first 172 and 356 days, on average by 19% and 30%, respectively, compared to the initial masses of the element in the litter. The mass of P in pine litter increased during the first 526 days of decomposition, on average by 48%. Net release of N from spruce litter, relative to the initial N amount, only began after 895 days of decomposition. Net release of N and P from pine litter, relative to the initial amounts of the elements, started only after 1097 days. In contrast, K, Mn, Ca, and Mg were released right from the beginning of the decomposition process.The results show that N and P import into decomposing plant litter is a quantitatively important process in temperate and boreal coniferous forests during the first stage of litter decomposition when N and P concentrations are low. Nutrient import alleviates stoichiometric imbalance between the microbial biomass and the litter and likely contributes to microbial nutrient acquisition.

Litter of plant origin is the main source of soil organic matter, and its physical and chemical quality and decomposition rates are key variables in the prediction and modelling of how litter-derived carbon (C) is cycling through the ecosystem. However, the biological control factors for decomposition are not well understood and often poorly represented in global C models. These are typically run using simple parameters, such as nitrogen (N) and lignin concentrations, characterizing the quality of the organic matter input to soils and its accessibility to decomposer organisms. Manganese (Mn) is a key component for the formation of manganese peroxidase (MnP), an important enzyme for lignin degradation. However, the functional role of Mn on plant litter decomposition has been rarely experimentally examined. Here, using a forest and a cropland site we studied, over 41 months, the effects of Mn fertilization on MnP activity and decomposition of eight substrates ranging in initial lignin concentrations from 9.8 to 44.6%. Asymptotic decomposition models fitted the mass loss data best and allowed us to separately compare the influence of Mn fertilization on different litter stages and pools. Across substrates, Mn fertilization stimulated decomposition rates of the late stage where lignin dominates decomposition, resulting in smaller fraction of slowly decomposing litter. The increased MnP activity caused by Mn fertilization provided the mechanism explaining the stimulated decomposition in the Mn-addition treatments.

Despite the importance of decomposition for biogeochemical cycles, it is still not clear how this process is affected by different forms of nitrogen (N). Equal amounts of N with different ratios of inorganic N : organic N (0 : 0, 10 : 0, 7 : 3, 5 : 5, 3 : 7, and 0 : 10) were added to the soil in a steppe. We studied the response of litter decomposition to different forms of N enrichment. The treatment with 30% organic N resulted in the fastest decomposition, which was higher than with inorganic N or organic N addition alone. Our results highlight the need for studies of N deposition on carbon cycles that consider different components in N deposition.

Despite the importance of plant litter decomposition for ecosystem nutrient cycling and soil fertility, it is still largely unknown how this biogeochemical process is affected by different forms of nitrogen (N). Numerous studies have investigated the effects of exogenous N addition on leaf litter decomposition, while the response of decomposing roots and their microbial communities to externally applied N is rarely studied. Fine roots, however, represent a key input to soil organic matter and understanding their decomposition under elevated atmospheric N deposition is important for predicting soil carbon (C) dynamics in response to changes in climatic conditions. In this study, we decomposed fine roots of five dominant grassland species for two years in field plots fertilized with different forms of N in a typical temperate grassland in Inner Mongolia. Ammonium nitrate was selected as inorganic N (IN), while urea and glycine were chosen as organic N (ON). Equal amounts of N (10 g N·m−2·yr−1) with different ratios of IN: ON (control, 10 : 0, 7 : 3, 5 : 5, 3 : 7, and 0 : 10) were added to the soil. Our results showed that all exogenous N additions, either IN or ON forms, stimulated the decomposition rates across species. Furthermore, the treatment with a mixture of IN and ON fertilizers led to the strongest responses in decomposition rate, which were, on average, 20% higher than control, and 12% higher than using just IN addition across the five studied species. Our results suggest that we need to consider the different components in N deposition when examining nitrogen deposition effects on terrestrial ecosystem carbon and nutrient cycles.

For decomposing foliar litter in forest systems the pattern for accumulated mass loss vs time varies among litter species and environments. Thus, decomposition may be complete (100% accumulated mass loss) or the rate may cease and proceed at such a low rate that a limit value for decomposition may be estimated. Such limit values have been found to range from 42% accumulated mass loss to 100%, resulting in stable fractions of 58 and 0%, respectively. To support a discussion about pattern we need causal relationships and I have used a conceptual model with three decomposition stages as a reference and discuss that in relation to two nutrients that influence the degradation of lignin and lignified tissue. Whereas a high nitrogen (N) concentration may have a stimulating effect on degradation of holocellulose in the early stage the same high N concentration may retard the degradation of lignified tissue in the late stage and the higher the N concentration the stronger the retardation. Through the enzyme manganese peroxidase (MnP), manganese (Mn) stimulates lignin degradation and thus the lignified fraction of litter. A high level of Mn appears to support a further degradation of the lignified tissue and thus results in a high limit value, whereas a high N has been related to a low value. The aim behind this review is to: (i) identify specific factors that may influence the pattern of accumulated mass loss and thus the type of regression model that describes the data; and (ii) present a possible climate influence on decomposition pattern. To this purpose we need to discuss a possible variation in pattern following the variation in concentrations of the two nutrients N and Mn. Concentrations of N and Mn in newly shed litter are both influenced by site climate, with N increasing and Mn decreasing with increasing mean annual temperature (MAT) and increasing annual actual evapotranspiration (AET). Thus, climate becomes at least an indirect regulating factor for decomposition pattern. Two main regression models are used to describe the patterns, namely the single exponential and a function separating a readily decomposed and a stable fraction, by estimating a limit value (asymptotic function). For the discussion on decomposition pattern a model parameter was used, namely estimated limit values with a so far observed range in variation from 42 to 100%. The stabilized litter fraction defined by the limit value may be used for estimating the accumulation rate of stable carbon (C) in soil.

The litterbag method (LB) widely used in decomposition studies was compared to a direct observation method (DO) of the litter collected in the forest floor in three European Scots pine forests. In this last method, needles were sorted out of blocks of the forest floor and separated into 5 categories using morphological criteria representing different layers and decomposition levels. The residence time of the needles in each layer was calculated using the litterfall input, the size of the layer in the forest floor and its level of decomposition. The mass loss of each layer was determined using the linear mass density method. To estimate decomposition rates, the DO method might be more advantageous than the LB method because it is done without any manipulation or microclimatic artifact and because it integrates spatial and temporal variability. It is also convenient because the sampling does not have to be done simultaneously on different sites to allow site comparisons. Indeed, the collected litter blocks include the whole site history until the needle collapses which has been shown to take place after a period of up to 10 years in the present study. After 3 years, the mass loss was lower measured by the DO method than by the LB method. The decrease of litter mass measured by the DO method was best described by a simple asymptotic function of time. This model has a mechanistic signification because it underlines the possible long-term accumulation of recalcitrant products which is not the case with the exponential function. This accumulation of recalcitrant products is a key process in the storage of carbon in the organic matter reservoir of forest soils. Nevertheless this method is time consuming and cannot replace the LB method for large data set collection. This paper recommends when using the LB method to be aware of its inherent bias that can lead to misestimating decomposition in the long run.

The objective of this work was to investigate the usefulness of near infrared reflectance spectroscopy (NIRS) in determining some C and N fractions of soils: labile compounds, microbial biomass, compounds derived from added 13C- and 15N-labelled straw. Soil samples were obtained from a previous experiment where soils were labelled by addition of 13C- and 15N-labelled wheat straw and incubated in coniferous forests in northern Sweden (64–60°N) and south France (43°N). The incubation lasted three years with 7–9 samplings at regular time steps and four replicates at each sampling (204 samples). Samples were scanned using a near infrared reflectance spectrophotometer (NIRSystem 6500). Calibrations were obtained by using a modified partial least squares regression technique with reference data on total C and N, 13C, 15N, control extract-C, -N, -13C and -15N, fumigated extract-C, -N, -13C and -15N, biomass-C, -N, -13C and -15N contents. Mathematical treatments of the absorbance data were first or second derivative with a gap from 4 to 10 nm. The standard error of calibration (SEC)-to-standard deviation of the reference measurements ratio was ≤0.2 for 10 models, namely total C and N, 13C, 15N, control extract-C, fumigated extract-C and -N, biomass-C and -N and biomass-15N models and therefore considered as very good. With an R2=0.955, the fumigated extract-15N model is also good. The standard error of performance calculated on the independent set of data and SEC were within 20% of each other for all the best equations except for the biomass-15N model. The ability of NIRS to detect 13C and 15N in total C and N and in the extracts is noteworthy, not because of its predictive function that is not really of interest in this case, but because it indicates that the spectra kept the signature of the properties of the organic matter derived from the straw even after two- or three-year decomposition. The incorporation of the 13C in the biomass was less well predicted than that of the 15N. This could indicate that the biomass derived from the straw was characterised by a particular protein or amino acid composition compared to the total biomass that includes a large proportion of dormant micro-organisms. The predictive ability of NIRS for microbial biomass-C and -N is particularly interesting because the conventional analyses are time consuming. In addition, NIRS allows detecting analytical errors.

The objective of this investigation was to assess the changes in chemical composition (lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash) of decomposing litter. Standard Pinus sylvestris needle litter, originating from southern Sweden, was incubated in litterbags at 15 sites selected from the Netherlands to south Spain. The changes in chemical composition of this litter were determined using near infrared reflectance spectroscopy. The hypothesis was that standard (chemically uniform) litter decomposing under a range of climates would show different dynamics of accumulation and loss of C-fractions, N, and ash, relative to mass loss. It was shown that, for a given mass-loss value (10, 20, 30, 40, or 50%), the proportion of lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash in the decomposing pine needles differed between sites. Lignin concentration in the litter residue at 50% mass loss ranged from approximately 26 to 43%, cellulose from 19 to 27%, hemicelluloses from 7 to 11%, non-structural compounds from 19 to 25%, N from 0.7 to 1.3%, and ash content from 1.4 to 10.1%. Lignin concentrations showed the highest range of variation. Lignin concentrations during decomposition were positively related to moisture factors as significant correlations were found with actual evapotranspiration and were improved in multiple regressions by the mean annual precipitation or the water surplus. Cellulose was degraded further at sites with high precipitation whereas hemicellulose degradation was related to temperature. This leads to the conclusion that the remaining organic matter produced by standard litter decomposition within the studied climatic range of variations tended to be more recalcitrant under wet and warm climatic conditions than under cold or dry climate.

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.

Mass loss of barley straw exposed at 10 cm depth in an unfertilized field plot was followed for 2 yr. Organic-chemical changes in the decomposing straw were also followed as well as the ingrowth of flourescein-diacetate-active (FDA) fungal mycelium and total mycelium. Mass loss was faster during the first year than in the second. Concentration of total solubles decreased initially to a low but fairly constant level which was maintained for 2 yr. Total solid polysaccharides increased initially, but began to decompose after about 200 days, while sulfuric-acid lignin, which also increased initially, later began to decompose slightly. The amount of FDA-active fungal mycelium remained at a low and constant level for the 2 yr while total fungal biomass started to increase after the first winter and continued for the rest of the investigation. Within 3 weeks about 30% of the N had been leached from the straw and after the first winter about 50% of the N was released. After that a net uptake took place and after 2 yr the total net release was about 25%. The relative amount of N increased continuously after the first winter and was linear to mass loss.

We have followed the decomposition of red clover root remains and described the process. In this investigation, red clover roots, divided into three diameter classes: 4 mm, were incubated in mesh bags in a loam soil at a temperature of 22°C and a moisture of pF 2–3 for up to 196 days. Analyses were made on the initial dead plant material and on mesh-bag contents of the 1–4 mm roots for the amounts of water and ethanol solubles, solid polysaccharides, lignin, ash, carbon, nitrogen and fungal mycelium. Organic matter mass loss was fast during the first 13 days (44%) and almost ceased after 30 days when about 29% of the organic material remained. There was no notable difference in mass- or nitrogen loss from roots of different diameters. The carbon-to-nitrogen ratio of the root remains decreased from initially 25–27 to 11–13 at the end of the incubation. The high rate of mass loss at the beginning is explained by the considerable initial concentration of water-soluble substances (31%), being the fraction which decreased the fastest (roots 1–4 mm φ). Glucans comprising 40% of the initial root material, were degraded linearly to total mass loss. Other polysaccharides (galactans, mannans, xylans, arabans and rhamnans) disappeared more slowly than the glucans. The amount of total fungal mycelium within the decomposing root remains increased during the incubation from 55 to 867m g −1 of organic material.

SEthylene formation in different litter components and soil layers from a Scots pine forest (site 1) and a mixed Scots pine and Norway spruce forest (site 2) is reported. The effects of temperature. moisture, length of incubation and anaerobic incubation on C2H4 formation in humus are also reported. The highest concentrations of C2H4 that accumulated in 24 h under ambient conditions in the laboratory were 3 and 17 parts/106 in humus from sites 1 and 2 respectively and 153 parts/106 in pine-needle litter. The most frequent microfungi from site I were tested for C2H4 formation on different media. A Mucor silvaticus isolate was found to form C2H4in the presence of methionine. The origin and significance of the C2H4 formed is discussed.

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.