Charcoal Application to Arable Soil: Effects on CO2 Emissions

Activated carbon and commercial household charcoal were added to soil in a 36-day incubation study at 20 °C measuring carbon dioxide evolution. The black carbon materials were found to decompose slowly, releasing between 1.4% and 0.8% of their carbon content per year, respectively. The main experiment tested whether the black carbon additions to soil (2% and 4% by mass) affected decomposition of selected substrates in soil, both respiration dynamics and total respiration. The results indicated that the black carbon materials tested had no effect on total respiration from added glucose. However, decomposition rates of amylose, xylan, casein, and ryegrass were reduced in soil with addition of activated carbon but were not significantly affected by household charcoal. A larger surface area of activated carbon than that of household charcoal, and thus a greater adsorption capacity for organic compounds and exo-enzymes required to break down water-insoluble substrates, may explain the results.     

Soil characteristics determine soil carbon and nitrogen availability during leaf litter decomposition regardless of litter quality

Climate and litter quality have been identified as major drivers of litter decomposition, but our knowledge of how soil characteristics (e.g. microbial community and chemical properties) determine carbon (C) and nitrogen (N) availability derived from the decomposition of litter of different qualities is still scarce. We conducted a microcosm experiment to evaluate how soils with contrasting microbial communities and soil properties (denoted Soils A and B hereafter, where Soil B has higher bacterial and fungal abundance, fungal:bacterial ratio, and organic C than Soil A) determine the availability of soil C (carbohydrates, proteins, amino acids and phenols) and N (dissolved organic and inorganic N, microbial biomass N and available N) during the decomposition of litter of contrasting quality (C:N ratios ranging from 20 to 102). We also evaluated the relative importance of soil characteristics and litter quality as drivers of C and N inputs to the soil during this process. Overall, higher soil C and N availability after litter decomposition was found in Soil B than in Soil A. Soil characteristics had a higher positive effect on soil C and N contents than litter quality during litter decomposition. We also found that changes in N availability and organic matter quality registered after litter decomposition, linked to different soil characteristics, were able to promote dissimilarities in the potential mineralization rates. In conclusion, our study provides evidence that soil characteristics (e.g. microbial communities and chemical properties) can be more important than litter quality in determining soil C and equally important for N availability during the decomposition of leaf litter.     

How temperature and aridity drive lignin decomposition along a latitudinal transect in western Siberia

Climate change drives a northward shift of biomes in high-latitude regions. This might have consequences on the decomposition of plant litter entering the soil, including its lignin component, which is one of the most abundant components of vascular plants. In order to elucidate the combined effect of climate and soil characteristics on the decomposition pattern of lignin, we investigated lignin contents and its degree of oxidative decomposition within soil profiles along a climosequence in western Siberia. Soil samples were collected from organic topsoil to mineral subsoil at six sites along a 1500-km latitudinal transect, stretching from tundra, through taiga and forest steppe to typical steppe. The stage of lignin degradation, as mirrored by decreasing organic carbon-normalized lignin contents and increasing oxidative alteration of the remnant lignin (acid-to-aldehyde ratios of vanillyl- and syringyl-units [(Ac/Al)_V and (Ac/Al)_S]) within soil horizons, increased from tundra to forest steppe and then decreased to the steppe. Principal component analysis, involving also climatic conditions such as mean annual temperature and aridity index, showed that the different states of lignin degradation between horizons related well to the activity of phenoloxidases and peroxidases, enzymes involved in lignin depolymerization that are produced primarily by fungi and less importantly by bacteria. The low microbial lignin decomposition in the tundra was likely due to low temperature and high soil moisture, which do not favour the fungi. Increasing temperature and decreasing soil moisture, facilitating a higher abundance of fungi, led to increased fungal lignin decomposition towards the forest-steppe biome, while drought and high pH might be responsible for the reduced lignin decomposition in the steppe. We infer that a shift of biomes to the north, driven by climate change, might promote lignin decomposition in the northern parts, whereas in the south a further retardation might be likely.     

Legacies of plant litter on carbon and nitrogen dynamics and the role of the soil community

There is increasing awareness of the importance of ecological legacies in contemporary ecosystem processes. Decomposition is regulated by a set of interacting hierarchically organized factors. As spatial and temporal scales decrease, decomposition is largely dependent on the quality of resources and the decomposer community, but whether and how these factors manifest via historical legacy effects is not well understood. We tested whether the history of plant litter inputs had short-term legacy effects on contemporary litter and soil organic matter carbon (C) and nitrogen (N) mineralization. Using a field/laboratory microcosm approach, we exposed soils to two litters of contrasting chemistry and, after adding fresh substrates, we monitored C and N dynamics. In a parallel experiment, we manipulated the soil community to reduce litter-history impacts on its composition and size to investigate whether the soil community could be an important contributor to legacy effects We found strong short-term litter legacy effects on contemporary litter and soil N mineralization, the duration of which was dependent on the contemporary substrate for decomposition. These strong effects were not consistent with the home field advantage phenomenon, as exposure to a specific litter did not favor the decomposition of the same litter when it was applied as a contemporary substrate. Reduction of the litter-history effects on soil biota decreased the impact of litter history on N immobilization, suggesting that plant litter impacts on the soil community may be an important component of plant litter legacies on N decomposition. In contrast to N, litter legacies appeared to be much less important for C decomposition, suggesting that legacy effects might uncouple contemporary C and N dynamics.     

Soil organic matter in urban soils: Estimation of elemental carbon by thermal oxidation and characterization of organic matter by solid-state C nuclear magnetic resonance (NMR) spectroscopy

To reduce soil destruction by urban sprawl, land use planning has to promote the use of soils within cities. As soil functions are now protected by law in Germany, urban soil quality has to be evaluated before soil management. We studied contributions from elemental carbon (EC) and soil organic matter (SOM) quality in topsoil horizons at seven sites in Stuttgart, Germany, differing in impurities by technogenic substrates. The most disturbed site was found at a disused railway area while high-density areas, public parks and garden areas showed varying degrees of disturbance by anthropogenic activities. For most soils, compounds derived from plant litter dominated organic matter (OM) quality characterized by nuclear magnetic resonance (NMR) spectroscopy. Although high contents of EC (up to 70% of soil organic carbon) were indicated by thermal oxidation, this was not confirmed by aromatic C intensities in NMR spectra. Only for the highly aromatic railway soil were results for elemental carbon by thermal oxidation and NMR similar. As other technogenic substrates beside EC like plastics may also contribute in the long-term to OM in urban soils, new analytical techniques are therefore required. This knowledge will promote the evaluation of urban soil properties and their sustainable use.     

Soil‐carbon preservation through habitat constraints and biological limitations on decomposer activity

We review recent experimental results on the role of soil biota in stabilizing or destabilizing soil organic matter (SOM). Specifically, we analyze how the differential substrate utilization of the various decomposer organisms contributes to a decorrelation of chemical stability, residence time, and carbon (C) age of organic substrates. Along soil depth profiles, a mismatch of C allocation and abundance of decomposer organisms is consistently observed, revealing that a relevant proportion of soil C is not subjected to efficient decomposition. Results from recent field and laboratory experiments suggest that (1) bacterial utilization of labile carbon compounds is limited by short‐distance transport processes and, therefore, can take place deep in the soil under conditions of effective local diffusion or convection. In contrast, (2) fungal utilization of phenolic substrates, including lignin, appears to be restricted to the upper soil layer due to the requirement for oxygen of the enzymatic reaction involved. (3) Carbon of any age is utilized by soil microorganisms, and microbial C is recycled in the microbial food web. Due to stoichiometric requirements of their metabolism, (4) soil animals tend to reduce the C concentration of SOM disproportionally, until it reaches a threshold level. The reviewed investigations provide new and quantitative evidence that different soil C pools underlie divergent biological constraints of decomposition. The specialization of decomposers towards different substrates and microhabitats leads to a relatively longer persistence of virtually all kinds of organic substrates in the nonpreferred soil spaces. We therefore propose to direct future research explicitly towards such biologically nonpreferred areas where decomposition rates are slow, or where decomposition is frequently interrupted, in order to assess the potential for long‐term preservation of C in the soil.     

Plant species richness leaves a legacy of enhanced root litter-induced decomposition in soil

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO₂ production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition.     

Carbon-to-nitrogen ratio is a poor predictor of low molecular weight organic nitrogen mineralization in soil

Carbon-to-nitrogen ratio (C:N) has frequently been shown to be a good predictor of the speed of organic residue decomposition and N mineralization in soil. While this relationship appears to work well for complex organic materials (e.g. plant litter), its applicability to smaller organic substrates containing N remains unknown. Here we evaluated whether the intrinsic properties of amino acids and peptides could be used to predict their rate of microbial uptake and subsequent N mineralization. In an agricultural grassland soil we found that C:N, molecular weight, aromaticity and sulphur content provided poor indicators of amino acid bioavailabilityand subsequent NH₄⁺ release into soil. We therefore hypothesize that the position of amino acids along microbial biosynthetic pathways together with internal demand for individual amino acids rather than their C or N content is the primary determinant of N mineralization.     

蚓粪施用方式对不同品种番茄生长和土壤肥力的影响

肥料的施用方式会影响有机肥料的分解和养分释放过程,且不同作物品种在养分需求方面也各有差异。蚓粪是一种能够改善土壤和促进植物生长的新型替代肥料。采用盆栽试验研究了蚓粪施用方式对不同番茄品种生长和土壤肥力的影响,盆栽试验设置为:两个番茄品种:金棚三号和艾瑞尔;4种施肥方式:单施化肥(CF);蚓粪表施(VS),即蚓粪与盆钵上部8 cm厚土壤混施;蚓粪中施(VL),即蚓粪均匀平铺紧邻8 cm处下方;蚓粪全土混施(VM),即蚓粪与所有土壤混合。结果表明:相比全土混施,蚓粪表施和中施更能增加番茄的茎叶生物量并且促进其对氮、钾的吸收;与单施化肥相比,蚓粪的效果更依赖于施用方式,蚓粪表施和全土混施比中施更能提高土壤pH和有机碳含量。在盛花期,无论番茄品种,土壤有效磷、速效钾含量都在蚓粪表施时最高;而在收获期,蚓粪表施和全土混施的有效磷含量高于中施处理。相比蚓粪中施和全土混施,表施降低了艾瑞尔的土壤矿质氮含量。总之,蚓粪的集中施用如表施和中施,能促进番茄生长及养分吸收,并且在不同品种之间效应一致。     

The effect of pre‐industrial charcoal kilns on chemical properties of forest soil of Wallonia,Belgium

In Wallonia, Belgium, intensive in situ charcoal production that was linked closely to pre‐industrial smelting and steel‐making affected a large part of the forested area in the late eighteenth century. Charcoal kiln relics can be detected under forest as domes of about 10 m in diameter, with the topsoil greatly enriched with charcoal residues. We sampled 19 charcoal kiln sites and the adjacent reference soil by soil horizon on four different soil types (Arenosols, Luvisols, Cambisols and Podzols). Data were analysed with linear mixed models to assess the effect of the charcoal kiln site on soil properties in relation to depth and soil conditions. We also addressed the evolution of soil properties over time by a comparison of the soil characteristics at a currently active kiln site. The charcoal‐rich topsoil has a larger C:N ratio and cation exchange capacity (CEC) per unit of organic carbon than the reference soil. The largest CECs per unit of carbon were observed on soil with coarser textures. On acidic soil, the increase in base saturation in the subsoil reflects the past liming effect of ash produced by wood charring, whereas the topsoil is re‐acidified. The acidity of carbonate‐rich Cambisols, however, is not reduced. Regardless of soil type, the kiln topsoil is greatly depleted in exchangeable K⁺ and available P, which may be attributed to the small affinity of the exchange complex of charcoal for K⁺ and a decrease in P availability with time. Therefore, we recommend further research on the long‐term effects of biochar on the dynamics of plant nutrients.     

Manioc peel and charcoal: a potential organic amendment for sustainable soil fertility in the tropics

In tropical areas, where crop production is limited by low soil quality, the development of techniques improving soil fertility without damage to the environment is a priority. In French Guiana, we used subsistence farmer plots on poor acidic soils to test the effect of different organic amendments, bitter manioc peel (M), sawdust (Sw) and charcoal (Ch), on soil nutrient content, earthworm abundance and yard-long bean (Vigna unguiculata sesquipedalis) production. The peregrine Pontoscolex corethrurus was the only earthworm species found. Pod production and plant growth were lowest in unamended soil. The application of a mixture of manioc peel and charcoal (M + Ch) improved legume production compared with other organic mixtures. It combined the favourable effects of manioc peel and charcoal. Manioc peel improved soil fertility through its low C:N ratio and its high P content, while charcoal decreased soil acidity and exchangeable Al and increased Ca and Mg availability, thus alleviating the possible toxic effects of Al on plant growth. The M + Ch treatment was favourable to P. corethrurus, the juvenile population of which reached a size comparable to that of the nearby uncultivated soil. The application of a mixture of manioc peel and charcoal, by improving crop production and soil fertility and enhancing earthworm activity, could be a potentially efficient organic manure for legume production in tropical areas where manioc is cultivated under slash-and-burn shifting agriculture.     

Temperature sensitivity of soil organic matter decomposition in southern and northern areas of the boreal forest zone

Much effort has been made to improve understanding of factors controlling the temperature dependence of soil organic matter (SOM) decomposition. The question of how soils formed in different geographical locations and conditions respond to temperature changes is still open. In addition to climate, residence times of soil organic matter are controlled by its decomposability and microbial community. In this work we hypothesized that the decomposition of SOM is adapted to the prevailing SOM quality and climatic conditions. This should result in different temperature vs. decomposition curves for northern and southern soils. We studied short-term temperature dependence of SOM decomposition near the northern and southern borders of the boreal forest zone using a Gaussian model. As carbon mineralization rate is driven by microbial activity, we focused on organic carbon fractions available to microbes and the size, composition and functioning of microbial communities in the soil. Despite differences in microbial community structure and behavior, similar amounts and qualities of the microbially available carbon led to similar temperature dependences of carbon mineralization in the north and south. The overall soil respiration rate level was higher in spruce forest sites than in pine forest sites irrespective of climate conditions. Our results do not mean that there is no risk of carbon losses from northern soils due to warming climate conditions. As temperature sensitivity of the decomposition increases with decreasing temperature regime, the proportional increase in the decomposition rate in northern latitudes might lead to significant carbon losses from the soils.     

土壤中黑碳对农药敌草隆的吸附-解吸迟滞行为研究

采用批处理振荡法和连续稀释法分别测定了敌草隆在人工添加黑碳土壤和自然形成的不同有机质和黑碳含量的土壤中的吸附一解吸行为。吸附结果表明，人工添加黑碳的土壤对敌草隆的吸附强度和吸附容量以及吸附等温线的非线性均随土壤黑碳添加浓度的增加而逐步增大；自然土壤的吸附容量和吸附强度随土壤总有机质含量增加而增加，但吸附等温线的非线性则与土壤中黑碳对有机质的相对含量有关，黑碳比例越高，等温线非线性越大。解吸实验结果表明，无论是人工添加黑碳的土壤还是自然土壤，对敌草隆的解吸迟滞作用均随土壤黑碳含量增高而愈明显。     

The priming effect of organic matter: a question of microbial competition?

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.     

Modelling soil organic matter decomposition and rainfall erosion in two tropical soils after forest clearing for permanent agriculture

A soil organic matter turnover model has been developed to analyse soil carbon (soil organic-C) loss caused by organic matter decomposition and rainfall erosion in soils used for permanent cultivation. It has been used to build up model profiles of five soils, one occurring in temperate and four in tropical regions, on the basis of estimates for ‘natural’ organic matter input. Organic matter input data for different systems of cultivation were used to model the long-term decomposition of soil organic-C in these model profiles. The modelling results show that soil organic matter decomposition in the tropics is three to four times faster than in temperate regions, and that there is a marked influence of soil type and soil climate. Simulated losses of organic-C in the tropical soils, not accounting for erosion are 31 to 50 per cent after 50 years and 43 to 63 per cent after 100 years of continuous cultivation. The simulated loss of soil organic-C when rainfall erosion is also allowed for is 40 to 80 per cent. Erosion caused an extra loss of at least 7 per cent after 100 years. The initial input of charcoal from forest burning is lost through erosion at a rate of 50 to almost 100 per cent, depending on the severity of erosion. The sensitivity of modelling results to variations in input data was also analysed. The losses of soil carbon were also used to calculate the global flux of CO₂ from soils. Soils are probably a small but not negligible source of CO₂.     

Charcoal mineralisation potential of microbial inocula from burned and unburned forest soil with and without substrate addition

Effects of fire on the functioning of the soil microbial community are largely unknown. In this study, we addressed the charcoal mineralisation potential of microbial inocula extracted from burned and unburned soil. The mineralisation of charcoal was analysed during a 1 month incubation experiment under controlled conditions with and without substrate addition. The aim of the study was to elucidate (1) the indirect effect of fire on the functioning of the soil microbial community in terms of charcoal degradation and (2) the possibility to stimulate this degradation by addition of two substrates of increasing complexity. Our conceptual approach included the monitoring of CO₂ emission from microcosms containing laboratory-made charcoal and microbial inocula from burned and unburned soil with and without ¹³C labelled glucose and cellulose.Our results showed higher charcoal mineralisation without substrate addition in microcosms with the inocula from unburned soil compared to burned soil. Charcoal mineralisation was stimulated by the addition of glucose, whereas cellulose addition did not induce a priming effect. We observed a higher stimulation of charcoal mineralisation induced by glucose for the inoculum from burned soil compared to the inoculum from unburned soil. We concluded that fire did affect the functioning of the soil microbial community in terms of charcoal degradation and that the important priming effect induced by glucose may be explained by an increase of the overall microbial activity, rather than selective stimulation of charcoal degrading microbial communities.     

Contribution of organic amendments to soil organic matter detected by thermogravimetry

Sustainable soil management requires reliable and accurate monitoring of changes in soil organic matter (SOM). However, despite the development of improved analytical techniques during the last decades, there are still limits in the detection of small changes in soil organic carbon content and SOM composition. This study focused on the detection of such changes under laboratory conditions by adding different organic amendments to soils. The model experiments consisted of artificially mixing soil samples from non‐fertilized plots of three German long‐term agricultural experiments in Bad Lauchstädt (silty loam), Grossbeeren (silty sand), and Müncheberg (loamy sand) with straw, farmyard manure, sheep faeces, and charcoal in quantities from 3 to 180 t ha^?1 each. In these mixtures we determined the organic carbon contents by elemental analysis and by thermal mass losses (TML) determined by thermogravimetry. The results confirmed the higher reliability of elemental analysis compared to TML for organic carbon content determination. The sensitivity of both methods was not sufficient to detect the changes in organic carbon content caused by small quantities of organic amendments (3 t ha^?1 or 0.1–0.4 g C kg^?1 soil). In the case of elemental analysis, the detectability of changes in carbon content increased with quantities of added amendments, but the method could not distinguish different types of organic amendments. On the contrary, the based on analysis of degradation temperatures, the TML allowed this discrimination together with their quantitative analysis. For example, added charcoal was not visible in TML from 320 to 330°C, which is used for carbon content determination. However, increasing quantities of charcoal were reflected in a higher TML around 520°C. Furthermore, differences between measured (with TML_110–550) and predicted mass loss on ignition using both organic carbon (with TML₃₃₀) and clay contents (with TML₁₄₀) were confirmed as a suitable indicator for detection of organic amendments in different types of soils. We conclude that thermogravimetry enables the sensitive detection of organic fertilizers and organic amendments in soils under arable land use.     

Rhizospheric influence on soil respiration and decomposition in a temperate Norway spruce stand

Assessments of terrestrial carbon fluxes require a thorough understanding of links between primary production, soil respiration and carbon loss through drainage. In this study, stem girdling was used to terminate autotrophic soil respiration including rhizosphere respiration and root exudation in a temperate Norway spruce stand. Rates of soil respiration and dissolved organic carbon (DOC) formation were measured in the second year after girdling, comparing an intact plant-rhizosphere continuum with an exclusive decomposer system. The molecular and isotopic composition of DOC in the soil solution was analysed with a coupled Py-GC/MS-C-IRMS system to distinguish between the carbon sources of dissolved carbon. Pyrolysis products were grouped according to their precursor origins: polysaccharides, proteins or of mixed origin (mainly derivates of lignins and proteins). When dead roots became available for decomposition, rates of heterotrophic soil respiration in girdling plots peaked at 6.5 μmol m⁻² s⁻¹, comparable to peak rates of total soil respiration (autotrophic and heterotrophic) in control plots, 6.1 μmol m⁻² s⁻¹. A significant response of soil respiration to temperature was found in control plots only, showing that an unlimiting supply of organic substrates for microbial respiration may mask any temperature effects. The enhanced decomposition in girdled plots was further supported by the isotopic composition of DOC in soil solution; all three precursor groups became isotopically enriched as the growing season progressed (polysaccharides by 2.3‰, proteins by 1.9‰, mixed origin group by 2.2‰). This indicates a trophic level shift due to incorporation of organic substrate into the microbial food chain. In the control plots’ mixed origin fraction, the isotopic composition changed over time from a signature resembling that of lignin (−28.9‰) to one similar of the protein fraction (−25.7‰). Significant temporal changes of structural DOC composition occurred in the girdling plots only. These results suggest that changes in the microbial community and in decomposition rates occurred in both girdled and control plots in the following ways: (i) increased substrate availability (dead roots) gave rise to generally enhanced performance of the decomposer community in girdled plots, (ii) root-derived exudates probably contributed to enhanced decomposition of recalcitrant lignin in the control plots and (iii) the structural composition of DOC seemed to be more a result of decomposition than of plant root exudation in all plots.     

Nodulated soybean enhances rhizosphere priming effects on soil organic matter decomposition more than non-nodulated soybean

The phenomenon that rhizosphere processes significantly control soil organic matter (SOM) decomposition, also termed rhizosphere priming effect (RPE), is now increasingly recognized as significant as the effects of soil temperature and moisture on SOM decomposition. However, the exact mechanisms responsible for RPE remain largely unknown. Particularly, some reports have suggested that the quality of rhizodeposits may play a significant role in causing different levels of RPE among various plant species. However, direct evidence for the “rhizodeposit quality hypothesis” has been lacking. Here we tested the hypothesis by investigating RPE on soil carbon (C) and nitrogen (N) mineralization of two soybean (Glycine max L. Merr.) isolines differing only in their ability to form nodules and to fix N₂, and thus differing in tissue N concentration and rhizodeposit quality. We used a continuous ¹³C-labeling method for measuring RPE on soil organic C decomposition, and employed an N-budgeting method for quantifying RPE on soil net N mineralization. We found that the rhizodeposits from nodulated soybean produced a stronger RPE (53% vs. 26%) on soil organic C decomposition than the rhizodeposits from non-nodulated soybean at the maturity stage when nodulated soybean had significantly higher plant tissue N concentration but similar plant biomass, while both soybean isolines produced a similar RPE (33–34%) at the vegetative stage when there was no difference in plant tissue N concentration or plant biomass. The levels of RPE on soil net N mineralization were similar between the two isolines, ranging from 25% at the vegetative stage to 38–46% at the maturity stage. Moreover, RPE on soil organic C decomposition was not linearly proportional to RPE on soil net N mineralization. These results indicate that higher rhizodeposit quality is one of the most likely causes to the higher RPE of the nodulated soybean compared to the non-nodulated soybean. Further investigations of rhizodeposit quality and quantity between the two soybean isolines are warranted to further test this rhizodeposit quality hypothesis.     

Diversity and function of decomposer fungi from a grassland soil

Despite the substantial interest to ecologists of the relationship between species diversity and ecosystem functioning, little is known about how the high species richness of decomposer (saprotrophic) fungi and their relative frequencies of occurrence influence the decomposition of organic matter. Three experiments were conducted to test the ability of culturable saprotrophic fungal isolates to utilise a range of artificial and more natural substrates that occur in organic matter, with the aims of (1) characterising the functional potential of ‘common’ and ‘occasional’ taxa in an upland grassland soil and (2) determining whether there was a high degree of apparent functional redundancy in these communities. ‘Function’ was defined as the ability of a fungal isolate to utilise broad categories of substrates (e.g. sugars, cellulose, lignin) that occur in organic matter and which change in proportion during decomposition. The terms ‘common/abundant/frequent’ and ‘occasional/infrequent’ usually referred here to the frequencies of occurrence of taxa estimated using Warcup soil plates. Accepting the difficulties of sampling fungi in soil, this appeared to be the most useful isolation method to produce a general picture of the microfungal community with an estimate of frequency of occurrence for every taxon obtained, and to provide cultures for use in function tests. The influence of this technique on the interpretation of the results is discussed.Forty-eight fungal isolates, obtained from an upland grassland in Roxburghshire, UK, were selected to cover the most ‘abundant’ taxa and a range of ‘occasionals’. Pure cultures of anamorphic fungi and members of the Zygomycota, Ascomycota and Basidiomycota were tested. Although there was apparently a high degree of functional redundancy (equivalence) in assemblages of culturable decomposer fungi, with ‘frequent’ and ‘infrequent’ taxa largely utilising the same substrates, the ‘infrequent’ taxa played important roles in decomposition. ‘Infrequent’ microfungi tested were potentially more active in decomposition than the ‘frequent’ taxa, i.e. several had a higher overall activity, were able to utilise a wider range of substrates and were more combative than the ‘abundant’ taxa. When ‘abundant’ and ‘occasional’ taxa from the same putative guild were inoculated together on grass litter, there was slight evidence of ‘positive’ indirect effects on decomposition and cellulose degradation. Some ‘negative’ effects on lignin degradation, probably as a result of combat, were observed.It is possible that the ‘occasional’ taxa increased the temporal resilience of the ecosystem process of decomposition, and were ‘waiting in the wings’ to replace the abundant taxa. Nevertheless, greater functional diversity could be associated with the uncultured taxa not studied here.