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Aims Long-term application of organic materials has been shown to significantly enhance the content of soil organic matter (SOM), underscoring the critical need to examine the components of soil organic carbon for a deeper understanding of SOM functionalities. Thus, the structural changes and microbial driving mechanisms of water extractable organic matter (WEOM), fulvic acid (FA) and humic acid (HA) were investigated in black soil by a long-term fertilization experiment. Methods This 33-year experiment comprises five treatments: no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer with low-rate straw (NPKJ1), chemical fertilizer with high-rate straw (NPKJ2), and chemical fertilizer with organic manure (NPKM). We also conducted a detailed study of WEOM, FA, HA, and the microbial community structure in both the 0–20 cm and 20–40 cm soil layers. Results Our findings indicate that organic material application primarily sourced WEOM, FA, and HA from microbial metabolism and plant-derived origins, exhibiting humus and aromatization characteristics with high molecular weight. WEOM was rich in fulvic acid-like and humic acid-like compounds, while FA and HA contained more protein-like components. Organic material use altered WEOM, FA, and HA structures by impacting soil microbial biomass carbon (MBC) and fungal/bacterial biomass. In 0–20 cm soil layer, SOM content was mainly influenced by humus, especially the HA fraction, whereas in 20–40 cm soil layer, it was predominantly affected by WEOM. Conclusions The present study emphasizes that the application of organic materials can influence the structure of microbial communities, thereby affecting the composition of WEOM, FA, and HA, consequently influencing the organic matter content in different soil layers.

• Litter decomposition plays a key role in nutrient cycling across ecosystems, yet to date, we lack a comprehensive understanding of the nonadditive decomposition effects in leaf litter mixing experiments. • To fill that gap, we compiled 69 individual studies with the aim to perform two meta-analyses on nonadditive effects. • We show that a significant synergistic effect (faster decomposition in mixtures than expected) occurs at a global scale, with an average increase of 3–5% in litter mixtures. In particular, low-quality litter in mixtures shows a significant synergistic effect, while additive effects are observed for high-quality species. Additionally, synergistic effects turn into antagonistic effects when soil fauna are absent or litter is in very late stages of decomposition (nearhumus). In contrast to temperate and tropical areas, studies in boreal regions show significant antagonistic effects. • Our two meta-analyses provide a systematic evaluation of nonadditive effects in mixed litter decomposition studies and show that litter quality alters the effects of litter mixing. Our results indicate that nutrient transfer, soil fauna and inhibitory secondary compounds can influence mixing effects. We also highlight that synergistic and antagonistic effects occur concurrently, and the final litter mixing effect results from the interplay between them.

Boreal forest soils store a major proportion of the global terrestrial carbon (C) and below‐ground inputs contribute as much as above‐ground plant litter to the total C stored in the soil. A better understanding of the dynamics and drivers of root‐associated fungal communities is essential to predict long‐term soil C storage and climate feedbacks in northern ecosystems. We used 454‐pyrosequencing to identify fungal communities across fine‐scaled soil profiles in a 5000 yr fire‐driven boreal forest chronosequence, with the aim of pinpointing shifts in fungal community composition that may underlie variation in below‐ground C sequestration. In early successional‐stage forests, higher abundance of cord‐forming ectomycorrhizal fungi (such as Cortinarius and Suillus species) was linked to rapid turnover of mycelial biomass and necromass, efficient nitrogen (N) mobilization and low C sequestration. In late successional‐stage forests, cord formers declined, while ericoid mycorrhizal ascomycetes continued to dominate, potentially facilitating long‐term humus build‐up through production of melanized hyphae that resist decomposition. Our results suggest that cord‐forming ectomycorrhizal fungi and ericoid mycorrhizal fungi play opposing roles in below‐ground C storage. We postulate that, by affecting turnover and decomposition of fungal tissues, mycorrhizal fungal identity and growth form are critical determinants of C and N sequestration in boreal forests.

Summary Communities of litter saprotrophic and root‐associated fungi are vertically separated within boreal forest soil profiles. It is unclear whether this depth partitioning is maintained exclusively by substrate‐mediated niche partitioning (i.e. distinct fundamental niches), or by competition for space and resources (i.e. distinct realized niches). Improved understanding of the mechanisms driving spatial partitioning of these fungal guilds is critical, as they modulate carbon and nutrient cycling in different ways. Under field settings, we tested the effects of substrate quality and the local fungal species pool at various depths in determining the potential of saprotrophic and mycorrhizal fungi to colonize and exploit organic matter. Natural substrates of three qualities – fresh or partly decomposed litter or humus – were incubated in the corresponding organic layers of a boreal forest soil profile in a fully factorial design. After one and two growing seasons, fungal community composition in the substrates was determined by 454‐pyrosequencing and decomposition was analyzed. Fungal community development during the course of the experiment was determined to similar degrees by vertical location of the substrates (24% of explained variation) and by substrate quality (20%), indicating that interference competition is a strong additional driver of the substrate‐dependent depth partitioning of fungal guilds in the system. During the first growing season, litter substrates decomposed slower when colonized by root‐associated communities than when colonized by communities of litter saprotrophs, whereas humus was only slightly decomposed by both fungal guilds. During the second season, certain basidiomycetes from both guilds were particularly efficient in localizing and exploiting their native organic substrates although displaced in the vertical profile. This validates that fungal community composition, rather than microclimatic factors, were responsible for observed depth‐related differences in decomposer activities during the first season. In conclusion, our results suggest that saprotrophic and root‐associated fungal guilds have overlapping fundamental niches with respect to colonization of substrates of different qualities, and that their substrate‐dependent depth partitioning in soils of ectomycorrhiza‐dominated ecosystems is reinforced by interference competition. Through competitive interactions, mycorrhizal fungi can thus indirectly regulate litter decomposition rates by restraining activities of more efficient litter saprotrophs. A lay summary is available for this article. Lay Summary

Soil organic carbon (C) is an essential component of the global C cycle. Processes that control its composition and dynamics over large scales are not well understood. Thus, our understanding of C cycling is incomplete, which makes it difficult to predict C gains and losses due to changes in climate, land use and management. Here we show that controls on the composition of organic C, the particulate, humus and resistant fractions, and the potential vulnerability of C to decomposition across Australia are distinct, scale-dependent and variable. We used machine-learning with 5,721 topsoil measurements to show that, continentally, the climate, soil properties (for example, total nitrogen and pH) and elevation are dominant controls. However, we found that such general assessments disregard underlying region-specific controls that affect the distribution of the organic C fractions and vulnerability. This can lead to misinterpretations that prejudice our understanding of soil C processes and dynamics. Regionally, climate is mediated through interactions with soil properties, mineralogy and topography. In some regions, climate is uninfluential. These results highlight the need for regional assessments of soil C dynamics and more local parameterization of biogeochemical and Earth system models. Our analysis propounds the development of region-specific strategies for effective C management and climate change mitigation.Soil geochemistry can be more important than climate in controlling carbon storage, its composition as well as stability, but controls are distinct, scale-dependent and variable, according to an analysis of topsoil measurements across Australia.

Key Points The symbiotic digestion of lignocellulose by termites involves the sequential activities of the host and its gut microbiota. The hindgut of termites is a microbial bioreactor that efficiently converts polymeric substrates to acetate and variable amounts of methane, with hydrogen as a central intermediate. The metabolic processes are strongly affected by the influx of oxygen into the gut periphery. Whereas primitive termites digest wood with the help of cellulolytic protists, the more advanced lineages have an entirely prokaryotic gut microbiota. Major shifts in the gut microbial community seem to reflect changes in digestive strategies and diet. The majority of termites are soil-feeding and mineralize peptides and other nitrogen-rich humus components. The consequences of the microbial processes in their highly alkaline guts for nitrogen cycling in tropical soils and greenhouse gas production are only scarcely investigated. It is becoming increasingly evident that the association of termites with gut bacteria not only functions in digestion but also enables the host to use the biosynthetic capacities of its symbionts as a nutritional resource. Termites are promising sources of novel microorganisms and catalytic capacities for the production of biofuels from lignocellulosic feedstock. However, the nature of the activities that are involved in the efficient digestion of lignified cell walls remains unclear. Termites depend on an intricate symbiosis with flagellated protists, archaea and bacteria in their guts for the digestion of lignocellulose. Here, Andreas Brune gives an overview of the diversity of the termite microbiota and highlights important microbial processes in the gut microecosystem and their implications for host nutrition. Their ability to degrade lignocellulose gives termites an important place in the carbon cycle. This ability relies on their partnership with a diverse community of bacterial, archaeal and eukaryotic gut symbionts, which break down the plant fibre and ferment the products to acetate and variable amounts of methane, with hydrogen as a central intermediate. In addition, termites rely on the biosynthetic capacities of their gut microbiota as a nutritional resource. The mineralization of humus components in the guts of soil-feeding species also contributes to nitrogen cycling in tropical soils. Lastly, the high efficiency of their minute intestinal bioreactors makes termites promising models for the industrial conversion of lignocellulose into microbial products and the production of biofuels.

Background and aims Rhizodeposited-carbon (C) plays an important role in regulating soil C concentrations and turnover, however, the distribution of rhizodeposited-C into different soil organic carbon (SOC) pools and how regulated by nitrogen (N) fertilization still remains elusive. Methods We applied five N fertilization rates (0, 10, 20, 40, and 60 mg N kg −1 soil) to rice ( Oryza sativa L.) with continuously labeled 13 CO 2 for 18 days, to measure 13 C allocation into plant tissues and soil C fractions. Results Relative to the unfertilized controls, the ratio of 13 C in plant aboveground shoot /belowground root increased as a result of N fertilization, and the contribution of rhizodeposited-C to SOC was increased by N fertilization, presumably due to the relatively high root biomass and exudates. Also, N fertilization increased 13 C incorporation into large aggregates (0.25–2.0 mm) and the humic acid fraction. Biological C immobilization might occur and preserve rhizodeposition following high rates of N addition, which regulates rhizodeposits and C cycling, thus determining the stabilization of rhizodeposits in the different SOC pools. Conclusion Rhizodeposited-C from rice plants and its distribution within SOC pools strongly depend upon N fertilization, thus determines C sequestration potential from the rice plants.

In northern forests, belowground sequestration of nitrogen (N) in complex organic pools restricts nutrient availability to plants. Oxidative extracellular enzymes produced by ectomycorrhizal fungi may aid plant N acquisition by providing access to N in macromolecular complexes. We test the hypotheses that ectomycorrhizal Cortinarius species produce Mn-dependent peroxidases, and that the activity of these enzymes declines at elevated concentrations of inorganic N. In a boreal pine forest and a sub-arctic birch forest, Cortinarius DNA was assessed by 454-sequencing of ITS amplicons and related to Mn-peroxidase activity in humus samples with- and without previous N amendment. Transcription of Cortinarius Mn-peroxidase genes was investigated in field samples. Phylogenetic analyses of Cortinarius peroxidase amplicons and genome sequences were performed. We found a significant co-localization of high peroxidase activity and DNA from Cortinarius species. Peroxidase activity was reduced by high ammonium concentrations. Amplification of mRNA sequences indicated transcription of Cortinarius Mn-peroxidase genes under field conditions. The Cortinarius glaucopus genome encodes 11 peroxidases - a number comparable to many white-rot wood decomposers. These results support the hypothesis that some ectomycorrhizal fungi - Cortinarius species in particular - may play an important role in decomposition of complex organic matter, linked to their mobilization of organically bound N.

Soils have the potential to accumulate heavy metals and the capacity to do so is strongly related the properties of each soil. Soil organic matter is a key factor in the retention, release, and bioavailability of heavy metals, and here we have determined the accumulation of heavy metals in various types of humus in the Rybnik Forest District in southern Poland. In a novel approach, we analyzed relationships between heavy metals within soil organic matter fractions and evaluated the role of organic fractions in mediating metal mobility. Specifically, we tested whether (i) the type of forest humus determines the heavy metal accumulation; (ii) heavy metals accumulation is associated with soil organic matter fractions; and (iii) heavy metals have an inhibitory influence on biochemical properties especially enzymes activity in different humus types. Four types of humus were sampled (mor, moder, moder-mull, mull), physically fractioned, and a number of chemical and biochemical properties were analyzed. Calculated geo-accumulation index (Igeo) and enrichment factor (EF) confirmed soil pollution with Cd and Pb. The type of humus differed in the accumulation of heavy metals, which is associated to the variable concentration of organic matter remaining at each decay class. We found no relationship between enzymatic activity and heavy metals concentration except for a positive correlation between urease activity and nickel concentration. Considering wider evidence, we propose a biogeochemical link between nickel deposition and the production of soil-borne urease in these forest soils.

Key recent developments in litter decomposition research are reviewed. Long-term inter-site experiments indicate that temperature and moisture influence early rates of litter decomposition primarily by determining the plants present, suggesting that climate change effects will be small unless they alter the plant forms present. Thresholds may exist at which single factors control decay rate. Litter decomposes faster where the litter type naturally occurs. Elevated CO₂ concentrations have little effect on litter decomposition rates. Plant tissues are not decay-resistant; it is microbial and biochemical transformations of materials into novel recalcitrant compounds rather than selective preservation of recalcitrant compounds that creates stable organic matter. Altering single characteristics of litter will not substantially alter decomposition rates. Nitrogen addition frequently leads to greater stabilization into humus through a combination of chemical reactions and enzyme inhibition. To sequester more C in soil, we need to consider not how to slow decomposition, but rather how to divert more litter into humus through microbial and chemical reactions rather than allowing it to decompose. The optimal strategy is to have litter transformed into humic substances and then chemically or physically protected in mineral soil. Adding N through fertilization and N-fixing plants is a feasible means of stimulating humification.