Interaction between tannins and fungal necromass stabilizes fungal residues in boreal forest soils

Soil microbial necromass carbon contributions to soil organic carbon after three decades of citrus cultivation

Introduction

Citrus is one of the most economically significant fruits globally, and soil organic carbon (SOC) plays a central role in maintaining soil health and fertility. Consequently, enhancing SOC content directly influences both the yield and quality of citrus crops. However, the sources of SOC in citrus orchards and their mechanisms of contribution remains poorly understood.

Methods

This study investigated citrus soils from orchards of varying planting ages by collecting 0-20 cm soil samples. We analyzed amino sugars, glomalin, particulate organic carbon (POC), and mineral-bound organic carbon (MAOC) to examine the source of microbial residue carbon and its contribution to SOC.

Results

The results revealed a significant decrease in microbial residue carbon (MNC), fungal residue carbon (FNC), and bacterial residue carbon (BNC) with increasing orchard age (p < 0.05). Specifically, the MNC in 30-year-old citrus soils was reduced by 46.27% compared to 10-year-old soils, FNC decreased by 45.61%, and BNC by 48.91%. The proportion of microbial residue carbon within SOC significantly decreased as planting years increased (p < 0.05), from 76.82 ± 2.84% in 10-year-old citrus soils to 20.54 ± 4.70% in 30-year-old soils. Furthermore, soil pH, NO₃--N and MAOC were the main factors controlling MNC. MNC showed a significant negative correlation with SOC, indicating a weakened microbial carbon pump function in citrus soils and an increased reliance on other carbon sources, such as plant-derived carbon. Although citrus cultivation had led to a decline in microbial residue carbon over time, it remained a primary source of organic carbon, with its contribution depending on the age of the orchard.

Discussion

These findings offered novel insights into the mechanisms through which intensive citrus cultivation influences microbial necromass contributions to SOC. This study also highlighted the negative impacts of long-term citrus cultivation on soil microbial necromass and offered recommendations for the rehabilitation of aging orchards.

Saprotrophic-mycorrhizal divide in stable isotope composition throughout the whole fungus: from mycelium to hymenophore

Mycorrhizal and saprotrophic macromycetes contribute strongly to the carbon and nitrogen cycles of forest ecosystems, often studied by tracing stable isotope composition of carbon and nitrogen. The phenomenon of the saprotrophic-mycorrhizal divide highlights the difference in the stable isotope composition of fruiting bodies of mycorrhizal and saprotrophic fungi. Much less is known about the isotopic composition of the mycelium, which plays an important role in the formation of the soil organic matter and fuels the fungal trophic channel in soil food webs. In this study, we assessed whether the saprotrophic-mycorrhizal divide in the natural δ13С and δ15N values can be traced throughout entire fungal organisms. This hypothesis was tested using 16 species of ectomycorrhizal and six species of saprotrophic basidiomycetous fungi. We showed that not only fruiting bodies, but also the mycelium of ectomycorrhizal and saprotrophic fungi differs in the δ13C and δ15N values. In both ectomycorrhizal and saprotrophic fungi, the δ13C and δ15N values increased from mycelium to hymenophores and correlated positively with the total N content in the corresponding tissues. The differences between ectomycorrhizal and saprotrophic mycelium can be used to reconstruct the fungal-driven belowground carbon and nitrogen allocation, and the contribution of saprotrophic and mycorrhizal fungi to soil food webs.

Effect of 9-year water and nitrogen additions on microbial necromass carbon content at different soil depths and its main influencing factors

Microbial necromass carbon (MNC) is a major source of soil organic carbon (SOC) pool, significantly influencing soil carbon sequestration. The effects of long-term water and nitrogen addition on MNC in soils at different depths, as well as their interactions, remain poorly understood. In this study, we examined the influence of water addition (+51.67 mm), nitrogen addition (25, 50, 100 kg N ha-1 yr-1), and their interactions on MNC accumulation at different soil depths in temperate grasslands. The addition of water and nitrogen over nine years has been observed to exhibit a decreasing trend in the MNC at different soil depths. Notably, MNC in the topsoil layer (0-10 cm) decreased significantly by 18.56 % under low nitrogen addition treatment, while MNC in the subsoil layer (40-60 cm) declined significantly by 27.19 % under high nitrogen addition treatment. Fungal microbial necromass carbon (FNC) contributed 3.25 times more to SOC than bacterial microbial necromass carbon (BNC). In the 0-10 cm soil layer, MNC is primarily governed by both microbial attributes and the physicochemical properties of the soil, in the 20-40 cm soil layer, the physicochemical properties of the soil predominantly control MNC, in the 40-60 cm layer, microbial characteristics exert a more significant influence on MNC. Collectively, our observations suggest that soil MNC decreased with the addition of water and nitrogen in the 0-60 cm soil slope, which could enable the accurate prediction of global change impacts on the accumulation of soil carbon, thus facilitating the implementation of strategies to augment soil carbon sequestration.

Restoration from grazing on the Tibetan plateau: Pathway-specific soil MAOC sequestration in meadow and peat wetlands

Mineral-associated organic matter (MAOM) constitutes a significant portion of carbon (C) pools in wetlands, determining their role as C sinks or sources. Nevertheless, detailed knowledge of the response of MAOM formation pathways to wetland restoration is lacking. Here, we collected 0-20 cm soil samples from two typical wetlands (meadow and peat wetlands) subjected to both grazing and restoration treatments (through micro dam construction) in the Zoige area. Lignin phenols and amino sugars within the MAOM fraction were analysed to clarify the MAOM formation pathways and their potential contributions to MAOM formation. The results demonstrated that both meadow and peat wetland restoration enhanced MAOM sequestration. Compared to grazed sites, restored meadow wetlands significantly increased the mineral-associated organic carbon (MAOC) content in soils but not the C content of the MAOM fraction; restored peat wetlands increased the C content of the MAOM fraction by 58% but not the MAOC content in soils. Notably, plant-derived C and microbial necromass C strongly and dominantly influenced the C content of the MAOM fraction in the meadow (path coefficient = 0.73) and peat wetlands (path coefficient = 0.75), respectively. Compared with those in the grazed peat wetlands, the MAOM fraction in restored peat wetlands contained 1.93, 1.61, and 2.09 times higher total, bacterial, and fungal necromass C contents, respectively. Moreover, soil properties influence lignin phenols and necromass C in conjunction with the C content of the MAOM fraction rather than directly. It seems reasonable to assume that meadow wetland restoration from grazing enhanced MAOM formation in soils via the accumulation of lignin phenols (through the "direct sorption" pathway). Peat wetland restoration from grazing enhanced MAOM formation in soils via the accumulation of lignin phenols (through the "direct sorption" pathway) and the "microbial turnover" pathway.

Characterization of organic materials in regulating soil carbon sequestration of urban greenspaces: Links to microbial necromass and community composition

Application of organic material is a win-win strategy that effectively boosts soil carbon (C) storage and promotes organic waste recycling in urban ecosystems. However, the divergent responses of C sequestration to organic materials and the underlying mechanisms remain unclear. Using molecular and litterbag methods, this study examined soil organic C (SOC) content, C sequestration efficiency of organic material (CSE-organic material), microbial necromass and community composition in urban greenspaces amended with different types of organic materials. The field experiment had seven treatments: addition of green waste (GreenWaste), green waste compost (GreenWasteCompost), biogas residue (BiogasResidue), biogas residue compost (BiogasResidueCompost), peat (PEAT), biochar (BioChar), and no organic material (Control). Organic materials after 16 months application increased SOC content by 34.1-87.0% compared with the Control and presented CSE-organic material in the order: BioChar > GreenWasteCompost > PEAT > BiogasResidueCompost > GreenWaste > BiogasResidue. Aromaticity index was positively correlated with CSE-organic material, indicating that aromatic C addition had a strong capacity to enhance C sequestration. Microbial necromass increased from 2.7 g C kg-1 in the Control to 3.9-5.0 g C kg-1 under organic materials addition. Bacterial necromass was enriched in the BiogasResidue and BiogasResidueCompost treatments, primarily because of sufficient substrate that facilitated the proliferation and activity of copiotrophic Firmicutes, whereas fungal necromass was 8.8-19.1% higher in the GreenWaste, GreenWasteCompost, and PEAT treatments than that in the BiogasResidue and BiogasResidueCompost treatments, accompanying by an increasing abundance of Ascomycota. However, the contribution of microbial necromass to the increased SOC was negatively correlated with CSE-organic material, suggesting that the input of recalcitrant C weakened the role of microbial necromass in C retention. Overall, this study reveals that the efficiency of organic materials application in promoting soil C accumulation depends largely on recalcitrant C rather than microbial necromass, which highlights the important role of organic waste acting as a C source in achieving urban sustainable development.

Global decline in microbial-derived carbon stocks with climate warming and its future projections

Soil organic carbon (SOC) represents the largest terrestrial pool of organic carbon and is indispensable for mitigating climate change and sustaining soil fertility. As a major component of stable SOC, microbial-derived carbon (MDC) accounts for approximately half of the total SOC and has repercussions on climate feedback. However, our understanding of the spatial and temporal dynamics of MDC stocks is limited, hindering assessments of the long-term impacts of global warming on persistent SOC sequestration in the soil‒atmosphere carbon cycle. Here, we compiled an extensive global dataset and employed ensemble machine learning techniques to forecast the spatial-temporal dynamics of MDC stocks across 93.4% of the total global land area from 1981 to 2018. Our findings revealed that for every 1°C increase in temperature, there was a global decrease of 6.7 Pg in the soil MDC stock within the predictable areas, equivalent to 1.4% of the total MDC stock or 0.9% of the atmospheric C pool. Tropical regions experienced the most substantial declines in MDC stocks. We further projected future MDC stocks for the next century based on shared socioeconomic pathways, showing a global decline in MDC stocks with a potential 6-37 Pg reduction by 2100 depending on future pathways. We recommend integrating the response of MDC stocks to warming into socioeconomic models to enhance confidence in selecting sustainable pathways.

A trait-based root acquisition-defence-decomposition framework in angiosperm tree species

To adapt to the complex belowground environment, plants make trade-offs between root resource acquisition and defence ability. This includes forming partnerships with different types of root associating microorganisms, such as arbuscular mycorrhizal and ectomycorrhizal fungi. These trade-offs, by mediating root chemistry, exert legacy effects on nutrient release during decomposition, which may, in turn, affect the ability of new roots to re-acquire resources, thereby generating a feedback loop. However, the linkages at the basis of this potential feedback loop remain largely unquantified. Here, we propose a trait-based root 'acquisition-defence-decomposition' conceptual framework and test the strength of relevant linkages across 90 angiosperm tree species. We show that, at the plant species level, the root-fungal symbiosis gradient within the root economics space, root chemical defence (condensed tannins), and root decomposition rate are closely linked, providing support to this framework. Beyond the dichotomy between arbuscular mycorrhizal-dominated versus ectomycorrhizal-dominated systems, we suggest a continuous shift in feedback loops, from 'high arbuscular mycorrhizal symbiosis-low defence-fast decomposition-inorganic nutrition' by evolutionarily ancient taxa to 'high ectomycorrhizal symbiosis-high defence-slow decomposition-organic nutrition' by more modern taxa. This 'acquisition-defence-decomposition' framework provides a foundation for testable hypotheses on multidimensional linkages between species' belowground strategies and ecosystem nutrient cycling in an evolutionary context.

Spatial variation and controls of soil microbial necromass carbon in a tropical montane rainforest

Soil microbial necromass carbon is an important component of the soil organic carbon (SOC) pool which helps to improve soil fertility and texture. However, the spatial pattern and variation mechanisms of fungal- and bacterial-derived necromass carbon at local scales in tropical rainforests are uncertain. This study showed that microbial necromass carbon and its proportion in SOC in tropical montane rainforest exhibited large spatial variation and significant autocorrelation, with significant high-high and low-low clustering patterns. Microbial necromass carbon accounted for approximately one-third of SOC, and the fungal-derived microbial necromass carbon and its proportion in SOC were, on average, approximately five times greater than those of bacterial-derived necromass. Structural equation models indicated that soil properties (SOC, total nitrogen, total phosphorus) and topographic features (elevation, convexity, and aspect) had significant positive effects on microbial necromass carbon concentrations, but negative effects on its proportions in SOC (especially the carbon:nitrogen ratio). Plant biomass also had significant negative effects on the proportion of microbial necromass carbon in SOC, but was not correlated with its concentration. The different spatial variation mechanisms of microbial necromass carbon and their proportions in SOC are possibly related to a slower accumulation rate of microbial necromass carbon than of plant-derived organic carbon. Geographic spatial correlations can significantly improve the microbial necromass carbon model fit, and low sampling resolution may lead to large uncertainties in estimating soil carbon dynamics at specific sites. Our work will be valuable for understanding microbial necromass carbon variation in tropical forests and soil carbon prediction model construction with microbial participation.

Tannins and Climate Change: Are Tannins Able To Stabilize Carbon in the Soil?

The interaction between tannins and proteins has been studied intensively for more than half a century as a result of its significance for various applications. In chemical ecology, tannins are involved in response to environmental stress, including biotic (pathogens and herbivores) and abiotic (e.g., drought) stress, and in carbon (C) and nutrient cycling. This perspective summarizes the newest insights into the role of tannins in soil processes, including the interaction with fungi leading to C stabilization. Recent knowledge presented here may help to optimize land management to increase or preserve soil C to mitigate climate change.

Vegetation restoration altered the soil organic carbon composition and favoured its stability in a Robinia pseudoacacia plantation

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration.

Carbon availability affects already large species-specific differences in chemical composition of ectomycorrhizal fungal mycelia in pure culture

Although ectomycorrhizal (ECM) contribution to soil organic matter processes receives increased attention, little is known about fundamental differences in chemical composition among species, and how that may be affected by carbon (C) availability. Here, we study how 16 species (incl. 19 isolates) grown in pure culture at three different C:N ratios (10:1, 20:1, and 40:1) vary in chemical structure, using Fourier transform infrared (FTIR) spectroscopy. We hypothesized that C availability impacts directly on chemical composition, expecting increased C availability to lead to more carbohydrates and less proteins in the mycelia. There were strong and significant effects of ECM species (R2 = 0.873 and P = 0.001) and large species-specific differences in chemical composition. Chemical composition also changed significantly with C availability, and increased C led to more polysaccharides and less proteins for many species, but not all. Understanding how chemical composition change with altered C availability is a first step towards understanding their role in organic matter accumulation and decomposition.

Peat loss collocates with a threshold in plant-mycorrhizal associations in drained peatlands encroached by trees

Drainage-induced encroachment by trees may have major effects on the carbon balance of northern peatlands, and responses of microbial communities are likely to play a central mechanistic role. We profiled the soil fungal community and estimated its genetic potential for the decay of lignin and phenolics (class II peroxidase potential) along peatland drainage gradients stretching from interior locations (undrained, open) to ditched locations (drained, forested). Mycorrhizal fungi dominated the community across the gradients. When moving towards ditches, the dominant type of mycorrhizal association abruptly shifted from ericoid mycorrhiza to ectomycorrhiza at c. 120 m from the ditches. This distance corresponded with increased peat loss, from which more than half may be attributed to oxidation. The ectomycorrhizal genus Cortinarius dominated at the drained end of the gradients and its relatively higher genetic potential to produce class II peroxidases (together with Mycena) was positively associated with peat humification and negatively with carbon-to-nitrogen ratio. Our study is consistent with a plant-soil feedback mechanism, driven by a shift in the mycorrhizal type of vegetation, that potentially mediates changes in aerobic decomposition during postdrainage succession. Such feedback may have long-term legacy effects upon postdrainage restoration efforts and implication for tree encroachment onto carbon-rich soils globally.

Depth-dependent effects of ericoid mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under arbuscular mycorrhizal trees

Plant mycorrhizal associations influence the accumulation and persistence of soil organic matter and could therefore shape ecosystem biogeochemical responses to global changes that are altering forest composition. For instance, arbuscular mycorrhizal (AM) tree dominance is increasing in temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond positively to canopy disturbances. Yet how shifts in the co-occurrence of trees and shrubs with different mycorrhizal associations will affect soil organic matter pools remains largely unknown. We examine the effects of ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial activity at different depths across gradients of AM versus ectomycorrhizal (EcM) tree dominance in three temperate forest sites. We find that ErM shrubs strongly modulate tree mycorrhizal dominance effects. In surface soils, ErM shrubs increase particulate organic matter accumulation and weaken the positive relationship between soil organic matter stocks and indicators of microbial activity. These effects are strongest under AM trees that lack fungal symbionts that can degrade organic matter. In subsurface soil organic matter pools, by contrast, tree mycorrhizal dominance effects are stronger than those of ErM shrubs. Ectomycorrhizal tree dominance has a negative influence on particulate and mineral-associated soil organic matter pools, and these effects are stronger for nitrogen than for carbon stocks. Our findings suggest that increasing co-occurrence of ErM shrubs and AM trees will enhance particulate organic matter accumulation in surface soils by suppressing microbial activity while having little influence on mineral-associated organic matter in subsurface soils. Our study highlights the importance of considering interactions between co-occurring plant mycorrhizal types, as well as their depth-dependent effects, for projecting changes in soil carbon and nitrogen stocks in response to compositional shifts in temperate forests driven by disturbances and global change.

Trade-offs in carbon-degrading enzyme activities limit long-term soil carbon sequestration with biochar addition

Biochar amendment is one of the most promising agricultural approaches to tackle climate change by enhancing soil carbon (C) sequestration. Microbial-mediated decomposition processes are fundamental for the fate and persistence of sequestered C in soil, but the underlying mechanisms are uncertain. Here, we synthesise 923 observations regarding the effects of biochar addition (over periods ranging from several weeks to several years) on soil C-degrading enzyme activities from 130 articles across five continents worldwide. Our results showed that biochar addition increased soil ligninase activity targeting complex phenolic macromolecules by 7.1%, but suppressed cellulase activity degrading simpler polysaccharides by 8.3%. These shifts in enzyme activities explained the most variation of changes in soil C sequestration across a wide range of climatic, edaphic and experimental conditions, with biochar-induced shift in ligninase:cellulase ratio correlating negatively with soil C sequestration. Specifically, short-term (<1 year) biochar addition significantly reduced cellulase activity by 4.6% and enhanced soil organic C sequestration by 87.5%, whereas no significant responses were observed for ligninase activity and ligninase:cellulase ratio. However, long-term (≥1 year) biochar addition significantly enhanced ligninase activity by 5.2% and ligninase:cellulase ratio by 36.1%, leading to a smaller increase in soil organic C sequestration (25.1%). These results suggest that shifts in enzyme activities increased ligninase:cellulase ratio with time after biochar addition, limiting long-term soil C sequestration with biochar addition. Our work provides novel evidence to explain the diminished soil C sequestration with long-term biochar addition and suggests that earlier studies may have overestimated soil C sequestration with biochar addition by failing to consider the physiological acclimation of soil microorganisms over time.

Climate warming alters the relative importance of plant root and microbial community in regulating the accumulation of soil microbial necromass carbon in a Tibetan alpine meadow

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8-year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low-level (+0-1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high-level (+1.5-2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming.

Contrasting plant-soil-microbial feedbacks stabilize vegetation types and uncouple topsoil C and N stocks across a subarctic-alpine landscape

Global vegetation regimes vary in belowground carbon (C) and nitrogen (N) dynamics. However, disentangling large-scale climatic controls from the effects of intrinsic plant-soil-microbial feedbacks on belowground processes is challenging. In local gradients with similar pedo-climatic conditions, effects of plant-microbial feedbacks may be isolated from large-scale drivers. Across a subarctic-alpine mosaic of historic grazing fields and surrounding heath and birch forest, we evaluated whether vegetation-specific plant-microbial feedbacks involved contrasting N cycling characteristics and C and N stocks in the organic topsoil. We sequenced soil fungi, quantified functional genes within the inorganic N cycle, and measured ¹⁵ N natural abundance. In grassland soils, large N stocks and low C : N ratios associated with fungal saprotrophs, archaeal ammonia oxidizers, and bacteria capable of respiratory ammonification, indicating maintained inorganic N cycling a century after abandoned reindeer grazing. Toward forest and heath, increasing abundance of mycorrhizal fungi co-occurred with transition to organic N cycling. However, ectomycorrhizal fungal decomposers correlated with small soil N and C stocks in forest, while root-associated ascomycetes associated with small N but large C stocks in heath, uncoupling C and N storage across vegetation types. We propose that contrasting, positive plant-microbial feedbacks stabilize vegetation trajectories, resulting in diverging soil C : N ratios at the landscape scale.

The complexities of proanthocyanidin biosynthesis and its regulation in plants

Proanthocyanidins (PAs) are natural flavan-3-ol polymers that contribute protection to plants under biotic and abiotic stress, benefits to human health, and bitterness and astringency to food products. They are also potential targets for carbon sequestration for climate mitigation. In recent years, from model species to commercial crops, research has moved closer to elucidating the flux control and channeling, subunit biosynthesis and polymerization, transport mechanisms, and regulatory networks involved in plant PA metabolism. This review extends the conventional understanding with recent findings that provide new insights to address lingering questions and focus strategies for manipulating PA traits in plants.

Do ectomycorrhizal exploration types reflect mycelial foraging strategies?

Ectomycorrhizal exploration types are commonly assumed to denote spatial foraging patterns and resource-related niches of extraradical mycelia. However, empirical evidence of the consistency of foraging strategies within exploration types is lacking. Here, we analysed ectomycorrhizal foraging patterns by incubating root-excluding ingrowth mesh bags filled with six different substrates in mature Picea abies forests. High-throughput sequencing was used to characterise ectomycorrhizal fungal communities in the mesh bags and on adjacent fine roots after one growing season. Contrary to expectations, many ectomycorrhizal genera of exploration types that are thought to produce little extraradical mycelium colonised ingrowth bags extensively, whereas genera commonly associated with ample mycelial production occurred sparsely in ingrowth bags relative to their abundance on roots. Previous assumptions about soil foraging patterns of exploration types do not seem to hold. Instead, we propose that variation in the proliferation of extraradical mycelium is related to intergeneric differences in mycelial longevity and the mobility of targeted resources.

The functional role of ericoid mycorrhizal plants and fungi on carbon and nitrogen dynamics in forests

Ericoid mycorrhizal (ErM) shrubs commonly occur in forest understories and could therefore alter arbuscular (AM) and/or ectomycorrhizal (EcM) tree effects on soil carbon and nitrogen dynamics. Specifically, ErM fungi have extensive organic matter decay capabilities, and ErM plant and fungal tissues have high concentrations of secondary compounds that can form persistent complexes in the soil. Together, these traits could contribute to organic matter accumulation and inorganic nutrient limitation. These effects could also differ in AM- vs EcM-dominated stands at multiple scales within and among forest biomes by, for instance, altering fungal guild interactions. Most work on ErM effects in forests has been conducted in boreal forests dominated by EcM trees. However, ErM plants occur in c. 96, 69 and 29% of boreal, temperate and tropical forests, respectively. Within tropical montane forests, the effects of ErM plants could be particularly pronounced because their traits are more distinct from AM than EcM trees. Because ErM fungi can function as free-living saprotrophs, they could also be more resilient to forest disturbances than obligate symbionts. Further consideration of ErM effects within and among forest biomes could improve our understanding of how cooccurring mycorrhizal types interact to collectively affect soil carbon and nitrogen dynamics under changing conditions.

Phosphorus addition decreases plant lignin but increases microbial necromass contribution to soil organic carbon in a subalpine forest

Increasing phosphorus (P) inputs induced by anthropogenic activities have increased P availability in soils considerably, with dramatic effects on carbon (C) cycling and storage. However, the underlying mechanisms via which P drives plant and microbial regulation of soil organic C (SOC) formation and stabilization remain unclear, hampering the accurate projection of soil C sequestration under future global change scenarios. Taking the advantage of an 8-year field experiment with increasing P addition levels in a subalpine forest on the eastern Tibetan Plateau, we explored plant C inputs, soil microbial communities, plant and microbial biomarkers, as well as SOC physical and chemical fractions. We found that continuous P addition reduced fine root biomass, but did not affect total SOC content. P addition decreased plant lignin contribution to SOC, primarily from declined vanillyl-type phenols, which was coincided with a reduction in methoxyl/N-alkyl C by 2.1%-5.5%. Despite a decline in lignin decomposition due to suppressed oxidase activity by P addition, the content of lignin-derived compounds decreased because of low C input from fine roots. In contrast, P addition increased microbial (mainly fungal) necromass and its contribution to SOC due to the slower necromass decomposition under reduced N-acquisition enzyme activity. The larger microbial necromass contribution to SOC corresponded with a 9.1%-12.4% increase in carbonyl C abundance. Moreover, P addition had no influence on the slow-cycing mineral-associated organic C pool, and SOC chemical stability indicated by aliphaticity and recalcitrance indices. Overall, P addition in the subalpine forest over 8 years influenced SOC composition through divergent alterations of plant- and microbial-derived C contributions, but did not shape SOC physical and chemical stability. Such findings may aid in accurately forecasting SOC dynamics and their potential feedbacks to climate change with future scenarios of increasing soil P availability in Earth system models.

Mycelium chemistry differs markedly between ectomycorrhizal and arbuscular mycorrhizal fungi

The chemical quality of soil carbon (C) inputs is a major factor controlling litter decomposition and soil C dynamics. Mycorrhizal fungi constitute one of the dominant pools of soil microbial C, while their litter quality (chemical proxies of litter decomposability) is understood poorly, leading to major uncertainties in estimating soil C dynamics. We examined litter decomposability of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungal species using samples obtained from in vitro cultivation. We showed that the chemical composition of AM and EM fungal mycelium differs significantly: EM fungi have higher concentrations of labile (water-soluble, ethanol-soluble) and recalcitrant (non-extractable) chemical components, while AM fungi have higher concentrations of acid-hydrolysable components. Our results imply that differences in decomposability traits among mycorrhizal fungal guilds represent a critically important driver of the soil C cycle, which could be as vital as is recognized for differences among aboveground plant litter.

Legacies of invertebrate exclusion and tree secondary metabolites control fungal communities in dead wood

During decomposition of organic matter, microbial communities may follow different successional trajectories depending on the initial environment and colonizers. The timing and order of the species arrival (assembly history) can lead to divergent communities through priority effects. We explored how assembly history and resource quality affected fungal communities and decay rate of decomposing wood, 1.5 and 4.5 years after tree felling. Additionally, we investigated the effect of invertebrate exclusion during the first two summers. We measured initial resource quality of bark and wood of aspen (Populus tremula) logs and surveyed the fungal communities by DNA metabarcoding at different times during succession. We found that gradients in fungal community composition were related to resource quality and discuss how this may reflect different fungal life history strategies. As with previous studies, the initial amount of bark tannins was negatively correlated with wood decomposition rate over 4.5 years. The initial fungal community explained variation in community composition after 1.5, but not 4.5 years, of succession. Although the assembly history of initial colonizers may cause alternate trajectories in successional communities, our results indicate that the communities may converge with the arrival of secondary colonizers. We also identified a strong legacy of invertebrate exclusion on fungal communities, even after 4.5 years of succession, thereby adding crucial knowledge on the importance of invertebrates in affecting fungal community development. By measuring and manipulating aspects of assembly history and resource quality that have rarely been studied, we expand our understanding of the complexity of fungal community dynamics.

Tree Diversity, Initial Litter Quality, and Site Conditions Drive Early-Stage Fine-Root Decomposition in European Forests

Decomposition of dead fine roots contributes significantly to nutrient cycling and soil organic matter stabilization. Most knowledge of tree fine-root decomposition stems from studies in monospecific stands or single-species litter, although most forests are mixed. Therefore, we assessed how tree species mixing affects fine-root litter mass loss and which role initial litter quality and environmental factors play. For this purpose, we determined fine-root decomposition of 13 common tree species in four European forest types ranging from boreal to Mediterranean climates. Litter incubations in 315 tree neighborhoods allowed for separating the effects of litter species from environmental influences and litter mixing (direct) from tree diversity (indirect). On average, mass loss of mixed-species litter was higher than those of single-species litter in monospecific neighborhoods. This was mainly attributable to indirect diversity effects, that is, alterations in microenvironmental conditions as a result of tree species mixing, rather than direct diversity effects, that is, litter mixing itself. Tree species mixing effects were relatively weak, and initial litter quality and environmental conditions were more important predictors of fine-root litter mass loss than tree diversity. We showed that tree species mixing can alter fine-root litter mass loss across large environmental gradients, but these effects are context-dependent and of moderate importance compared to environmental influences. Interactions between species identity and site conditions need to be considered to explain diversity effects on fine-root decomposition.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Mycorrhizal roots slow the decay of belowground litters in a temperate hardwood forest

There is increasing evidence that plant roots and mycorrhizal fungi, whether living or dead, play a central role in soil carbon (C) cycling. Root-mycorrhizal-microbial interactions can both suppress and enhance litter decay, with the net result dependent upon belowground nutrient acquisition strategies and soil nutrient availability. We measured the net effect of living roots and mycorrhizal fungi on the decay of dead roots and fungal hyphae in a hardwood forest dominated by either sugar maple (Acer saccharum) or white oak (Quercus alba) trees. Root and fungal litter were allowed to decompose within root-ingrowth bags and root-exclusion cores. In conjunction with root effects on decay, we assessed foraging responses and root induced changes in soil moisture, nitrogen (N) availability and enzyme activity. After 1 year, maple root production increased, and mycorrhizal fungal colonization decreased in the presence of decaying litter. In addition, we found that actively foraging roots suppressed the decay of root litter (- 14%) more than fungal litter (- 3%), and suppression of root decay was stronger for oak (- 20%) than maple roots (- 8%). Suppressive effects of oak roots on decay were greatest when roots also reduced soil N availability, which corresponded with reductions in hydrolytic enzyme activity and enhanced oxidative enzyme activities. These findings further our understanding of context-dependent drivers of root-mycorrhizal-microbial interactions and demonstrate that such interactions can play an underappreciated role in soil organic matter accumulation and turnover in temperate forests.

Reindeer control over subarctic treeline alters soil fungal communities with potential consequences for soil carbon storage

The climate-driven encroachment of shrubs into the Arctic is accompanied by shifts in soil fungal communities that could contribute to a net release of carbon from tundra soils. At the same time, arctic grazers are known to prevent the establishment of deciduous shrubs and, under certain conditions, promote the dominance of evergreen shrubs. As these different vegetation types associate with contrasting fungal communities, the belowground consequences of climate change could vary among grazing regimes. Yet, at present, the impact of grazing on soil fungal communities and their links to soil carbon have remained speculative. Here we tested how soil fungal community composition, diversity and function depend on tree vicinity and long-term reindeer grazing regime and assessed how the fungal communities relate to organic soil carbon stocks in an alpine treeline ecotone in Northern Scandinavia. We determined soil carbon stocks and characterized soil fungal communities directly underneath and >3 m away from mountain birches (Betula pubescens ssp. czerepanovii) in two adjacent 55-year-old grazing regimes with or without summer grazing by reindeer (Rangifer tarandus). We show that the area exposed to year-round grazing dominated by evergreen dwarf shrubs had higher soil C:N ratio, higher fungal abundance and lower fungal diversity compared with the area with only winter grazing and higher abundance of mountain birch. Although soil carbon stocks did not differ between the grazing regimes, stocks were positively associated with root-associated ascomycetes, typical to the year-round grazing regime, and negatively associated with free-living saprotrophs, typical to the winter grazing regime. These findings suggest that when grazers promote dominance of evergreen dwarf shrubs, they induce shifts in soil fungal communities that increase soil carbon sequestration in the long term. Thus, to predict climate-driven changes in soil carbon, grazer-induced shifts in vegetation and soil fungal communities need to be accounted for.

How do boreal forest soils store carbon?

Boreal forests store a globally significant pool of carbon (C), mainly in tree biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms underlying C stabilization are not well understood. Here, recently discovered mechanisms behind SOM stabilization, their level of understanding, interrelations, and future directions in the field are provided. A recently unraveled mechanism behind C stabilization via interaction of root-derived tannins with fungal necromass emphasizing fungal necromass chemistry is brought forth. The long-lasting dogma of the stability of SOM on minerals is challenged and the newest insights from the field of soil fauna and their influence on SOM stabilization are provided. In conclusion, mechanisms unraveled during the last decade are crucial steps forward to draw a holistic view of the main drivers of SOM stabilization.

Root-Derived Proteases as a Plant Tool to Access Soil Organic Nitrogen; Current Stage of Knowledge and Controversies

Anthropogenic deterioration of the global nitrogen (N) cycle emerges mainly from overuse of inorganic N fertilizers in nutrient-limited cropping systems. To counteract a further dysregulation of the N cycle, we need to improve plant nitrogen use efficiency. This aim may be reached via unravelling all plant mechanisms to access soil N, with special attention to the dominating high-molecular-mass N pool. Traditionally, we believe that inorganic N is the only plant-available N pool, however, more recent studies point to acquisition of organic N compounds, i.e., amino acids, short peptides, and proteins. The least known mechanism of plants to increase the N uptake is a direct increase of soil proteolysis via root-derived proteases. This paper provides a review of the knowledge about root-derived proteases and also controversies behind this phenomenon.

A tipping point in carbon storage when forest expands into tundra is related to mycorrhizal recycling of nitrogen

Tundra ecosystems are global belowground sinks for atmospheric CO₂ . Ongoing warming-induced encroachment by shrubs and trees risks turning this sink into a CO₂ source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long-term carbon (C) and nitrogen (N) stocks, we studied small-scale soil depth profiles of fungal communities and C-N dynamics across a subarctic-alpine forest-heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree-associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic-alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

Root presence modifies the long-term decomposition dynamics of fungal necromass and the associated microbial communities in a boreal forest

Recent studies have highlighted that dead fungal mycelium represents an important fraction of soil carbon (C) and nitrogen (N) inputs and stocks. Consequently, identifying the microbial communities and the ecological factors that govern the decomposition of fungal necromass will provide critical insight into how fungal organic matter (OM) affects forest soil C and nutrient cycles. Here, we examined the microbial communities colonising fungal necromass during a multiyear decomposition experiment in a boreal forest, which included incubation bags with different mesh sizes to manipulate both plant root and microbial decomposer group access. Necromass-associated bacterial and fungal communities were taxonomically and functionally rich throughout the 30 months of incubation, with increasing abundances of oligotrophic bacteria and root-associated fungi (i.e., ectomycorrhizal, ericoid mycorrhizal and endophytic fungi) in the late stages of decomposition in the mesh bags to which they had access. Necromass-associated β-glucosidase activity was highest at 6 months, while leucine aminopeptidase peptidase was highest at 18 months. Based on an asymptotic decomposition model, root presence was associated with an initial faster rate of fungal necromass decomposition, but resulted in higher amounts of fungal necromass retained at later sampling times. Collectively, these results indicate that microbial community composition and enzyme activities on decomposing fungal necromass remain dynamic years after initial input, and that roots and their associated fungal symbionts result in the slowing of microbial necromass turnover with time.

Experimental nitrogen fertilisation globally accelerates, then slows decomposition of leaf litter

Plant litter decomposition is a central process in the carbon (C) cycle and sensitive to ongoing anthropogenic nitrogen (N) fertilisation. Previous syntheses evaluating the effect of N fertilisation on litter decomposition relied largely on models that define a constant rate of mass loss throughout decomposition, which may mask hypothesised shifts in the effect of N fertilisation on litter decomposition dynamics. In this meta-analysis, we compared the performance of four empirical decomposition models and showed that N fertilisation consistently accelerates early-stage but slows late-stage decomposition when the model structure allows for flexibility in decomposition rates through time. Within a particular substrate, early-stage N-stimulation of decomposition was associated with reduced rates of late-stage decay. Because the products of early- vs. late-stage decomposition are stabilised in soils through distinct chemical and physical mechanisms, N-induced changes in the litter decomposition process may influence the formation and cycling of soil C, the largest terrestrial C pool.

Mycorrhizal and environmental controls over root trait-decomposition linkage of woody trees

Traits are critical in predicting decomposition that fuels carbon and nutrient cycling in ecosystems. However, our understanding of root trait-decomposition linkage, and especially its dependence on mycorrhizal type and environmental context, remains limited. We explored the control of morphological and chemical (carbon- and nutrient-related) traits over decomposition of absorptive roots in 30 tree species associated with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi in temperate and subtropical forests in China. Carbon-related traits (acid-unhydrolysable residue (AUR) and cellulose concentrations) had predominant control of root decomposition in AM species while nutrient-related traits (magnesium concentration) predominately controlled that in ECM species. Thicker absorptive roots decomposed faster in AM species as a result of their lower AUR concentrations, but more slowly in ECM angiosperm species potentially as a result of their higher magnesium concentrations. Root decomposition was linked to root nutrient economy in both forests while root diameter-decomposition coordination emerged only in the subtropical forest where root diameter and decomposition presented similar cross-species variations. Our findings suggest that root trait-decomposition linkages differ strongly with mycorrhizal type and environment, and that root diameter can predict decomposition but in opposing directions and with contrasting mechanisms for AM and ECM species.

MYB134-RNAi poplar plants show reduced tannin synthesis in leaves but not roots, and increased susceptibility to oxidative stress

The importance of the poplar MYB134 gene in controlling condensed tannin (CT) biosynthesis was tested by suppressing its expression using RNA interference (RNAi). MYB134-RNAi plants grew normally but showed reduced accumulation of stress-induced CTs in leaves. RNA-seq analysis indicated that flavonoid- and CT-related genes, as well as additional CT regulators, were strongly and specifically down-regulated by MYB134 suppression. This confirmed that the primary MYB134 target is the leaf flavonoid and CT pathway. Root CT accumulation was not impacted by MYB suppression, suggesting that additional CT regulators are active in roots and emphasizing the complexity of the regulation of CTs in poplar. To test the effect of CT down-regulation on oxidative stress resistance, leaves of MYB134-RNAi and control plants were exposed to the reactive oxygen species generator methyl viologen. MYB134-RNAi leaves sustained significantly more photosystem II damage, as seen in reduced chlorophyll fluorescence, compared with wild-type leaves. MYB134-RNAi leaves also contained more hydrogen peroxide, a reactive oxygen species, compared with the wild type. Our data thus corroborate the hypothesis that CT can act as an antioxidant in vivo and protect against oxidative stress. Overall, MYB134 was shown to be a central player in the regulation of CT synthesis in leaves.

Mechanisms of Carbon Sequestration in Highly Organic Ecosystems - Importance of Chemical Ecology

Organic matter decomposition plays a major role in the cycling of carbon (C) and nutrients in terrestrial ecosystems across the globe. Climate change accelerates the decomposition rate to potentially increase the release of greenhouse gases and further enhance global warming in the future. However, fractions of organic matter vary in turnover times and parts are stabilized in soils for longer time periods (C sequestration). Overall, a better understanding of the mechanisms underlying C sequestration is needed for the development of effective mitigation policies to reduce land-based production of greenhouse gases. Known mechanisms of C sequestration include the recalcitrance of C input, interactions with soil minerals, aggregate formation, as well as its regulation via abiotic factors. In this Minireview, we discuss the mechanisms behind C sequestration including the recently emerging significance of biochemical interactions between organic matter inputs that lead to C stabilization.

Above- and belowground linkages shape responses of mountain vegetation to climate change

Upward shifts of mountain vegetation lag behind rates of climate warming, partly related to interconnected changes belowground. Here, we unravel above- and belowground linkages by drawing insights from short-term experimental manipulations and elevation gradient studies. Soils will likely gain carbon in early successional ecosystems, while losing carbon as forest expands upward, and the slow, high-elevation soil development will constrain warming-induced vegetation shifts. Current approaches fail to predict the pace of these changes and how much they will be modified by interactions among plants and soil biota. Integrating mountain soils and their biota into monitoring programs, combined with innovative comparative and experimental approaches, will be crucial to overcome the paucity of belowground data and to better understand mountain ecosystem dynamics and their feedbacks to climate.

Plant roots increase both decomposition and stable organic matter formation in boreal forest soil

Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N build-up. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome-encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade-off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and -omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.

Nitrogen dynamics of decomposing Scots pine needle litter depends on colonizing fungal species

In boreal ecosystems plant production is often limited by low availability of nitrogen. Nitrogen retention in below-ground organic pools plays an important role in restricting recirculation to plants and thereby hampers forest production. Saprotrophic fungi are commonly assigned to different decomposer strategies, but how these relate to nitrogen cycling remains to be understood. Decomposition of Scots pine needle litter was studied in axenic microcosms with the ligninolytic litter decomposing basidiomycete Gymnopus androsaceus or the stress tolerant ascomycete Chalara longipes. Changes in chemical composition were followed by 13C CP/MAS NMR spectroscopy and nitrogen dynamics was assessed by the addition of a 15N tracer. Decomposition by C. longipes resulted in nitrogen retention in non-hydrolysable organic matter, enriched in aromatic and alkylic compounds, whereas the ligninolytic G. androsaceus was able to access this pool, counteracting nitrogen retention. Our observations suggest that differences in decomposing strategies between fungal species play an important role in regulating nitrogen retention and release during litter decomposition, implying that fungal community composition may impact nitrogen cycling at the ecosystem level.