Belowground Carbon Allocation Research Articles

Jasmonic acid and heat stress induce high volatile organic compound emissions in Picea abies from needles, but not from roots.

Plants emit diverse volatile organic compounds (VOCs) from their leaves and roots for protection against biotic and abiotic stress. An important signaling cascade activated by aboveground herbivory is the jasmonic acid (JA) pathway that stimulates the production of VOCs. So far it remains unclear if the activation of this pathway also leads to enhanced VOC emissions from conifer roots, and how the interplay of above- and belowground defenses in plants are affected by multiple stressors. Therefore, we simultaneously analyzed needle and root VOC emissions of Picea abies saplings, as well as CO2 and H2O fluxes in response to aboveground JA treatment, heat stress and their interaction in a controlled climate chamber experiment. Continuous online VOC measurements by PTR-TOF-MS showed an inverse pattern of total needle and root VOC emissions, when plants were treated with JA and heat. While needle sesquiterpene emissions increased nine-fold one day after JA application, total root VOC emissions decreased. This was mainly due to reduced emissions of acetone and monoterpenes by roots. In response to aboveground JA treatment, root total carbon emitted as VOCs decreased from 31% to only 4%. While VOC emissions aboveground increased, net CO2 assimilation strongly declined due to JA treatment, resulting in net respiration during the day. Interestingly, root respiration was not affected by aboveground JA application. Under heat the effect of JA on VOC emissions of needles and roots was less pronounced. The buffering effect of heat on VOC emissions following JA treatment points towards an impaired defense reaction of the plants under multiple stress. Our results indicate efficient resource allocation within the plant to protect threatened tissues by a rather local VOC release. Roots may only be affected indirectly by reduced belowground carbon allocation, but are not involved directly in the JA-induced stress response.

Decreasing carbon allocation belowground in alpine meadow soils by shrubification

Distribution of shrubs expanding in grasslands – shrubification – is ongoing worldwide in grasslands and is common on the Qinghai–Tibetan Plateau (QTP). But the consequences of shrubification for plant carbon (C) input and fate in soil are unclear. We used 13C pulse labelling in a meadow on the QTP to compare photosynthetic capacity and photosynthate distribution in shoots, roots, soil and microbial functional groups between herbaceous plants (herbs) and shrubby Potentilla fruticosa. During 3 h of labeling in 13CO2 atmosphere, the 13C amount assimilated by shrubs (0.81 g C/m−2) was only 38 % of that by herbs. Over 8 days after labeling, 13C amount respired jointly by roots and soil microorganisms under shrubs (0.049 g m−2) was less than half of that under herbs. The mean residence time of 13C for respiration jointly by roots and rhizosphere microorganisms was longer under shrubs (0.61 day) than under herbs (0.44 day). Within 22 days after labelling, 13C amounts in roots, soil, and microorganisms were consistently smaller under shrubs than those under herbs. Consequently, shrub P. fruticosa had not only smaller photosynthetic potential, but also allocated less photosynthate belowground and slowed down C cycling in soil compared with herbs. The distribution of total 13C in microbial functional groups indicated by PLFA analysis was similar between herbs and shrubs. Averaged over vegetation patch types, gram negative bacteria and AMF accounted for 22 % and 4 % of the total microbial PLFAs in the 0–20 cm, respectively, but these two functional groups took up 51 % and 23 % of the total 13C absorbed by microorganisms, respectively. This indicates that gram negative bacteria and AMF are major consumers of rhizodeposits. Concluding, shrubification leads to smaller C allocation belowground and slows down C cycling in the soil.

Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year.

Tree species differ in their carbon (C) allocation strategies during environmental change. Disentangling species-specific strategies and contribution to the C balance of mixed forests requires observations at the individual tree level. We measured a complete set of C pools and fluxes at the tree level in five tree species, conifers and broadleaves, co-existing in a mature evergreen mixed Mediterranean forest. Our study period included a drought year followed by an above-average wet year, offering an opportunity to test the effect of water availability on tree C allocation. We found that in comparison to the wet year, C uptake was lower in the dry year, C use was the same, and allocation to belowground sinks was higher. Among the five major C sinks, respiration was the largest (ca. 60%), while root exudation (ca. 10%) and reproduction (ca. 2%) were those that increased the most in the dry year. Most trees relied on stored starch for maintaining a stable soluble sugars balance, but no significant differences were detected in aboveground storage between dry and wet years. The detailed tree-level analysis of nonstructural carbohydrates and δ13 C dynamics suggest interspecific differences in C allocation among fluxes and tissues, specifically in response to the varying water availability. Overall, our findings shed light on mixed forest physiological responses to drought, an increasing phenomenon under the ongoing climate change.

Severe and mild drought cause distinct phylogenetically linked shifts in the blue grama (Bouteloua gracilis) rhizobiome

Plants rely on a diverse rhizobiome to regulate nutrient acquisition and plant health. With increasing severity and frequency of droughts worldwide due to climate change, untangling the relationships between plants and their rhizobiomes is vital to maintaining agricultural productivity and protecting ecosystem diversity. While some plant physiological responses to drought are generally conserved, patterns of root exudation (release of small metabolites shown to influence microbes) and the consequential effects on the plant rhizobiome can differ widely across plant species under drought. To address this knowledge gap, we conducted a greenhouse study using blue grama (Bouteloua gracilis), a drought-tolerant C4 grass native to shortgrass prairie across North American plains, as a model organism to study the effect of increasing drought severity (ambient, mild drought, severe drought) on root exudation and the rhizobiome. Our previous results demonstrated physiological effects of increasing drought severity including an increase in belowground carbon allocation through root exudation and shifts in root exudate composition concurrent with the gradient of drought severity. This work is focused on the rhizobiome community structure using targeted sequencing and found that mild and severe drought resulted in unique shifts in the bacterial + archaeal and fungal communities relative to ambient, non-droughted controls. Specifically, using the change in relative abundance between ambient and drought conditions for each ZOTU as a surrogate for population-scale drought tolerance (e.g., as a response trait), we found that rhizobiome response to drought was non-randomly distributed across the phylogenies of both communities, suggesting that Planctomycetota, Thermoproteota (formerly Thaumarchaeota), and the Glomeromycota were the primary clades driving these changes. Correlation analyses indicated weak correlations between droughted community composition and a select few root exudate compounds previously implicated in plant drought responses including pyruvic acid, D-glucose, and myoinositol. This study demonstrates the variable impacts of drought severity on the composition of the blue grama rhizobiome and provides a platform for hypothesis generation for targeted functional studies of specific taxa involved in plant-microbe drought responses.

Vertical distribution patterns of community biomass, carbon and nitrogen content in grasslands on the eastern Qinghai–Tibet Plateau

The accumulation, transport and distribution of photosynthates within plants not only affect the ratio of shoot-to-root biomass, but also change the distribution pattern of nutrient elements in different organs and tissues. A full understanding of the allocation patterns of biomass and nutrient elements within plants, is helpful to strengthen our understanding of the above- and belowground ecosystem functions. However, at present, we do not know enough about the aboveground–belowground allocation of biomass, carbon (C) and nitrogen (N) contents, and, in particular, the corresponding vertical distribution pattern belowground and its environmental impact. At the community scale, the changes in vegetation community attributes measured in the field can reflect the long-term adaptation and subsequent adaptation strategies of vegetation to the regional environment and environmental changes. In this study, the vertical distribution patterns of community biomass and the carbon and nitrogen contents with soil depth in an alpine grassland on the eastern Qinghai–Tibet Plateau were investigated based on the transect method. Our results show the following: (1) Alpine grasslands can respond to differences in hydrothermal resource availability by changing biomass allocation patterns. In low-temperature and high-humidity environments, aboveground and belowground biomass allocation supports the isometric growth hypothesis. However, in environments with less soil moisture or high temperature and humidity, the aboveground and belowground biomass distributions support the allometric allocation hypothesis. (2) The vertical distribution patterns of the C content, N content and carbon: nitrogen ratio (C:N) within plants are different. From the leaves to the root system, with the increase in root depth, the C content demonstrated a vertical distribution pattern of first decreasing and then increasing, and the N% demonstrated a gradual decreasing distribution pattern, while the C:N ratio demonstrated an exponential increasing distribution pattern. (3) The change rate in C%, N% and C:N related to the soil depth was affected by environmental hydrothermal conditions. C% and C:N ratio changed more rapidly with root depth under low temperature and high humidity. The change rate of N% was mainly affected by temperature and changed more rapidly with root depth at low temperatures. (4) When exploring the relationship between the content of elements within plants, it is necessary to combine both the above- and belowground compartments. There was a nonlinear quadratic relationship between the carbon and nitrogen contents within plants.

Extraradical hyphae exhibit more plastic nutrient-acquisition strategies than roots under nitrogen enrichment in ectomycorrhiza-dominated forests.

Ectomycorrhizal (ECM) functional traits related to nutrient acquisition are impacted by nitrogen (N) deposition. However, less is known about whether these nutrient-acquisition traits associated with roots and hyphae differentially respond to increased N deposition in ECM-dominated forests with different initial N status. We conducted a chronic N addition experiment (25 kg N ha-1 year-1 ) in two ECM-dominated forests with contrasting initial N status, that is, a Pinus armandii forest (with relatively low N availability) and a Picea asperata forest (with relatively high N availability), to assess nutrient-mining and nutrient-foraging strategies associated with roots and hyphae under N addition. We show that nutrient-acquisition strategies of roots and hyphae differently respond to increased N addition. Root nutrient-acquisition strategies showed a consistent response to N addition, regardless of initial forest nutrient status, shifting from organic N mining toward inorganic N foraging. In contrast, the hyphal nutrient-acquisition strategy showed diverse responses to N addition depending on initial forest N status. In the Pinus armandii forest, trees increased belowground carbon (C) allocation to ECM fungi thus enhancing hyphal N-mining capacity under increased N availability. By comparison, in the Picea asperata forest, ECM fungi enhanced both capacities of P foraging and P mining in response to N-induced P limitation. In conclusion, our results demonstrate that ECM fungal hyphae exhibit greater plasticity in nutrient-mining and nutrient-foraging strategies than roots do in response to changes of nutrient status induced by N deposition. This study highlights the importance of ECM associations in tree acclimation and forest function stability under changing environments.

Root exudation patterns of Chinese fir after thinning relating to root characteristics and soil conditions

Root exudates are crucial to belowground carbon allocation and nutrient cycling in forest ecosystems. Few studies have focused on the effects of forest management activities, such as thinning, on root exudation rates of mature woody species in subtropical regions. In this study, we aimed to determine three kinds of root exudation rates (i.e., per mass, length, and area) in situ under three thinning intensities: Control (unthinned), light thinning intensity (LIT, felled 30% of individual trees), and heavy thinning intensity (HIT, felled 70% of individual trees), in a 29-year-old Chinese fir plantation. We found that root exudation rates increased after thinning and showed significant seasonal dynamics, with the highest and lowest rates being in summer and winter, respectively. The root exudation rates increased with microbial biomass C and N. Positive relationships between the root exudation rates, number of root tips, and root vitalities were observed. The root exudation rates increased with root diameter and decreasing specific root area, suggesting that resource-conservative roots of Chinese fir were biased toward accelerating root exudation rate for nutrient acquisition, instead of optimizing root morphology. In addition, thinning reduced overall soil total carbon content in a Chinese fir plantation. The soil total C content was higher in HIT than in LIT, indicating that HIT could reduce soil C loss by root exudation rates. These findings elucidate how thinning affects fine root exudation rates by altering soil conditions and root characteristics while contributing to our ability to predict belowground C allocation and nutrient cycling in response to silvicultural treatments.

Phosphorus regulates ectomycorrhizal fungi biomass production in a Norway spruce forest

Abstract. Ectomycorrhizal fungi (EMF) are important components of soil microbial communities, and EMF biomass can potentially increase carbon (C) stocks by accumulating in the soils as necromass and producing recalcitrant structures. EMF growth depends on the C allocated belowground by the host trees, and the nutrient limitation on tree growth is expected to influence this allocation. Therefore, studying EMF production and understanding the factors that regulates it in natural soils are important to understand C cycling in forests. Fungal mycelium collected from ingrowth mesh bags is commonly used to estimate EMF biomass, but these measurements might not reflect the total EMF production since turnover rates of the hyphae are not considered. Here we estimated EMF production and turnover in response to P fertilization (applied as superphosphate) in a Norway spruce forest where nitrogen (N) deposition has resulted in phosphorus (P) limitation of plant production by using a combination of mesh bags with different incubation periods and with Bayesian inferences. To test how localized patches of N and P influence EMF production and turnover we amended some bags with a nitrogen source (methylene urea) or P source (apatite). Additionally, the Bayesian model tested the effect of seasonality (time of mesh-bag harvesting) on EMF production and turnover. We found that turnover of EMF was not affected by P fertilization or mesh-bag amendment. P fertilization had a negative effect on EMF production in all the mesh-bag amendments, suggesting a reduced belowground C allocation to the EMF when P limitation is alleviated. Apatite amendment significantly increased EMF biomass production in comparison with the pure quartz bags in the control plots but not in the P-fertilized plots. This indicates that P-rich patches enhance EMF production in P-limited forests, but not when P is not limiting. Urea amendment had a generally positive effect on EMF production, but this was significantly reduced by P fertilization, suggesting that a decrease in EMF production due to the alleviated P limitation will affect N foraging. Seasonality had a significant effect on EMF production, and the differences registered between the treatments were higher during the warmer months and disappeared at the end of the growing season. Many studies highlight the importance of N for regulating belowground C allocation to EMF in northern coniferous forests, but here we show that the P status of the forest can be equally important for belowground carbon allocation to EMF production in areas with high N deposition.

Spatial and seasonal variations in soil respiration in a subtropical forest in Okinawa, Japan

AbstractTo clarify soil respiration (soil CO2 efflux, Rs) characteristics in a subtropical evergreen broad‐leaved natural forest in Yambaru, Okinawa, Japan, we examined spatiotemporal variation in Rs and its determining factors. We then compared yearly Rs with the value in other forests. The spatial variation in Rs (coefficient of variation [CV] = 38.9%) was not significantly related to temperature or soil water content but was evidently dependent on ground surface litter coverage. Rs was greater in summer (ca. 7–10 μmol m−2 s−1), and its seasonal variation was exponentially related to soil temperature (Q10 = 2.16). As a function of soil temperature, we estimated a yearly mean stand‐scale Rs of 5.17 μmol m−2 s−1, and a total carbon efflux from the soil of 1959 g C m−2 year−1 for 2014. Despite showing similar seasonal patterns as those in temporal forests, the Rs in this ecosystem is very high throughout the year, and the yearly value is much higher for natural mature forests. A mass balance approach suggests that the large amount of belowground carbon allocation of plants contributed to the high CO2 emissions from the soils.

Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass.

Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 °C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

Perennial Crops Can Compensate for Low Soil Carbon Inputs from Maize in Ley-Arable Systems.

(1) Background: Soil organic carbon (SOC) in agricultural soils plays a crucial role in mitigating global climate change but also, and maybe more importantly, in soil fertility and thus food security. Therefore, the influence of contrasting cropping systems on SOC not only in the topsoil, but also in the subsoil, needs to be understood. (2) Methods: In this study, we analyzed SOC content and δ13C values from a crop rotation experiment for biogas production, established in southern Germany in 2004. We compared two crop rotations, differing in their proportions of maize (0 vs. 50%) and perennial legume-grass leys as main crops (75 vs. 25%). Maize was cultivated with an undersown white clover. Both rotations had an unfertilized variant and a variant that was fertilized with biogas digestate according to the nutrient demand of crops. Sixteen years after the experiment was established, the effects of crop rotation, fertilization, and soil depth on SOC were analyzed. Furthermore, we defined a simple carbon balance model to estimate the dynamics of δ13C in soil. Simulations were compared to topsoil data (0-30 cm) from 2009, 2017, and 2020, and to subsoil data (30-60 cm) from 2020. (3) Results: Crop rotation and soil depth had significant effects, but fertilization had no effect on SOC content and δ13C. SOC significantly differed between the two crop rotations regarding δ13C in both depths but not regarding content. Annual enrichment in C4 (maize) carbon was 290, 34, 353, and 70 kg C ha-1 per maize year in the topsoil and subsoil of the unfertilized and fertilized treatments, respectively. These amounts corresponded to carbon turnover rates of 0.8, 0.3, 0.9, and 0.5% per maize year. Despite there being 50% maize in the rotation, maize carbon only accounted for 20% of the observed carbon sequestration in the topsoil. Even with pre-defined parameter values, the simple carbon model reproduced observed δ13C well. The optimization of model parameters decreased the carbon use efficiency of digestate carbon in the soil, as well as the response of belowground carbon allocation to increased aboveground productivity of maize. (4) Conclusions: Two main findings resulted from this combination of measurement and modelling: (i) the retention of digestate carbon in soil was low and its effect on δ13C was negligible, and (ii) soil carbon inputs from maize only responded slightly to increased above-ground productivity. We conclude that SOC stocks in silage maize rotations can be preserved or enhanced if leys with perennial crops are included that compensate for the comparably low maize carbon inputs.

Management practice and soil properties affect plant productivity and root biomass in endophyte‐symbiotic and endophyte‐free meadow fescue grasses

AbstractIntroductionPesticides are increasingly used in intensely managed agro‐environments, with an increasingly acknowledged impact on crop production, root establishment and plant resilience. At the same time management practices are intensified with the goal to maximize productivity.Materials and MethodsIn a greenhouse, we studied the effects of three mowing regimes (uncut, and cutting 5 or 15 cm) employed three times during the season on root and shoot biomass and chlorophyll content of the cool‐season grass Festuca pratensis (meadow fescue) growing in soil with a history of glyphosate‐based herbicide (GBH) use, the corresponding control soil, and sterilized control soil. Half of the plants hosted a systemic and vertically transmitted fungal endophyte, Epichloë uncinata, which is known to promote host grass growth.ResultsEndophyte symbiosis did not affect any tested plant parameters. Cutting the plants to 5 cm decreased both root and cumulative shoot biomass. Herbicide soil history, together with intense cutting (5 cm), caused a decrease in shoot biomass and lowered the chlorophyll content. Surprisingly, soil sterilization boosted shoot biomass and chlorophyll concentrations during less intense cutting (15 cm) and noncutting when compared to the control soil. Root biomass reduced in uncut plants when growing in soil with a history of glyphosate use.ConclusionOur results indicate that GBH residues in the soil can diminish shoot biomass when grass is frequently cut. Decreased root biomass caused by soil glyphosate history goes together with a reduction of carbon allocation belowground, which decreases grassland resilience to climate warming, increasingly occurring droughts and extreme weather events.

Cropland intensification mediates the radiative balance of greenhouse gas emissions and soil carbon sequestration in maize systems of sub-Saharan Africa.

Sub-Saharan Africa (SSA) must undertake proper cropland intensification for higher crop yields while minimizing climate impacts. Unfortunately, no studies have simultaneously quantified greenhouse gas (GHG; CO2 , CH4 , and N2 O) emissions and soil organic carbon (SOC) change in SSA croplands, leaving it a blind spot in the accounting of global warming potential (GWP). Here, based on 2-year field monitoring of soil emissions of CO2 , CH4 , and N2 O, as well as SOC changes in two contrasting soil types (sandy vs. clayey), we provided the first, full accounting of GWP for maize systems in response to cropland intensifications (increasing nitrogen rates and in combination with crop residue return) in SSA. To corroborate our field observations on SOC change (i.e., 2-year, a short duration), we implemented a process-oriented model parameterized with field data to simulate SOC dynamic over time. We further tested the generality of our findings by including a literature synthesis of SOC change across maize-based systems in SSA. We found that nitrogen application reduced SOC loss, likely through increased biomass yield and consequently belowground carbon allocation. Residue return switched the direction of SOC change from loss to gain; such a benefit (SOC sequestration) was not compromised by CH4 emissions (negligible) nor outweighed by the amplified N2 O emissions, and contributed to negative net GWP. Overall, we show encouraging results that, combining residue and fertilizer-nitrogen input allowed for sequestering 82-284 kg of CO2 -eq per Mg of maize grain produced across two soils. All analyses pointed to an advantage of sandy over clayey soils in achieving higher SOC sequestration targets, and thus call for a re-evaluation on the potential of sandy soils in SOC sequestration across SSA croplands. Our findings carry important implications for developing viable intensification practices for SSA croplands in mitigating climate change while securing food production.

Long-Term Grazing Intensity Impacts Belowground Carbon Allocation and Mycorrhizas Revealed by 13CO2 Pulse Labeling

Despite the importance of grasslands for carbon storage and climate regulation, there is uncertainty about the effect of livestock grazing intensity on aboveground carbon assimilation and belowground carbon partitioning. Furthermore, the relationship between belowground carbon allocation and arbuscular mycorrhizal fungi, which serve as a conduit for carbon movement through the plant and soil, is unclear. To investigate this, we used an in situ 13C stable isotope pulse-chase labeling approach in plots under seven rates of sheep grazing intensity in a steppe grassland in northern China. We quantified the allocation of carbon to plants, soil, and soil-respired CO2 along with measurements of mycorrhizal hyphal density in the soil. With increasing grazing intensity, carbon assimilation per unit shoot biomass increased significantly, whereas carbon allocation to roots marginally decreased. Soil-respired CO2 appeared to be independent of grazing intensity. Mycorrhizal hyphal density decreased with increasing grazing intensity and was correlated significantly with new carbon input to roots 2 d after labeling and marginally related to that of soil 1 d after the 13C-CO2 pulse. Our study suggests that grazing intensity alters the distribution of carbon among different carbon pools within the plant-soil system. The results also underscored the key role of mycorrhizas as a fast route for carbon transfer from plant to soil.

Differential Response of Soil Respiration and Total Belowground Carbon Allocation to Simulated Nitrogen and Phosphorus Deposition in Moso Bamboo Forests

Elevated nitrogen (N) and phosphorus (P) depositions have greatly affected belowground carbon processes in forest ecosystems. However, open questions still remained on the effects of N and P depositions on belowground carbon processes, including soil respiration (RS), its source components—autotrophic respiration (RA) and heterotrophic respiration (RH), and total belowground carbon allocation (TBCA) in Moso bamboo forests—one of the most important forest types with wide distributions in subtropical China. To fill this knowledge gap, a two-year N, P, and NP experiment was conducted in Moso bamboo forests. Results showed that RS, RA, and RH had a strong seasonal variability and were exponentially correlated with soil temperature. N and P depositions did not change RS and RA. However, P deposition increased RH due to the stimulation of microbial activities, indicating a significant soil carbon loss under P deposition. N and P depositions did not affect TBCA. However, NP deposition significantly increased root carbon-use efficiency. Net ecosystem production (NEP) varied from 198 ± 104 to 529 ± 225 g C m−2 year−1, indicating that Moso bamboo is an important carbon sink. P deposition marginally decreased NEP, while N and NP depositions did not affect NEP, which indicates that N deposition alleviated the suppression of P deposition on NEP. These findings highlight the inconsistent responses of RA, RH, and NEP to N, P, and NP depositions, which should be differently considered to increase the accuracy of predicting belowground carbon dynamics.

Drought and interspecific competition increase belowground carbon allocation for nitrogen acquisition in monocultures and mixtures of Trifolium repens and Lolium perenne

Drought and interspecific competition increase belowground carbon allocation for nitrogen acquisition in monocultures and mixtures of Trifolium repens and Lolium perenne

Ericoid shrubs shape fungal communities and suppress organic matter decomposition in boreal forests.

Mycorrhizal fungi associated with boreal trees and ericaceous shrubs are central actors in organic matter (OM) accumulation through their belowground carbon allocation, their potential capacity to mine organic matter for nitrogen (N) and their ability to suppress saprotrophs. Yet, interactions between co-occurring ectomycorrhizal fungi (EMF), ericoid mycorrhizal fungi (ERI), and saprotrophs are poorly understood. We used a long-term (19 yr) plant functional group manipulation experiment with removals of tree roots, ericaceous shrubs and mosses and analysed the responses of different fungal guilds (assessed by metabarcoding) and their interactions in relation to OM quality (assessed by mid-infrared spectroscopy and nuclear magnetic resonance) and decomposition (litter mesh-bags) across a 5000-yr post-fire boreal forest chronosequence. We found that the removal of ericaceous shrubs and associated ERI changed the composition of EMF communities, with larger effects occurring at earlier stages of the chronosequence. Removal of shrubs was associated with enhanced N availability, litter decomposition and enrichment of the recalcitrant OM fraction. We conclude that increasing abundance of slow-growing ericaceous shrubs and the associated fungi contributes to increasing nutrient limitation, impaired decomposition and progressive OM accumulation in boreal forests, particularly towards later successional stages. These results are indicative of the contrasting roles of EMF and ERI in regulating belowground OM storage.

Nitrogen addition and defoliation alter belowground carbon allocation with consequences for plant nitrogen uptake and soil organic carbon decomposition

Grassland plants allocate photosynthetically fixed carbon (C) belowground to root biomass and rhizodeposition, but also to support arbuscular mycorrhizal fungi (AMF). These C allocation pathways could increase nutrient scavenging, but also mining of nutrients through enhanced organic matter decomposition. While important for grassland ecosystem functioning, methodological constraints have limited our ability to measure these processes under field conditions. We used 13CO2 and 15N pulse labelling methods to examine belowground C allocation to root biomass production, rhizodeposition and AMF colonisation during peak plant growth in a grassland field experiment after three years of N fertilisation (0 and 40 kg N ha−1 year−1) and defoliation frequency treatments (“low” and “high”, with 3–4 and 6–8 simulated grazing events per year, mimicking moderate and intense grazing, respectively). Moreover, we quantified the consequences for plant nitrogen (N) uptake and decomposition of soil organic C (SOC). Nitrogen fertilisation increased rhizodeposition and AMF colonisation (by 63 % and 54 %), but reduced root biomass (by 25 %). With high defoliation frequency, AMF colonisation increased (by 60 %), but both root biomass and rhizodeposition declined (by 35 % and 58 %). Plant N uptake was highest without N fertilisation and low defoliation frequency, and positively related to root biomass and the number of root tips. Therefore, when N supply is low and the capacity to produce C through photosynthesis is high, belowground C allocation to root production and associated root tips was important to scavenge for N in the soil. In contrast, the strong positive relationship between the rate of rhizodeposition and SOC decomposition, suggests that rhizodeposition may help plants to mine for nutrients locked in SOC. Taken together, the results of this study suggest that belowground C allocation pathways affected by N fertilisation and defoliation frequency affect plant N scavenging and mining with important consequences for long-term grassland C dynamics.

Reduced chemodiversity suppresses rhizosphere microbiome functioning in the mono-cropped agroecosystems

BackgroundRhizodeposits regulate rhizosphere interactions, processes, nutrient and energy flow, and plant-microbe communication and thus play a vital role in maintaining soil and plant health. However, it remains unclear whether and how alteration in belowground carbon allocation and chemodiversity of rhizodeposits influences microbiome functioning in the rhizosphere ecosystems. To address this research gap, we investigated the relationship of rhizosphere carbon allocation and chemodiversity with microbiome biodiversity and functioning during peanut (Arachis hypogaea) continuous mono-cropping. After continuously labeling plants with 13CO2, we studied the chemodiversity and composition of rhizodeposits, along with the composition and diversity of active rhizosphere microbiome using metabolomic, amplicon, and shotgun metagenomic sequencing approaches based on DNA stable-isotope probing (DNA-SIP).ResultsOur results indicated that enrichment and depletion of rhizodeposits and active microbial taxa varied across plant growth stages and cropping durations. Specifically, a gradual decrease in the rhizosphere carbon allocation, chemodiversity, biodiversity and abundance of plant-beneficial taxa (such as Gemmatimonas, Streptomyces, Ramlibacter, and Lysobacter), and functional gene pathways (such as quorum sensing and biosynthesis of antibiotics) was observed with years of mono-cropping. We detected significant and strong correlations between rhizodeposits and rhizosphere microbiome biodiversity and functioning, though these were regulated by different ecological processes. For instance, rhizodeposits and active bacterial communities were mainly governed by deterministic and stochastic processes, respectively. Overall, the reduction in carbon deposition and chemodiversity during peanut continuous mono-cropping tended to suppress microbial biodiversity and its functions in the rhizosphere ecosystem.ConclusionsOur results, for the first time, provide the evidence underlying the mechanism of rhizosphere microbiome malfunctioning in mono-cropped systems. Our study opens new avenues to deeply disentangle the complex plant-microbe interactions from the perspective of rhizodeposits chemodiversity and composition and will serve to guide future microbiome research for improving the functioning and services of soil ecosystems.2t8oyWpyXt9Xr4Hhf-zbjPVideo abstract

Long-term compost amendment modulates wheat genotype differences in belowground carbon allocation, microbial rhizosphere recruitment and nitrogen acquisition

The implementation of soil health-promoting practices, such as cover cropping and compost application, has important implications for nutrient cycling and management in agroecosystems. At the same time, plant belowground carbon (C) allocation patterns can influence nutrient cycling and availability in soil through changes to the microbial community, but the effects may depend on the crop genotype and management practices in place. We evaluated belowground C allocation patterns using 13C labeling and root architecture in two genotypes of winter wheat (Triticum aestivum) with different levels of exudation and belowground allocation strategies in soils with contrasting compost amendment legacy (108.7 Mg ha−1 every 2 years over 10 years vs. no compost). We also measured microbial community structure and function in the rhizosphere and quantified uptake of residue-derived N from 15N-labelled cover crop residues. We found an interactive effect between soil management and genotype, where in the no-compost soil, the high-exudation genotype (Snowmass) increased exudation by over 4-fold, while the low-exudate genotype (Byrd) increased only 2-fold. While we did not observe genotype differences in rhizosphere enzyme activity or dissolved N pools, residue N uptake was 1.8 times greater for Snowmass in the compost-amended soil. There were more rhizosphere microbial taxa associated with the high-exudate genotype (Snowmass); nine bacterial and seven fungal families were indicative of Snowmass, versus one bacterial and four fungal families for Byrd. Our results suggest that the high-exudation strategy can influence the rhizosphere microbial community, and lead to greater short-term residue N uptake in high SOM soil. By directly linking root architecture, exudation, microbial communities, and N mineralization and uptake dynamics, this work demonstrates that plasticity in root C allocation is genotype-specific and influences microbial communities and nutrient cycling depending on the soil health context.