Vidya Suseela

Cover crops (CCs) are widely used to improve soil health, but their species-specific effects on microbial functional diversity and redundancy remain poorly understood. We evaluated how monocultures of field pea (Pisum sativum), forage radish (Raphanus sativus), and cereal rye (Secale cereale), as well as a three-species mixture (3spp) and a five-species mixture (5spp), influence rhizosphere bacterial and fungal communities in a field experiment. Amplicon sequencing and predictive functional profiling were used to assess microbial diversity, composition, functional potential, and functional redundancy (FR). Microbial alpha diversity showed little change across treatments, but community composition and predicted functional profiles were strongly influenced by CC identity. The 3spp rhizobiome showed the highest bacterial FR, broad metabolic capacity, and greater network connectivity, suggesting increased functional stability of all treatments. In contrast, the 5spp rhizobiome showed reduced bacterial FR and more widespread functional depletion, likely due to imbalances in plant functional types. Monocultures showed species-specific patterns, with rye supporting functionally efficient communities and radish promoting more competitive interactions and reduced functional diversity. Fungal communities responded differently from bacterial communities. While fungal taxonomic shifts were limited, they showed stronger compositional differences among treatments and more stable functional profiles. Notably, fungal FR was highest in the 5spp rhizobiome. Overall, these findings highlight that balanced plant functional composition, rather than greater species richness alone, is important for shaping rhizosphere microbial function and redundancy. Balanced mixtures promoted higher bacterial redundancy, while more diverse but functionally imbalanced mixtures did not consistently enhance microbial function.

Drought can strongly affect the rhizosphere microbiome that performs important ecosystem functions. Functional redundancy (FR), in which different microbes can perform similar functions, might help plants withstand drought stress in agroecosystems. However, the effect of different cover crop (CC) plant functional types (PFTs) in monocultures and mixtures on FR, and microbial stability under drought is not well characterized. We studied how drought and CC PFTs shape bacterial and fungal rhizosphere communities using amplicon sequencing from three monocultures of legume (field pea), grass (cereal rye), and brassica (forage radish), a three-species mixture, and a no CC control. Bacterial diversity and predicted functions were more negatively affected by drought than predicted fungal traits, which remained relatively stable. Although we hypothesized that higher plant diversity would enhance microbial response to drought, the three-species mixture did not maintain FR or network complexity under drought as effectively as monocultures. Under ambient conditions, the mixture supported greater microbial diversity, yet it experienced the most significant decline in FR under drought. Among the monocultures, rye harbored the most stable microbial community between conditions, retaining FR and sustaining complex microbial interaction networks. In contrast, radish and pea communities changed more under drought, showing reduced FR and simplified network structures. These findings indicate that the identity of the PFT has a more substantial influence on microbial FR than species richness alone. Selecting appropriate CC species, rather than simply increasing diversity, may be a more effective strategy for maintaining soil microbial functions and supporting plant performance under drought conditions.

Summary: Plant litter decomposition sustains ecosystem productivity and modulates soil carbon cycling. Drought directly impacts decomposition by decreasing soil moisture and indirectly by altering plant tissue chemistry, an aspect that is less explored in decomposition studies. To elucidate the above interactive effects of altered precipitation on litter decomposition, we conducted an in situ and reciprocal decomposition experiment at a climate manipulation experiment (BACE) in MA, USA, using litter from Quercus rubra and Quercus velutina exposed to ambient, dry, and wet precipitation treatments for 4 yr at BACE. Plant tissues exposed to dry (litter origin) treatment were enriched in alkyl compounds and lignin in both species. Litter of Q. rubra from the dry treatment initially decomposed faster in the dry treatment than in the wet treatment. Similarly, litter of Q. rubra from the wet treatment initially decomposed faster in the wet treatment than in the dry treatment. Interestingly, litter of both species from the dry treatment, when decomposed in dry, ambient, and wet treatments, had more alkyl compounds, representing a legacy effect of the drought‐induced altered initial litter chemistry that persisted throughout the 2 yr of decomposition. Our study provides novel insights that drought stress‐induced changes in litter chemistry can influence mass loss and create a legacy effect on the persistence of recalcitrant compounds during decomposition.

Phosphorus (P), an essential nutrient, is apparently unavailable to plants due to strong sorption in soils. Plants with shallow root systems and high surface area exhibit high P acquisition efficiency (PAE). Arbuscular mycorrhizal fungal (AMF) symbiosis can also enhance PAE. However, whether AMF symbiosis will equally benefit crop accessions with contrasting root traits is less known. We selected sorghum accessions that varied in root traits to evaluate P uptake strategies and assessed changes in root traits, acid‐phosphatase activity, primary and specialised metabolome in the presence of AMF, and under limited and stratified P availability. Our results revealed that regardless of the inherent accession differences in root traits, all accessions had higher shoot P and biomass with AMF inoculation. AMF‐inoculated plants had lower specific root length, higher hyphal length and acid phosphatase activity than the non‐inoculated control, indicating that plants can enhance PAE with AMF, irrespective of inherent accession differences. The AMF induced similar changes in root metabolome, where AMF‐inoculated plants had higher organic acids and specialised metabolites necessary for a functional symbiosis. Our results emphasise the critical role of AMF in efficient P uptake regardless of inherent root traits, which should be considered while selecting crop accessions for improved PAE.

Mycorrhizas are widespread below‐ground symbioses formed between plant roots and soil fungi. This plant–fungal partnership impacts terrestrial ecosystems by mediating plant performance and biogeochemical processes. The influence of mycorrhizas on plant and ecosystem functioning is ultimately driven by the biological processes that regulate plant–mycorrhizal interactions. Although convergent patterns in morphological and genetic traits of mycorrhizas have been well‐documented and reflect key selection forces that shape the biology of mycorrhizas, generalizable traits of mycorrhizal‐associated root metabolome, which are more intimately linked to plant and ecosystem functioning, remain unexplored. Here, we compared mycorrhizal‐associated metabolome alterations across multiple plant–mycorrhizal fungus combinations. Specifically, we inoculated a phylogenetically diverse set of temperate tree species with either arbuscular mycorrhizal or ectomycorrhizal fungi (the two major mycorrhizal lifestyles). Using comprehensive metabolomics approaches, we then assessed the metabolome in mycorrhizal and non‐mycorrhizal roots and the corresponding leaves. Comparing across multiple plant–mycorrhizal fungus combinations, our data revealed metabolite alterations unique to mycorrhizal lifestyle as well as those common across plant–fungus combinations irrespective of lifestyles. Roots colonized by arbuscular mycorrhizal and ectomycorrhizal fungi accumulated different sets of carbohydrates, reflecting unique carbon allocation strategies for mycorrhizas. Arbuscular mycorrhizal roots accumulated cyclic polyols (e.g. inositols) inaccessible to their fungal partners, suggesting tight regulation of carbon partitioning. Such accumulation did not occur in ectomycorrhizal‐colonized roots, which instead accrued acyclic polyols (e.g. mannitol and arabitol) that were undetected in non‐mycorrhizal roots and likely of fungal origin. Mycorrhizas also altered specialized metabolism, featuring frequent increases in flavan‐3‐ols (e.g. catechins, gallocatechins, and their oligomers) but decreases in flavanols irrespective of mycorrhizal lifestyles, suggesting tactical reconfiguration of specialized metabolites to both facilitate and restrain the symbiont. These characteristic metabolite alterations were largely root specific and were not mirrored in leaves. Synthesis. Using multiple plant–mycorrhizal systems and metabolomics approaches, our study demonstrates that part of the metabolite alterations occurring during root–mycorrhizal interactions were relatively common across plant–mycorrhizal systems, with implications for both carbon partitioning and tissue protection strategies important for successful symbiosis. These generalizable patterns appear robust to the phylogenetic history of host plants and thus may be widespread in land plants.

Summary: Lignin is an important root chemical component that is widely used in biogeochemical models to predict root decomposition. Across ecological studies, lignin abundance has been characterized using both proximate and lignin‐specific methods, without much understanding of their comparability. This uncertainty in estimating lignin limits our ability to comprehend the mechanisms regulating root decomposition and to integrate lignin data for large‐scale syntheses. We compared five methods of estimating lignin abundance and composition in fine roots across 34 phylogenetically diverse tree species. We also assessed the feasibility of high‐throughput techniques for fast‐screening of root lignin. Although acid‐insoluble fraction (AIF) has been used to infer root lignin and decomposition, AIF‐defined lignin content was disconnected from the lignin abundance estimated by techniques that specifically measure lignin‐derived monomers. While lignin‐specific techniques indicated lignin contents of 2–10% (w/w) in roots, AIF‐defined lignin contents were c. 5–10‐fold higher, and their interspecific variation was found to be largely unrelated to that determined using lignin‐specific techniques. High‐throughput pyrolysis–gas chromatography–mass spectrometry, when combined with quantitative modeling, accurately predicted lignin abundance and composition, highlighting its feasibility for quicker assessment of lignin in roots. We demonstrate that AIF should be interpreted separately from lignin in fine roots as its abundance is unrelated to that of lignin polymers. This study provides the basis for informed decision‐making with respect to lignin methodology in ecology.

Cover crops (CCs) can increase soil organic carbon (SOC) sequestration by providing additional OC residues, recruiting beneficial soil microbiota, and improving soil aggregation and structure. The various CC species that belong to distinct plant functional types (PFTs) may differentially impact SOC formation and stabilization. Biogeochemical theory suggests that selection of PFTs with distinct litter quality (C:N ratio) should influence the pathways and magnitude of SOC sequestration. Yet, we lack knowledge on the effect of CCs from different PFTs on the quantity and composition of physiochemical pools of SOC. We sampled soils under monocultures of three CC PFTs (legume [crimson clover]; grass [triticale]; and brassica [canola]) and a mixture of these three species, from a long‐term CC experiment in Pennsylvania, USA. We measured C content in bulk soil and C content and composition in contrasting physical fractions: particulate organic matter, POM; and mineral‐associated organic matter, MAOM. The bulk SOC content was higher in all CC treatments compared to the fallow. Compared to the legume, monocultures of grass and brassica with lower litter quality (wider C:N) had higher proportion of plant‐derived C in POM, indicating selective preservation of complex structural plant compounds. In contrast, soils under legumes had greater accumulation of microbial‐derived C in MAOM. Our results for the first time, revealed that the mixture contributed to a higher concentration of plant‐derived compounds in POM relative to the legume, and a greater accumulation of microbial‐derived C in MAOM compared to monocultures of grass and brassica. Mixtures with all three PFTs can thus increase the short‐ and long‐term SOC persistence balancing the contrasting effects on the chemistries in POM and MAOM imposed by monoculture CC PFTs. Thus, despite different cumulative C inputs in CC treatments from different PFTs, the total SOC stocks did not vary between CC PFTs, rather PFTs impacted whether C accumulated in POM or MAOM fractions. This highlights that CCs of different PFTs may shift the dominant SOC formation pathways (POM vs. MAOM), subsequently impacting short‐ and long‐term SOC stabilization and stocks. Our work provides a strong applied field test of biogeochemical theory linking litter quality to pathways of C accrual in soil.

SUMMARY: Rhizobiome confer stress tolerance to ruderal plants, yet their ability to alleviate stress in crops is widely debated, and the associated mechanisms are poorly understood. We monitored the drought tolerance of maize (Zea mays) as influenced by the cross‐inoculation of rhizobiota from a congeneric ruderal grassAndropogon virginicus(andropogon‐inoculum), and rhizobiota from organic farm maintained under mesic condition (organic‐inoculum). Across drought treatments (40% field capacity), maize that receivedandropogon‐inoculumproduced two‐fold greater biomass. This drought tolerance translated to a similar leaf metabolomic composition as that of the well‐watered control (80% field capacity) and reduced oxidative damage, despite a lower activity of antioxidant enzymes. At a morphological‐level, drought tolerance was associated with an increase in specific root length and surface area facilitated by the homeostasis of phytohormones promoting root branching. At a proteome‐level, the drought tolerance was associated with upregulation of proteins related to glutathione metabolism and endoplasmic reticulum‐associated degradation process. Fungal taxa belonging to Ascomycota, Mortierellomycota, Archaeorhizomycetes, Dothideomycetes, and Agaricomycetes inandropogon‐inoculumwere identified as potential indicators of drought tolerance. Our study provides a mechanistic understanding of the rhizobiome‐facilitated drought tolerance and demonstrates a better path to utilize plant–rhizobiome associations to enhance drought tolerance in crops.