Root nitrogen reallocation: what makes it matter?

Root nitrogen (N) reallocation involves remobilization of root N-storage pools to support shoot growth. Representing a critical yet underexplored facet of plant function, we developed innovative frameworks to elucidate its connections with key ecosystem components. First, root N reallocation increases with plant species richness and N-acquisition strategies, driven by competitive stimulation of plant N demand and synergies in N uptake. Second, competitive root traits and mycorrhizal symbioses, which enhance N foraging and uptake, exhibit trade-offs with root N reallocation. Furthermore, root N reallocation is attenuated by N-supply attributes such as increasing litter quality, soil fungi-to-bacteria ratios, and microbial recruitment in the hyphosphere/rhizosphere. These frameworks provide new insights and research avenues for understanding the ecological roles of root N reallocation.

Root traits in response to frequent fires: Implications for belowground carbon dynamics in fire-prone savannas.

Predicting how belowground carbon storage reflects changes in aboveground vegetation biomass is an unresolved challenge in most ecosystems. This is especially true for fire-prone savannas, where frequent fires shape the fraction of carbon allocated to root traits for post-fire vegetation recovery. Here I review evidence on how root traits may respond to frequent fires and propose to leverage root traits to infer belowground carbon dynamics in fire-prone savannas. Evidently, we still lack an understanding of trade-offs in root acquisitive vs. conservative traits in response to frequent fires, nor have we determined which root traits are functionally important to mediate belowground carbon dynamics in a frequently burned environment. Focusing research efforts along these topics should improve our understanding of savanna carbon cycling under future changes in fire regimes.

Linking root exudation to belowground economic traits for resource acquisition.

The concept of a root economics space (RES) is increasingly adopted to explore root trait variation and belowground resource-acquisition strategies. Much progress has been made on interactions of root morphology and mycorrhizal symbioses. However, root exudation, with a significant carbon (C) cost (c. 5-21% of total photosynthetically fixed C) to enhance resource acquisition, remains a missing link in this RES. Here, we argue that incorporating root exudation into the structure of RES is key to a holistic understanding of soil nutrient acquisition. We highlight the different functional roles of root exudates in soil phosphorus (P) and nitrogen (N) acquisition. Thereafter, we synthesize emerging evidence that illustrates how root exudation interacts with root morphology and mycorrhizal symbioses at the level of species and individual plant and argue contrasting patterns in species evolved in P-impoverished vs N-limited environments. Finally, we propose a new conceptual framework, integrating three groups of root functional traits to better capture the complexity of belowground resource-acquisition strategies. Such a deeper understanding of the integrated and dynamic interactions of root morphology, root exudation, and mycorrhizal symbioses will provide valuable insights into the mechanisms underlying species coexistence and how to explore belowground interactions for sustainable managed systems.

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Fine roots are sensitive to changes in the soil environment, and play an important role in plant growth and development. To clarify the relationship between fine root traits and rhizosphere soil nutrient characteristics, fine roots of trees belonging to different diameter classes in six-year-old Zenia insignis plantation were sampled. The results showed that root biomass, root length density and root volume density increased with the increases of diameter class. Specific root length and specific root area showed the trend of first rising and then falling and rising again with the increases of diameter class. Root tissue density did not change with diameter class. There were significant diffe-rences in soil pH, water content, total carbon, total phosphorus, ammonium nitrogen, nitrate nitrogen and total available nitrogen contents of rhizosphere soil belonging to different diameter classes. The concentrations of soil total carbon, total nitrogen, nitrate nitrogen and total available nitrogen in the rhizosphere soil of large diameter trees were relatively higher, while the soil water content, total phosphorus and ammonium nitrogen contents of small diameter trees were relatively higher. The concentrations of soil total nitrogen, total carbon, nitrate nitrogen and total available nitrogen were significantly positively correlated with root biomass, root length density and root volume density. The concentrations of soil total phosphorus was significantly positively correlated with root tissue density of fine roots, but negatively correlated with specific root length and specific root area. Soil water content was significantly positively correlated with root biomass and root volume density. Soil pH was significantly positively correlated with the specific root length and specific root area of fine roots, but negatively correlated with root tissue density. Our results provide scientific basis for the selection of excellent germplasm resources of Z. insignis.

Tradeoffs among root morphology, exudation and mycorrhizal symbioses for phosphorus-acquisition strategies of 16 crop species.

Plant roots exhibit diverse root functional traits to enable soil phosphorus (P) acquisition, including changes in root morphology, root exudation and mycorrhizal symbioses. Yet, whether these traits are differently coordinated among crop species to enhance P acquisition is unclear. Here, eight root functional traits for P acquisition were characterized in 16 major herbaceous crop species grown in a glasshouse under limiting and adequate soil P availability. We found substantial interspecific variation in root functional traits among species. Those with thinner roots showed more root branching and less first-order root length, and had consistently lower colonization by arbuscular mycorrhizal fungi (AMF), fewer rhizosheath carboxylates and reduced acid phosphatase activity. In response to limiting soil P, species with thinner roots showed a stronger response in root branching, first-order root length and specific root length of the whole root system, Conversely, species with thicker roots exhibited higher colonization by AMF and/or more P-mobilizing exudates in the rhizosheath. We conclude that, at the species level, tradeoffs occur among the three groups of root functional traits we examined. Root diameter is a good predictor of the relative expression of these traits and how they change when P is limiting.

Hierarchical traits distances explain grassland Fabaceae species' ecological niches distances.

Fabaceae species play a key role in ecosystem functioning through their capacity to fix atmospheric nitrogen via their symbiosis with Rhizobium bacteria. To increase benefits of using Fabaceae in agricultural systems, it is necessary to find ways to evaluate species or genotypes having potential adaptations to sub-optimal growth conditions. We evaluated the relevance of phylogenetic distance, absolute trait distance and hierarchical trait distance for comparing the adaptation of 13 grassland Fabaceae species to different habitats, i.e., ecological niches. We measured a wide range of functional traits (root traits, leaf traits, and whole plant traits) in these species. Species phylogenetic and ecological distances were assessed from a species-level phylogenetic tree and species' ecological indicator values, respectively. We demonstrated that differences in ecological niches between grassland Fabaceae species were related more to their hierarchical trait distances than to their phylogenetic distances. We showed that grassland Fabaceae functional traits tend to converge among species with the same ecological requirements. Species with acquisitive root strategies (thin roots, shallow root systems) are competitive species adapted to non-stressful meadows, while conservative ones (coarse roots, deep root systems) are able to tolerate stressful continental climates. In contrast, acquisitive species appeared to be able to tolerate low soil-P availability, while conservative ones need high P availability. Finally we highlight that traits converge along the ecological gradient, providing the assumption that species with similar root-trait values are better able to coexist, regardless of their phylogenetic distance.