Nutrient Foraging Research Articles

Pseudomonas aeruginosa faces a fitness trade-off between mucosal colonization and antibiotic tolerance during airway infection.

Pseudomonas aeruginosa frequently causes antibiotic-recalcitrant pneumonia, but the mechanisms driving its adaptation during human infections remain unclear. To reveal the selective pressures and adaptation strategies at the mucosal surface, here we investigated P. aeruginosa growth and antibiotic tolerance in tissue-engineered airways by transposon insertion sequencing (Tn-seq). Metabolic modelling based on Tn-seq data revealed the nutritional requirements for P. aeruginosa growth, highlighting reliance on glucose and lactate and varying requirements for amino acid biosynthesis. Tn-seq also revealed selection against biofilm formation during mucosal growth in the absence of antibiotics. Live imaging in engineered organoids showed that biofilm-dwelling cells remained sessile while colonizing the mucosal surface, limiting nutrient foraging and reduced growth. Conversely, biofilm formation increased antibiotic tolerance at the mucosal surface. Moreover, mutants with exacerbated biofilm phenotypes protected less tolerant but more cytotoxic strains, contributing to phenotypic heterogeneity. P. aeruginosa must therefore navigate conflicting physical and biological selective pressures to establish chronic infections.

Available nitrogen and enzyme activity in rhizosphere soil dominate the changes in fine-root nutrient foraging strategies during plantation development

The variation in fine-root traits in response to soil resources (i.e., fine-root nutrient foraging strategy) is critical for plants to adapt to environmental changes and even win in intra- or interspecific resource competition. However, the patterns and driving mechanisms that change in fine-root traits and nutrient foraging strategies during the development of plantations remain unclear. We analyzed the relationships among fine-root traits, rhizosphere soil nutrient variables, and enzyme activities at four stages of Pinus massoniana plantations: 18 years (young), 30 years (middle-aged), 43 years (mature), and 63 years (overmature). We found that specific root area (SRA) and specific root length (SRL), which represent the speed of nutrient acquisition, decreased with stand development. Root tissue density (RTD) and specific root tip number (SRT), which represent the cost of nutrient foraging, decreased with stand development after peaking in the mature and middle-aged forests, respectively. Compared with young stands, fine-root organic carbon and total phosphorus contents decreased by 23 % in the overmature forest, but total nitrogen increased by nearly 50 %. Principal component analysis (PCA) showed that those fine-root traits significantly varied from young to overmature stands on PC1, and such a shift was consistent with changes in the growth period from fast (in the young forest) to slow (in the overmature forest) for P. massoniana. We suggest that those changes in fine-root traits along the PC1 represents a change in the fine-root foraging strategy of P. massoniana. This strategy (i.e., PC1) was negatively correlated with rhizosphere soil total nutrients (i.e., soil organic carbon and total nitrogen) and carbon (C)-nitrogen (N)-phosphorus (P) stoichiometry but positively correlated with available carbon, nitrogen, and C and N cycling-related enzyme activities. Specifically, the variation in rhizosphere soil NH4+ had the highest amount of explication (72 %) for the variation in the root nutrient foraging strategy. These results demonstrate that the intensity of the fine-root response to soil resources is related to the structure of rhizosphere soil available nutrients (especially soil available nitrogen) as well as carbon and nitrogen conversion enzyme activities during plantation development. Such relationships can reshape fine-root traits and change the fine-root nutrient foraging strategies during the development of plantations.

Root diameter, host specificity and arbuscular mycorrhizal fungal community composition among native and exotic plant species.

Plant root systems rely on a functionally diverse range of arbuscular mycorrhizal fungi to, among other benefits, extend their nutrient foraging. Extended nutrient foraging is likely of greatest importance to coarse-rooted plants, yet few studies have examined the link between root traits and arbuscular mycorrhizal fungal community composition. Here, we examine the relationship between root diameter and the composition of arbuscular mycorrhizal fungal communities in a range of native and exotic plant species. We characterized the arbuscular mycorrhizal fungal communities of 30 co-occurring native and exotic montane grassland/shrubland plant species in New Zealand. We found that plant root diameter and native/exotic status both strongly correlated with arbuscular mycorrhizal fungal community composition. Coarse-rooted plants had a lower diversity of mycorrhizal fungi compared with fine-rooted plants and associated less with generalist fungal partners. Exotic plants had a lower diversity of fungi and fewer associations with nondominant families of arbuscular mycorrhizal fungi compared with native plants. These observational patterns suggest that plants may differentially associate with fungal partners based on their root traits, with coarse-rooted plants being more specific in their associations. Furthermore, exotic plants may associate with dominant arbuscular mycorrhizal fungal taxa as a strategy in invasion.

Scions impact biomass allocation and root enzymatic activity of rootstocks in grafted melon and watermelon plants.

Vegetable grafting is increasingly recognized as an effective and sustainable plant production alternative. Grafted plants usually show increased uptake of water and minerals compared with self-rooted plants, mostly thought a consequence of the vigorous rootstocks selected. However, while studies frequently addressed the effects of rootstocks on the performance of scions, knowledge on the influences of scions on biomass allocation, morphology, and metabolic activity of roots is rare. In particular, the plasticity of root traits affecting resource acquisition and its efficiency remains poorly understood. Two different rootstock species, Cucurbita maxima × Cucurbita moschata and Lagenaria siceraria, were grafted in combination with melon (Cucumis melo) and watermelon (Citrullus lanatus). Self-grafted rootstocks were used as control. Plant biomass and root traits were determined after destructive harvesting 30 and/or 60 days after grafting. Traits included biomass allocation, leaf and root morphology, potential activities of four extracellular enzymes on root tips and basal root segments, and root respiration. Successfully grafted scions increase the ratio of root to whole plant dry matter (RMF), and increased ratios of root length to whole plant dry matter (RLR) and to plant leaf area (RL : LA). In contrast, morphological root traits such as diameter, tissue density, and specific root length remain surprisingly stable, and thus scion-induced changes of those traits may only play a minor role for the beneficial effects of grafting in Cucurbitaceae. Incompatibility in melon/L. siceraria grafts, however, was likely responsible for the reduced root growth in combination with clear changes in root morphological traits. Reduced root respiration rates seem to be the effects of a non-compatible rootstock–scion combination rather than an active, C-efficiency increasing acclimation. In contrast, heterografts with melon and watermelon frequently resulted in root-stock-specific, often enhanced potential enzymatic activities of acid phosphatase, β-glucosidase, leucine-amino-peptidase, and N-acetyl-glucosaminidase both at root tips and basal parts of lateral roots—presenting a potential and complementary mechanism of grafted plants to enhance nutrient foraging. The studied melon and watermelon scions may thus increase the nutrient foraging capacity of grafted plants by fostering the relative allocation of C to the root system, and enhancing the extracellular enzymatic activities governed by roots or their rhizobiome.

Responses of absorptive root and mycorrhizal colonization of Chinese fir (Cunninghamia lanceolata) to varied environmental conditions

Root branching and mycorrhizal symbioses are two major mechanisms for soil resource acquisition by trees. Understanding the relationship between these two mechanisms and their responses to varied environmental conditions is crucial for predicting the responses of foraging strategies of roots to environmental changes. This study was conducted in 11 Chinese fir (Cunninghamia lanceolata) plantations distributed in different environmental conditions in Subtropical China to assess the relationship between root tip traits related to nutrient foraging (branching ratio of 1st-order roots to 2nd-order roots, arbuscular mycorrhizal (AM) colonization) and their environmental variables, including mean annual precipitation (MAP), mean annual temperature (MAT), soil C, soil N, soil P, and soil pH. Root branching was more sensitive to environmental conditions than mycorrhizal symbioses. The branching ratio and AM colonization of Chinese fir were significantly related to several environmental variables. The branching ratios were positively correlated with MAT, but negatively correlated with soil C, soil N, and soil pH (P < 0.05), suggesting that harsh environmental conditions can promote absorptive root branching. To our surprise, the AM colonization of absorptive roots was not as sensitive to environmental factors as branching ratio. However, the AM colonization of absorptive roots was positively correlated with soil pH (P < 0.1), indicating that soil acidity controls mycorrhizal symbioses. Moreover, the branching ratio was significantly negatively correlated with AM colonization (P < 0.05). Our results confirmed that environmental conditions significantly regulate fine root branching and its mycorrhizal symbioses, but with different controlling variables. The negatively correlated relationship of branching ratio and AM colonization shows that environmental factors regulate absorptive root traits in different ways.

Long-term soil warming alters fine root dynamics and morphology, and their ectomycorrhizal fungal community in a temperate forest soil.

Climate warming is predicted to affect temperate forests severely, but the response of fine roots, key to plant nutrition, water uptake, soil carbon, and nutrient cycling is unclear. Understanding how fine roots will respond to increasing temperature is a prerequisite for predicting the functioning of forests in a warmer climate. We studied the response of fine roots and their ectomycorrhizal (EcM) fungal and root-associated bacterial communities to soil warming by 4°C in a mixed spruce-beech forest in the Austrian Limestone Alps after 8 and 14years of soil warming, respectively. Fine root biomass (FRB) and fine root production were 17% and 128% higher in the warmed plots, respectively, after 14years. The increase in FRB (13%) was not significant after 8years of treatment, whereas specific root length, specific root area, and root tip density were significantly higher in warmed plots at both sampling occasions. Soil warming did not affect EcM exploration types and diversity, but changed their community composition, with an increase in the relative abundance of Cenoccocum at 0-10cm soil depth, a drought-stress-tolerant genus, and an increase in short- and long-distance exploration types like Sebacina and Boletus at 10-20cm soil depth. Warming increased the root-associated bacterial diversity but did not affect their community composition. Soil warming did not affect nutrient concentrations of fine roots, though we found indications of limited soil phosphorus (P) and potassium (K) availability. Our findings suggest that, in the studied ecosystem, global warming could persistently increase soil carbon inputs due to accelerated fine root growth and turnover, and could simultaneously alter fine root morphology and EcM fungal community composition toward improved nutrient foraging.

Morphological Variation in Absorptive Roots in Downy Birch (Betula pubescens) and Norway Spruce (Picea abies) Forests Growing on Drained Peat Soils

Peatland drainage based on ditch systems is a widely used forestry management practice in the boreal and hemiboreal forests to improve tree growth. This study investigated the morphological variation in absorptive roots (first- and second-order roots) across the distance gradient from the ditch with four sampling plots (5, 15, 40, and 80 m) in six drained peatland forests dominated by Downy birch and Norway spruce. The dominating tree species had a significant effect on the variation in absorptive root morphological traits. The absorptive roots of birch were thinner with a higher specific root area and length (SRA and SRL), higher branching intensity (BI), and lower root tissue density (RTD) than spruce. The distance from the ditch affected the absorptive root morphological traits (especially SRA and RTD), but this effect was not dependent on tree species and was directionally consistent between birch and spruce. With increased distance from the ditch (from plot 5 to plot 80), the mean SRA increased by about 10% in birch and 5% in spruce; by contrast, the mean RTD decreased by about 10% in both tree species, indicating a potential shift in nutrient foraging. However, soil physical and chemical properties were not dependent on the distance from the ditch. We found a species-specific response in absorptive root morphological traits to soil properties such as peat depth, pH, and temperature. Our results should be considered when evaluating the importance of morphological changes in absorptive roots when trees acclimate to a changing climate.

Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs.

Root proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition. Near-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg-1 soil. Both WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients. In addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.

A potassium-sensing niche in Arabidopsis roots orchestrates signaling and adaptation responses to maintain nutrient homeostasis.

Organismal homeostasis of the essential ion K+ requires sensing of its availability, efficient uptake, and defined distribution. Understanding plant K+ nutrition is essential to advance sustainable agriculture, but the mechanisms underlying K+ sensing and the orchestration of downstream responses have remained largely elusive. Here, we report where plants sense K+ deprivation and how this translates into spatially defined ROS signals to govern specific downstream responses. We define the organ-scale K+ pattern of roots and identify a postmeristematic K+-sensing niche (KSN) where rapid K+ decline and Ca2+ signals coincide. Moreover, we outline a bifurcating low-K+-signaling axis of CIF peptide-activated SGN3-LKS4/SGN1 receptor complexes that convey low-K+-triggered phosphorylation of the NADPH oxidases RBOHC, RBOHD, and RBOHF. The resulting ROS signals simultaneously convey HAK5 K+ uptake-transporter induction and accelerated Casparian strip maturation. Collectively, these mechanisms synchronize developmental differentiation and transcriptome reprogramming for maintaining K+ homeostasis and optimizing nutrient foraging by roots.

The evolution of root branching: increasing the level of plasticity

Plant roots and root systems are indispensable for water and nutrient foraging, and are a major evolutionary achievement for plants to cope with dry land conditions. The ability of roots to branch contributes substantially to their capacity to explore the soil for water and nutrients, and led ~400 million years ago to the successful colonization of land by plants, eventually even in arid regions. During this colonization, different forms of root branching evolved, reinforcing step by step the phenotypic plasticity of the root system. Whereas the lycophytes, the most ancient land plants with roots, only branch at the root tip, ferns are able to form roots laterally in a fixed pattern along the main root. Finally, roots of seed plants show the highest phenotypic plasticity, because lateral roots can possibly, dependent on internal and/or external signals, be produced at almost any position along the main root. The competence to form lateral roots in seed plants is based on the presence of internal cell files with stem cell-like features. Despite the dissimilarities between the different clades, a number of genetic modules seem to be co-opted in order to acquire root branching capacity. In this review, starting from the lateral root pathways in seed plants, we review root branching in the different land plant lineages and discuss the hitherto described genetic modules that contribute to their root branching capacity. We try to obtain insight into how land plants have acquired an increasing root branching plasticity during evolution that contributed to the successful colonization of our planet by seed plants.

Nitrogen deposition and decreased precipitation does not change total nitrogen uptake in a temperate forest

Decreased precipitation and increased anthropogenical by derived nitrogen (N) are important climate change factors that alter the availability of soil water and N which are crucial to root function and morphological traits. However, these factors are seldom explored in forests. To clarify how altered precipitation and N addition affect the uptake of organic and inorganic N by fine roots, a field hydroponic experiment using brief 15N exposures was conducted in a temperate forest in northern China. The root traits related to nutrient foraging (root morphology and mycorrhizal colonization) were measured simultaneously. Our results showed that all three tree species preferred ammonium (NH4+) over glycine and nitrate (NO3−), and NH4+ contributed 73% to the total N uptake from the soil. Uptake of glycine was higher than that of NO3−. Decreased precipitation, N addition, and their interaction increased NH4+ uptake rate compared with the control. Decreased precipitation decreased the glycine and NO3− uptake rate. Moreover, N addition, decreased precipitation and their interaction changed root morphological traits and significantly decreased mycorrhizal colonization. Although our treatments resulted in changes to the root traits and the forms of N uptake by plants, the total amount of N uptake did not change among all treatments. We conclude that although fine root traits of dominant tree species in temperate forests have high plasticity in response to climate change, nutrient balance in plants causes the total amount of N uptake to remain unchanged.

Acclimation of Fine Root Systems to Soil Warming: Comparison of an Experimental Setup and a Natural Soil Temperature Gradient

Global warming is predicted to impact high-latitude and high-altitude forests severely, jeopardizing their overall functioning and carbon storage, both of which depend on the warming response of tree fine root systems. This paper investigates the effect of soil warming on the biomass, morphology and colonizing ectomycorrhizal community of spruce fine and absorptive fine roots. We compare the responses of spruce roots growing at a man-made long-term soil warming (+ 4°C) experiment to results obtained from a geothermal soil temperature gradient (+ 1 to + 14°C) extending to the forest die-off edge, to shed light on the generalizability of the warming response and reveal any thresholds in acclimation ability. Trees in warmer soils formed longer and less-branched absorptive roots with higher specific root length and area, and lower root tissue density in both spruce stands, irrespective of warming method and location. Soil warming at the experimental warming site also supported the occurrence of a more varied EcM community and an increase in the abundance of Tomentella spp., indicating a shift in nutrient foraging. Fine and absorptive fine root biomass decreased toward warmer soil, with a sharp reduction occurring between + 4 and + 6°C from the ambient and leading to the collapse of the fine root system at the geothermal gradient. At the experimental warming site, the applied + 4°C warming had no effect on fine and absorptive fine root biomass. The similar fine root responses at the two warming sites suggest that the observations possibly reflect general acclimation patterns in spruce forests to global warming.

Root diameter predicts the extramatrical hyphal exploration distance of the ectomycorrhizal fungal community

AbstractLocally coexisting tree species of temperate forests often vary widely in the diameter of their absorptive roots, resulting in contrasting strategies of root foraging within soil nutrient hot spots. We hypothesized that root diameter would also influence the extramatrical hyphal exploration distance of the mycorrhizal fungal community due to coevolution of the plant and fungal partners leading to functional complementarity. We collected absorptive roots from mature trees of nine ectomycorrhizal tree species in replicated monoculture plots of a plantation where the soil is relatively homogeneous. The identities of ectomycorrhizal fungal taxa and their relative abundances were determined by DNA sequencing. The hyphal exploration type (i.e., contact, short‐distance, medium‐distance or long‐distance) was assigned to each taxon, allowing us to calculate an index of the abundance‐weighted mean extramatrical hyphal exploration distance of the fungal community. Overall, there was a significant, positive correlation between root diameter and our index of hyphal exploration distance such that tree species with the thicker roots were associated with fungi with the longer hyphal exploration distance. Moreover, we found that root diameter negatively correlated with the proportion of contact ectomycorrhizal fungal taxa and positively correlated with the proportion of medium‐distance ectomycorrhizal fungal taxa. Our results suggest that among temperate ectomycorrhizal tree species, root diameter selects for communities of mycorrhizal fungi of specific exploration distance in a way that is consistent with root‐fungal functional complementarity in nutrient foraging.

Barnyardgrass Root Recognition Behaviour for Rice Allelopathy

Recent studies have demonstrated that the presence of belowground neighbours induces varied morphological and biochemical responses in plants. Plant allelopathic activity is elicited by the presence of competitor seedlings or competitor root exudates. However, it is unknown whether allelopathy also influences root recognition behaviour in weed–crop interaction. To assess barnyardgrass response to the presence of allelopathic rice roots, we conducted a greenhouse experiment of barnyardgrass–rice mixed culture, including barnyardgrass monoculture, barnyardgrass mixed with the allelopathic rice line PI312777 and barnyardgrass mixed with the nonallelopathic rice cultivar Liaojing-9. Our results showed that the presence of allelopathic rice roots enhanced root allocation and tissue density (RTD) of barnyardgrass, whereas it decreased root biomass, total root length, specific root length (SRL) and topological index (TI), compared to barnyardgrass grown in monoculture; moreover, there was a significant correlation of topological index with root foraging precision and competition. Therefore, the presence of allelopathic rice roots affected the barnyardgrass root morphology, nutrient foraging and competition, suggesting that allelopathy plays a key role in root recognition behaviour of barnyardgrass–rice competitive interaction.

No evidence for nutrient foraging in root-sprouting clonal plants

Clonality is defined as vegetative reproduction via the production of ramets, which are, at least initially, connected by spacers. In general, there are three types of spacers of two origins. Whereas stolons are aboveground spacers, rhizomes are belowground spacers; however, both of stem origin. The third type of spacers are roots in root-sprouting plants. The possibility of foraging in clonal plants has attracted broad interest among ecologists but has been experimentally documented only for stoloniferous clonal plants foraging for light. Foraging for belowground resources has yet to be demonstrated, perhaps because tests of foraging have focused on clonal plants that spread laterally via stolons or rhizomes, i.e. stem organs. Lateral spread based on sprouting roots has not been considered even though, in addition to functioning as conduits between ramets, root spacers are able to sense and take up nutrients. We therefore hypothesized that root-sprouting clonal plants may be able to directly react to environmental heterogeneity and exhibit nutrient foraging. To test this hypothesis, we conducted two experiments with root-sprouters in nutrient-heterogeneous and -homogeneous environments. We found that plants produced more biomass when growing in a heterogeneous environment than in a homogeneous environment and that root biomass was greater in the nutrient-rich patches than in nutrient-poor patches. However, the number of ramets did not differ between patches in the heterogeneous environment. We conclude that plants whose clonality is based on roots, similarly as plants whose clonality is based on stolons or rhizomes, do not exhibit accumulation of ramets in nutrient-rich patches. Foraging at the organ level, i.e. by roots, seems to be more probable in this clonal group. To analyse how clonal plants with different clonal strategies perceive and react to environmental heterogeneity, researchers must account for the high variability in clonal growth forms and in scales of environmental heterogeneity.

Nutrient foraging by mycorrhizas: From species functional traits to ecosystem processes

Abstract Plant roots and the associated mycorrhizal fungal hyphae often selectively proliferate into patchily distributed soil nutrient hotspots, but interactions between these two components of a mycorrhizal root system are usually ignored or experimentally isolated in nutrient foraging studies. From studies in which both roots and mycorrhizal hyphae had access to nutrient hotspots, we compiled data on root foraging precision (increase in roots in nutrient hotspots relative to outside hotspots) of plant species from different ecosystems, ranging from temperate grasslands to subtropical forests. We found that root foraging precision across the wide range of plant species was strongly influenced by root morphology and mycorrhizal type. The precision of root nutrient foraging, as a plant functional trait, may coordinate with other root traits that are related to the economics of nutrient acquisition. High foraging precision is expected to associate with the strategy of fast return on the investment in roots, such as low construction cost, high metabolic rate and rapid turnover. Nutrient foraging by mycorrhizal fungi alone may be influenced by functional traits such as hyphal exploration distance, hyphal turnover, and hyphal uptake capacity and efficiency, but such data are limited to a small portion of mycorrhizal fungal species. We propose a conceptual framework in which to simulate nitrogen and phosphorus acquisition from both nutrient hotspots and outside hotspots in mixed‐species plant communities. Simulation outputs suggest that plant species with varying root morphology and mycorrhizal type can be adaptive to a range of nutrient heterogeneity. Although there are still knowledge gaps related to nutrient foraging, as well as many unexplored plant and fungal species, we suggest that scaling nutrient foraging from individual plants to communities would advance understanding of plant species interactions and below‐ground ecosystem function. A plain language summary is available for this article.

Differences in nutrient foraging among Trifolium subterraneum cultivars deliver improved P-acquisition efficiency

The growth and root morphology responses to soil phosphorus (P) fertility by five cultivars of Trifolium subterraneum (a temperate annual pasture legume) were examined to assess whether differences in root morphology and/or root acclimation to P stress influenced P-acquisition by the clover varieties, or their critical P requirements (i.e. the rate of P supply needed for maximum shoot yield). The clovers were grown as microswards in soil with P stratified in a topsoil layer to mimic growth conditions and soil P availability in a pasture. Yield and P content of shoots, and roots from the topsoil and subsoil layers was determined after 5 weeks growth in a controlled-environment cabinet. The lengths, diameters, and root hair lengths of nutrient foraging roots from the topsoil layer were quantified. The shoot yield of the cultivars was similar when grown with high soil P fertility. However, the cultivars varied up to 1.5-fold in their ability to yield at low levels of soil P supply, and by 1.6-fold in their critical P requirements. All cultivars acclimated to low P soil by increasing root length density in the topsoil but those that yielded relatively well did so by maintaining a large root hair cylinder volume (i.e. they explored more soil) under low soil P conditions. This was associated with maintenance of dry matter allocations to topsoil roots and higher specific root lengths. Both factors assisted development of high root length density for nutrient foraging. Root acclimation responses to P were compared among the cultivars at equivalent relative shoot yields to assess the influence of plant P stress as a trigger for nutrient foraging. The least P-efficient cultivars slowed their allocation of dry matter to foraging roots at lower levels of P stress. The results suggest variation within T. subterraneum for root proliferation and specific root length could be targeted in breeding for improved P-acquisition efficiency.

Nutrient foraging strategies are associated with productivity and population growth in forest shrubs.

Temperate deciduous forest understoreys are experiencing widespread changes in community composition, concurrent with increases in rates of nitrogen supply. These shifts in plant abundance may be driven by interspecific differences in nutrient foraging (i.e. conservative vs. acquisitive strategies) and, thus, adaptation to contemporary nutrient loading conditions. This study sought to determine if interspecific differences in nutrient foraging could help explain patterns of shrub success and decline in eastern North American forests. Using plants grown in a common garden, fine root traits associated with nutrient foraging were measured for six shrub species. Traits included the mean and skewness of the root diameter distribution, specific root length (SRL), C:N ratio, root tissue density, arbuscular mycorrhizal colonization and foraging precision. Above- and below-ground productivity were also determined for the same plants, and population growth rates were estimated using data from a long-term study of community dynamics. Root traits were compared among species and associations among root traits, measures of productivity and rates of population growth were evaluated. Species fell into groups having thick or thin root forms, which correspond to conservative vs. acquisitive nutrient foraging strategies. Interspecific variation in root morphology and tissue construction correlated with measures of productivity and rates of cover expansion. Of the four species with acquisitive traits, three were introduced species that have become invasive in recent decades, and the fourth was a weedy native. In contrast, the two species with conservative traits were historically dominant shrubs that have declined in abundance in eastern North American forests. In forest understoreys of eastern North America, elevated nutrient availability may impose a filter on species success in addition to above-ground processes such as herbivory and overstorey canopy conditions. Shrubs that have root traits associated with rapid uptake of soil nutrients may be more likely to increase in abundance, while species without such traits may be less likely to keep pace with more productive species.

Nitrogen addition regulates tradeoff between root capture and foliar resorption of nitrogen and phosphorus in a subtropical pine plantation

Increased nitrogen availability affects root capture and leaf resorption of nitrogen and phosphorus, and the corresponding acquisition-conservation tradeoff in slash pine forests. Plants may maintain their stoichiometric homeostasis via regulating root nutrient capture (acquisition) and leaf nutrient resorption (conservation). However, how increased nitrogen (N) availability affects root capture and leaf resorption of N and phosphorus (P), and the corresponding acquisition-conservation tradeoff remains unclear. We quantified N and P concentrations and their stoichiometric ratios of five nutrient pools (soils, absorptive fine-roots, transport fine-roots, green needles, and senesced needles) to investigate simultaneous responses of root capture (nutrient foraging potential and translocation) and needle resorption (efficiency and proficiency) to different rates and forms of N addition in a slash pine plantation in subtropical China. N addition increased N concentration in senesced needles, but reduced N capture capacity. N addition reduced P concentration in senesced needles (improved P resorption proficiency), but did not affect P capture capacity. N addition exerted no significant influence on resorption efficiency of N and P. N rate and form effects occurred only in nutrient foraging (N foraging was more responsive to high N rate and P foraging was more responsive to ammonium-based N addition), but not in nutrient translocation and resorption. The decreased root N capture capacity and increased senesced needle N concentration imply an alleviation of N limitation to plant growth and a behavior of plants to economize on cost for nutrient acquisition and conservation. P resorption rather than capture strategy was employed, indicating a more conservative use of P in response to exacerbated P limitation. These findings jointly suggest that the allocation of effort towards capture and resorption strategy depends on the strength and type of nutrient limitation. Our results have implications for understanding the acquisition-conservation tradeoff in plant nutrient economy and the mechanisms that plants maintain their stoichiometric homeostasis in the context of N loading.

Absorptive roots trait plasticity explains the variation of root foraging strategies in Cunninghamia lanceolata

The architecture and morphology of absorptive roots show substantial plasticity in response to forest management practices. These traits are known to play important roles in the acquisition of soil resources by trees. However, the effects of nutrient addition, thinning and pruning on absorptive root traits and their feedback to root foraging strategies remain unclear.We investigated the values and plasticity of traits related to nutrient foraging (root architecture, growth and morphology) for first- and second-order roots (absorptive roots) following nitrogen (N) addition, phosphorous (P) addition, thinning and pruning treatments in a young Chinese fir (Cunninghamia lanceolata) plantation.We measured twelve traits of absorptive roots under the five treatments (control, N addition, P addition, thinning and pruning) and determined relationships between the values and plasticity of root traits and stem growth rate. We demonstrated clear patterns of root traits and their plasticity in response to the treatments. N and P addition increased root biomass (B) and root tissue density (RTD). Thinning and pruning led to larger specific root length (SRL) and root nitrogen concentrations (N), but resulted in lower root length (L) and root length density (RLD). Principal component analysis of the measured traits and plasticity provided evidence for two suites of traits related to resource acquisition and conservation strategies among treatments. The trait syndromes exhibiting resource acquisition strategy (SRL and N) is arrayed well along the positive part of the first axis, whereas the opposite trait syndromes with resource conservation strategy (root diameter and RTD) is along the negative part of the first axis. The first axis also separates nutrient-induced treatments (N and P addition) from light-mediated treatments (pruning and thinning). Furthermore, first-order root exhibited higher foraging sensitivity and precision (expressed as relative fine root length difference) in response to P addition than to the other treatments. The foraging sensitivity and precision, plus B, L and RLD showed that first-order roots are more responsive to environment than second-order roots across the treatments. Stem growth rate was correlated positively with absorptive root traits (biomass, root surface area index, root length density, and root tip number) after thinning, but negatively with these traits after N and P addition.These findings demonstrate that C. lanceolata finely tuned root foraging strategies between first- and second-order root traits and their plasticity at the intraspecific level in response to forest management practices. Further studies may explore nutrient-induced and light-mediated foraging strategies between absorptive roots across root branch orders in mature Chinese fir plantation.