The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

Soils harbor enormously diverse bacterial populations, and soil bacterial communities can vary greatly in composition across space. However, our understanding of the specific changes in soil bacterial community structure that occur across larger spatial scales is limited because most previous work has focused on either surveying a relatively small number of soils in detail or analyzing a larger number of soils with techniques that provide little detail about the phylogenetic structure of the bacterial communities. Here we used a bar-coded pyrosequencing technique to characterize bacterial communities in 88 soils from across North and South America, obtaining an average of 1,501 sequences per soil. We found that overall bacterial community composition, as measured by pairwise UniFrac distances, was significantly correlated with differences in soil pH (r = 0.79), largely driven by changes in the relative abundances of Acidobacteria, Actinobacteria, and Bacteroidetes across the range of soil pHs. In addition, soil pH explains a significant portion of the variability associated with observed changes in the phylogenetic structure within each dominant lineage. The overall phylogenetic diversity of the bacterial communities was also correlated with soil pH (R2 = 0.50), with peak diversity in soils with near-neutral pHs. Together, these results suggest that the structure of soil bacterial communities is predictable, to some degree, across larger spatial scales, and the effect of soil pH on bacterial community composition is evident at even relatively coarse levels of taxonomic resolution.

/pdf/pyrosequencing-based-assessment-of-soil-ph-as-a-predictor-of-1g7rafh378.pdf

Pyrosequencing-based assessment of soil pH as a predictor of soil bacterial community structure at the continental scale.

Publisher Summary In this chapter, the advances that have been made in understanding the ecology of the mineral nutrition of wild plants from terrestrial ecosystems have been reviewed. This chapter is organized along three lines. First, the issues of nutrient-limited plant growth and nutrient uptake, with special emphasis on the importance of the uptake of nutrients in organic form—both by mycorrhizal and by non-mycorrhizal plants—and the importance of symbiotic nitrogen fixation is treated. In addition, the influence of allocation patterns on mineral nutrient uptake is described. Next, a few of the nutritional aspects of leaf functioning and how nutrients are used for biomass production by the plant are explored. That is done by studying the nutrient use efficiency (NUE) of plants and the various components of NUE. Finally, the feedback of plant species to soil nutrient availability by reviewing patterns in litter decomposition and nutrient mineralization is investigated. The chapter concludes with a synthesis of the various aspects of the mineral nutrition of wild plants. The chapter ends with a conceptual description of plant strategies with respect to mineral nutrition.

The Mineral Nutrition of Wild Plants Revisited: A Re-evaluation of Processes and Patterns

Although it is generally accepted that plant community composition is key for predicting rates of ecosystem processes in the face of global change, microbial community composition is often ignored in ecosystem modeling. To address this issue, we review recent experiments and assess whether microbial community composition is resistant, resilient, or functionally redundant in response to four different disturbances. We find that the composition of most microbial groups is sensitive and not immediately resilient to disturbance, regardless of taxonomic breadth of the group or the type of disturbance. Other studies demonstrate that changes in composition are often associated with changes in ecosystem process rates. Thus, changes in microbial communities due to disturbance may directly affect ecosystem processes. Based on these relationships, we propose a simple framework to incorporate microbial community composition into ecosystem process models. We conclude that this effort would benefit from more empirical data on the links among microbial phylogeny, physiological traits, and disturbance responses. These relationships will determine how readily microbial community composition can be used to predict the responses of ecosystem processes to global change.

https://www.pnas.org/content/pnas/105/Supplement_1/11512.full.pdf

Resistance, resilience, and redundancy in microbial communities

Contents

 Summary

I Introduction
II Variability of N : P ratios in response to nutrient  supply
III Critical N : P ratios as indicators of nutrient  limitation
IV Interspecific variation in N : P ratios
V Vegetation properties in relation to N : P ratios
VI Implications of N : P ratios for human impacts  on ecosystems
VII Conclusions

 Acknowledgements


 References



Summary
Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios – those of plants whose growth is equally limited by N and P – depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios 20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2004.01192.x

N : P ratios in terrestrial plants: variation and functional significance

Unravelling rhizosphere–microbial interactions: opportunities and limitations

Summary The role of carbon (C) and nitrogen (N) storage by trees will be discussed in terms of uncoupling their growth from resource acquisition. There are profound differences between the physiology of C and N storage. C storage acts as a short-term, temporary buffer when photosynthesis cannot meet current sink demand and remobilization is sink driven. However, the majority of C allocated to non-structural carbohydrates such as starch is not reused so is in fact sequestered, not stored. In contrast, N storage is seasonally programmed, closely linked to tree phenology and operates at temporal scales of months to years, with remobilization being source driven. We examine the ecological significance of N storage and remobilization in terms of regulating plant N use efficiency, allowing trees to uncouple seasonal growth from N uptake by roots and allowing recovery from disturbances such as browsing damage. We also briefly consider the importance of N storage and remobilization in regulating how trees will likely respond to rising atmospheric carbon dioxide concentrations. Most studies of N storage and remobilization have been restricted to small trees growing in a controlled environment where 15 N can be used easily as a tracer for mineral N. We highlight the need to describe and quantify these processes for adult trees in situ where most root N uptake occurs via ectomycorrhizal partners, an approach that now appears feasible for deciduous trees through quantification of the flux of remobilized N in their xylem. This opens new possibilities for studying interactions between N and C allocation in trees and associated mycorrhizal partners, which are likely to be crucial in regulating the response of trees to many aspects of global environmental change.

/pdf/nitrogen-storage-and-remobilization-by-trees-4yiktgrcde.pdf

Nitrogen storage and remobilization by trees: ecophysiological relevance in a changing world

Assessing shifts in microbial community structure across a range of grasslands of differing management intensity using CLPP, PLFA and community DNA techniques

Abstraet. Accumulation of nitrogen (N) by plants in response to N supply outstripping demand is contrasted with storage of N, which itnplies that N in one tissue can be reused for the growth or maintenance of another. Storage can, therefore, occur in N-deficient plants; accutnulation can not. The consequence of accumulation and storage of N is considered, particularly in relation to the reproductive growth of annual plants, which can often use a great deal of stored N. Nitrate and proteins are the fonns of N most often stored in vegetative tissues and, quantitatively, ribulose 1,5bisphosphate carboxylase/oxygenase is often the most important protein store. While storing nitrate will be less costly to the plant in terms of energy, protein stores offer several possible advantages. These advantages are (i) maximizing the potential for carbon assimilation, (ii) avoiding problems with the regulation of leaf turgor and (iii) allowing the reduction on nitrate to occur in the young, fully illuminated leaf.

The accumulation and storage of nitrogen by herbaceous plants

Internal cycling of nitrogen has been shown to be a major source of nitrogen used for the seasonal growth of both evergreen and deciduous trees providing up to 90% of N used for leaf growth of some species. The processes of internal cycling comprise seasonal nitrogen storage, followed by remobilisation during either periods of growth (e.g. in the spring) or during leaf senescence. The ecophysiology of these processes is reviewed, along with the methods used to quantify their contribution to tree growth.

Nitrogen budget studies have been widely used to estimate internal cycling, particularly in relation to soil fertility. These studies have shown that as trees develop their rate of N uptake decreases, but as they grow their storage capacity increases, However, budget studies are imprecise and have not always quantified remobilisation adequately. An alternative approach has been the use of 15N to quantify N uptake and partitioning, allowing precise measurements of N storage and remobilisation to be made. The use of isotopes has allowed experiments to be run which have shown that environmental factors such as soil fertility influence the amount of N stored, but have no direct influence upon the amount of N remobilised. These methods are discussed in light of recent research on N remobilisation, which has provided an understanding of the processes of storage and remobilisation which potentially allows direct measurements to be made in field grown trees for the first time.

Peter Millard

Papers

Unravelling rhizosphere–microbial interactions: opportunities and limitations

Nitrogen storage and remobilization by trees: ecophysiological relevance in a changing world

Assessing shifts in microbial community structure across a range of grasslands of differing management intensity using CLPP, PLFA and community DNA techniques

The accumulation and storage of nitrogen by herbaceous plants

Ecophysiology of the internal cycling of nitrogen for tree growth