The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

Root systems of most land plants form arbuscular mycorrhizal (AM) symbioses in the field, and these contribute to nutrient uptake. AM roots have two pathways for nutrient absorption, directly through the root epidermis and root hairs and via AM fungal hyphae into root cortical cells, where arbuscules or hyphal coils provide symbiotic interfaces. New physiological and molecular evidence shows that for phosphorus the mycorrhizal pathway (MP) is operational regardless of plant growth responses (positive or negative). Amounts delivered cannot be determined from plant nutrient contents because when responses are negative the contribution of the direct pathway (DP) is reduced. Nitrogen (N) is also delivered to roots via an MP, but the contribution to total N requirement and the costs to the plant are not clear. The functional interplay between activities of the DP and MP has important implications for consideration of AM symbioses in ecological, agronomic, and evolutionary contexts.

/pdf/roles-of-arbuscular-mycorrhizas-in-plant-nutrition-and-4crmwp9flj.pdf

Roles of Arbuscular Mycorrhizas in Plant Nutrition and Growth: New Paradigms from Cellular to Ecosystem Scales

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora.

Nutrient uptake in mycorrhizal symbiosis

CONTENTS I . Introduction . 11. Factors which influence microbial biomass . . . ( I ) Soil chemical factors . . . . . . . ( a ) Carbon and nitrogen limitation . . . . ( b ) Residue and nutrient amendment . . . (c) Influence of plants . . . . . . ( d ) Soil pH . . . . . . . . (e) Heavy metals and pesticides . . . . (2) Soil physical factors . . . . . . . ( a ) Soil texture and structure ( b ) Soil moisture and temperature (c) Seasonal fluctuations . . . . . . 111. Global distribution of microbial biomass . . . ( I ) Relationship with soil chemical factors and macroclimate

A comparative assessment of factors which influence microbial biomass carbon and nitrogen levels in soil

The beneficial effects of mycorrhizae on plant growth have often been related to the increase in the uptake of immobile nutrients, especially phosphorus (P). In this review the mechanisms for the increase in the uptake of P by mycorrhizae and the sources of soil P for mycorrhizal and non-mycorrhizal plants are examined. Various mechanisms have been suggested for the increase in the uptake of P by mycorrhizal plants. These include: exploration of larger soil volume; faster movement of P into mycorrhizal hyphae; and solubilization of soil phosphorus. Exploration of larger soil volume by mycorrhizal plants is achieved by decreasing the distance that P ions must diffuse to plant roots and by increasing the surface area for absorption. Faster movement of P into mycorrhizal hyphae is achieved by increasing the affinity for P ions and by decreasing the threshold concentration required for absorption of P. Solubilization of soil P is achieved by the release of organic acids and phosphatase enzymes. Mycorrhizal plants have been shown to increase the uptake of poorly soluble P sources, such as iron and aluminium phosphate and rock phosphates. However, studies in which the soil P has been labelled with radioactive 32P indicated that both mycorrhizal and non-mycorrhizal plants utilized the similarly labelled P sources in soil.

A critical review on the role of mycorrhizal fungi in the uptake of phosphorus by plants

Summary
Mycorrhizal and non-mycorrhizal sorghum [Sorghum bicolor (L.) Moench] plants were grown in soil treated with one of three rates of (15NH4)2SO4, fertilizer. Mycorrhizal plants did not significantly differ from non-mycorrhizal plants in total plant dry weight, total plant N, or percent utilization of the applied N by the whole plant at any fertilizer rate. However, the atom percent 15N excess in shoots, roots and the whole plant, and percent N derived from fertilizer were significantly higher for non-mycorrhizal plants at the intermediate level of fertilizer N. The percent N derived from native soil sources, and ‘A’ values were significantly higher for mycorrhizal plants, compared to non-mycorrhizal plants, at the intermediate but not low or high fertilizer rates. The results suggest that, at the intermediate fertilizer rate, where plants produced the most growth, mycorrhizal sorghum plants derived N from a source that was less available to non-mycorrhizal plants.

Nitrogen sources and ‘a’ values for vesicular–arbuscular and non-mycorrhizal sorghum grown at three rates of 15n-ammonium sulphate*†

(1) Perturbations were performed on organisms in a below-ground trophic food web in a semi-arid grassland, using five separate biocide treatments to observe changes in trophic structures, interactions, and nutrient cycling. Changes in N mineralization and trophic interactions as predicted on the basis of simple predator-prey microcosm studies were observed following removal of particular groups. (2) Five biocides: streptomycin (bactericide), captan and PCNB (fungicides), carbofuran (insecticide-nematicide), and cygon (acaricide) were applied in situ to soil in cylinders containing predominantly blue grama grass. The response of microbes, fungal grazers, soil inorganic N and plants were followed monthly between April and October 1982. (3) Grazing of bacteria or fungi by predators resulted in one or more of the following occurrences: (i) increased soil inorganic N, decreased predator populations following reduction of prey or increased plant growth after reduction of nematodes; (ii) reduction of one group of decomposers, e.g. bacteria, allowed a second decomposer group, e.g., fungi, to increase in numbers; (iii) compensatory responses of microbial feeders, e.g. decreases in bacterial feeders (protozoa and bacterivorous nematodes) were followed by compensatory increases in fungal feeders, which increased following the increase in their fungal food supply. (4) Continuing changes in nitrogen cycling were not observed, presumably because the function of the reduced group was compensated by increased numbers of the second group performing a similar function. Nematode-VAM interactions must be considered in food web and nutrient flows, as the percentage of VAM colonization of plant roots increased markedly when nematode populations were decreased by nematicide treatment. Production of predator biomass was as important as microbial and plant production in determining nutrient flow in this system.

Trophic interactions and nitrogen cycling in a semi-arid grassland soil. II: System responses to removal of different groups of soil microbes or fauna

Trophic interactions and nitrogen cycling in a semi-arid grassland soil. I: Seasonal dynamics of the natural populations, their interactions and effects on nitrogen cycling

Summary
A mixture of bacteria and a vesicular-arbuscular mycorrhizal fungus isolated from field-collected sods of blue grama (Bouteloua gracilis (H.B.K.) Griffiths) were tested for their interaction in the rhizosphere of pot-grown blue grama plants. Populations of the inoculated bacterial species and actinomycete populations, as influenced by the presence or absence of Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, were enumerated by dilution plate counts from rhizosphere and non-rhizosphere soil samples. Total bacterial counts and the population of one bacterial species in the non-rhizosphere soil of pots containing plants were significantly greater than in soil of pots without plants. The population of two bacterial species and actinomycetes were not significantly different in the non-rhizosphere soil of both mycorrhizal and non-mycorrhizal plant treatments when compared to the soil of pots without plants. In the rhizosphere of mycorrhizal plants, the total bacterial population and colony counts of one of the four bacterial isolates, when expressed as colony-forming units (CFU) per gram of root dry weight, were significantly reduced compared with controls. The numbers of CFU per gram of rhizosphere soil of one bacterial species were significantly increased by the presence of the mycorrhizal fungus. Although no significant negative correlation was observed between populations of bacterial species in the rhizosphere soils, significant positive correlations between specific bacterial populations depended on whether or not the roots were mycorrhizal.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1984.tb03590.x

Rhizosphere bacterial population responses to root colonization by a vesicular-arbuscular mycorrh1zal fungus *†

SUMMARY
Celery (Apium graveolens L.) plants were grown in pots in which the root system was separated from the soil in a side chamber by a fine mesh screen. The side chamber was treated with either an organic (ground plant tissue) or inorganic [(NH4)2SO4] source of 15N. Mycorrhizal (Glomus mosseae) and control (non-mycorrhizal) plants were exposed to 15N over a period of 30 days (inorganic-15N) or 88 days (organic-16N). Mycorrhizal and control plants did not differ in shoot dry weight or shoot P content. Dry weight of root was reduced in the mycorrhiza treatments. Mycorrhizal plants derived significantly (P= 0.01) more 16N, from both N sources, than did control plants. In the inorganic-N treatment, 15N in mycorrhizal plants was significantly (P= 0.001) and positively correlated with percent mycorrhizal fungal colonization (r= 0.58), number of hyphal crossings (±10 μ diameter) through the mesh into the area of 15N placement (r= 0.76), total length of hyphae per gram of soil (r= 0.74), and length of hyphae of 5 μ diameter in the soil (r= 0.77). No correlations were found between the 16N content of mycorrhizal plants and any parameter in the organic-N treatment. The 16N content of control plants was not correlated with hyphal length in the outer chamber and there were no hyphal crossings of the size ( 10 μ diameter) which was counted for the mycorrhiza treatments. The presence of the organic matter (ground plant tissue) increased the total length of saprophytic hyphae per gram of soil but decreased the number of vesicular-arbuscular mycorrhizal fungal hyphae crossing into the area of 16N placement. The mean flux of N through the hyphae of G. mosseae was 7.42 × 10−8 mol N cm−2 s−1 for the inorganic-N treatment over a 30-day period, and 1.74 × 10−8 mol N cm−2 s−1 for the organic-N treatment over an 88-day period.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1983.tb03506.x

Hyphal uptake and transport of nitrogen from two 15n‐labelled sources by glomus mosseae, a vesicular‐arbuscular mycorrhizal fungus *

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