Impact of endomycorrhizal fungi on plant trace element uptake and nutrition

by Neil I. Ward PhD, Karen Stead, John Reeves

 

Abstract

Several endomycorrhizal plant nutrient trials were undertaken from 1987-98 at Eastleigh, Forest of Dean, England. The mycorrhizal inoculum contained three species, namely, Glomus mosseae, Glomus calendonium, and E3 spores and mycelium in a peat carrier. The plants grown were standard commercially available broad beans, peas, runner benns, carrots, parsnip, onion, parsley, ryegrass, clover and potatoes. Treatments included no addition, added mycorrhizal inoculum with and without added fertiliser or rock phosphate and wood ash. The main findings were: (1) the sterilisation of soil has a significant effect on endomycorrhizal activity and trace element uptake: (2) endomycorrhizal effect on trace element uptake varies with plant species (foodstuff); (3) addition of fertiliser and rock phosphate to soil reduces mycorrhizal colonisation and plant trace element content (except phosphorous); and (4) mycorrhiza enhances the trace element plant uptake following the addition of wood ash.

 

Introduction

Mycorrhizae are a group of fungi living in a symbiotic, or mutually beneficial, relationship with most land plants. These fungi draw sugars from the plant and, in exchange, act as a root extension and thereby enhance the plant uptake of trace elements and other nutrients. The physiological processes stimulated by mycorrhizal symbiosis include increased plant growth, more rapid and uniform growth, increased stress resistance, enhanced disease resistance or tolerance to root pathogens, increased size, colour and number of flowers and enhanced nutrient composition.1-4 Early studies showed remarkable crop yield increases after the pre-inoculated addition to plants of vesicular-arbuscular mycorrhiza (VAM) to maize, barley and wheat grown on nutrient deficient soils.1 An explanation for this is the fact that mycorrhizae extend up to 7 cm from the root surfaces, thereby increasing the volume of soil from which nutrients can be extracted.5 This function is disproportionately important for nutrients that have narrow diffusion zones around roots, particularly phosphorus, zinc and copper.6 Several factors have been found to influence mycorrhizal fungal activity, including changes in soil texture and chemistry, temperature and pH, moisture and organic matter content and the application of lime, fertiliser and chemical pesticides.6-8 Most studies have reported the effects of minor elements (especially phosphorus, potassium, calcium, magnesium and sulphur) on mycorrhizal activity and farm crop yields. Only a few reported studies have evaluated the trace element changes in plants or related foodstuffs and mycorrhizal infection.6,9-13 To date, iron, zinc, caesium, copper, cobalt, selenium, manganese and boron have been studied. This paper reports the findings of mycorrhizal plant nutrient trials undertaken from 1987-98 at Eastleigh, Forest of Dean, England.

 

Methods

The Eastleigh research site consisted of a plot of unsterilised soil (sandy-loam, pH 7.2, organic matter content, typically 8.2%) which had remained uncultivated for at least five years. The mycorrhizal inoculum (Rothamsted. Herts., UK) used contained three species, namely Glomus mosseae, Glomus calendonium. and E3 spores and mycelium in a peat carrier. The plants grown were standard commercially available broad beans, peas, runner beans, carrots, parsnip, onion, parsley, ryegrass, clover and potatoes.

 

Treatments included no addition, added mycorrhizal inoculum with and without added fertiliser or rock phosphate and wood ash. Mycorrhiza was introduced at root level (6-10 cm) using a double-drill depth relative to the plant seed. NPK fertiliser (8.25.25), rock phosphate (Gafsa) and wood ash (produced by burning local native hardwood) were branded along the rows as a surface treatment at the equivalent of two cwt per acre. Seeds were planted in mid-March and harvesting was undertaken at traditional times, for example, beans (mid-July), carrots (end August), parsnips and potatoes (October-November). All plant materials were carefully cleaned to remove surface soil, weighed and stored at 4°C until delivery to the laboratory. All samples were dried at 110°C for three days, weighed, and dry-ashed at 450°C for twelve hours. The homogenised ash (~0.25g ash weight) was dissolved in 1 ml 12M Aristar™ nitric acid and diluted with 1% nitric acid before analysis by inductively coupled plasma mass spectrometry (ICP-MS) with 100 mg/1 indium internal standardisation. Quality control analysis was performed using certified reference materials.

 

Results and discussion

Table 1 summarises the zinc uptake by plant species as a function of mycorrhizal innoculum treatment. The results are presented as 'normalised values', with no addition (NM) representing the baseline plant nutrient content, shown as 100 units. Plants indicating a positive effect following treatment have values >100, and conversely, reduced trace element uptake is represented by values <100.

 

Plants grown with no added mycorrhizal innoculum but with added NPK fertiliser (F), or rock phosphate (RP) and wood ash (A) all show reduced plant uptake of trace elements, especially zinc and copper, with a significant variation between plant species (or foodstuffs). Only broad beans show a small zinc reduction following treatment. Rock phosphate has a major influence on zinc uptake by root vegetables (parsnip>carrot~potato>onion). Mycorrhizal innoculum addition (M) has a dramatic effect on increasing trace element uptake (especially zinc) in all plant species, with the order of the effect being: broad beans>peas>potato~-onion>carrot>parsnip~ryegrass and clover. Mycorrhiza enhances the trace element uptake in conjunction with wood ash (M+A). In contrast, the addition of fertiliser (M+F) and rock phosphate (M+RP) to soil reduces mycorrhizal colonisation and most plant trace element content. The only element to show raised levels following treatment is phosphorus, especially with the addition of both F and RP.

 

In addition to studying the impact of endomycorrhizal fungi on plant trace element uptake some data was obtained on germination rates and crop yields. As an example, after a period of six weeks post-treatment the germination rate for broad beans was: no addition (NM) 78%, fertiliser addition (NM+F) 82%, mycorrhizal innoculum addition (M) 94%, and mycorrhizal innoculum and fertiliser addition (M ( F) 78%. Similarly crop yield data for carrots are reported in Table 2 as fresh weight of carrots (kg) per equal row length.

 

Table 1: Zinc uptake by plants with and without endomyorrhizal treatment*

 

Treatment

**

Broad bean

Pea

Carrot

Potato

Parsnip

Onion

Ryegrass & clover

NM

100

100

100

100

100

100

100

NM + F

95

59

59

60

58

65

37

NM + RP

92

73

47

49

45

54

92

NM + A

91

70

65

69

50

57

77

M

134

125

118

121

114

120

113

M + F

32

42

51

60

53

51

44

V. + RP

74

73

58

68

102

100

98

M + A

112

93

68

77

144

84

81

* 'Normalised values': 100 units for baseline plant nutrient content (NM)

** See Table 2 for notation of treatments

 

 

Table 2: Crop yield* data for carrots with and without endomycorrhizal treatment.                 

Treatment

Symbol

Crop yield

No addition

NM

7.5

+ Fertiliser

NM + F

3.5

+ Rock phosphate

NM + RP

2.5

+ Wood ash

NM + A

4.5

Mycorrhiza

M

12

+ Fertiliser

M + F

13

+ Rock phosphate

M + RP

10

+ Wood ash

M + A

13

* Fresh weight of carrots (kg per equal row length)

 

 

Summary

These results show the importance of endomycorrhizal colonisation of agricultural soils. In particular, the benefits to plant nutrient uptake and thereby plant health and nutrient value are obvious. However, as shown by the various treatment profiles, the addition of other common agricultural media, namely fertiliser, rock phosphate and wood ash can result in both beneficial and harmful effects on the activity of mycorrhizae. More extensive studies need to be undertaken, especially in relation to the impact of environmental stress conditions (drought, extreme winter) and under the influence of chemical pollutant loadings.

 

 

 

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