Soil parameters and plant responses associated with arbuscular mycorrhizas from contrasting grassland management regimes

Agriculture, Ecosystems and Environment 73 (1999) 245–255

Soil parameters and plant responses associated with arbuscular mycorrhizas from contrasting grassland management regimes W.R. Eason a,∗ , J. Scullion b , E.P. Scott b a

Institute for Grassland and Environmental Research, Plas Goggerdan, Aberystwyth SY23 3EB, UK b Institute of Biological Sciences, University of Wales, Aberystwyth SY23 3DE, UK Received 02 July 1998; received in revised form 21 January 1999; accepted 26 February 1999

Abstract Increased interest in sustainable low-input forms of agriculture has focused attention on the role of beneficial soil microsymbionts, such as arbuscular mycorrhizal fungi (AMF), in plant productivity and health. Root colonisation by AMF and AMF spore density was significantly lower in grassland soils with a history of high-input, conventional management than in soils with a low-input or organic management. Spores of AMF isolated from contrasting management regimes, taken from a total of 24 sites at 13 farms, were used in a series of inoculation trials involving three host plants (Allium ameloprasum L., Trifolium repens L. and Lolium perenne L.). The mean yield response (measured as shoot dry weight) was significantly greater when host plants were inoculated with spores taken from farms with organic rather than high-input management for both Allium and Trifolium hosts. Very low levels of root infection in Lolium hosts excluded these plants from further analysis. Not all organic farms had highly effective AMF associations however, and some highly effective AMF isolates were found in high-input systems. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Arbuscular mycorrhizal fungi; Grassland; Organic management; United Kingdom

1. Introduction The role of soil micro-organisms in sustainable agricultural systems will become increasingly important where synthetic inputs such as fertilisers and chemical pest control agents are reduced or omitted. In UK there is particular interest in organic systems as an alternative farming strategy, with strictly regulated standards with regard to the level of synthetic inputs allowed (UKROFS, 1991). In conventional high-input systems there are continual disturbances to the soil system (e.g. the addition of chemicals) which may affect intrinsic ∗ Corresponding author. Tel.: +44-01970-823000; fax: +4401970-828357; e-mail: [email protected]

abiotic and biotic soil factors possibly leading to longterm soil degradation (Bethlenfalvay and Linderman, 1992). Arbuscular mycorrhizal fungi (AMF) may play a critical role in low-input or organic systems because of their role in linking plant and soil processes. They are non-pathogenic, symbiotic fungi infecting plant roots of most terrestrial plants. The fungus obtains carbohydrates from the host plant and the plant obtains nutrients that the fungal hyphae take up from the soil. The likely main benefit to the host plant is the uptake of immobile soil nutrients, in particular phosphorus, because the hyphae avoid nutrient depleted zones that build up around plant roots (Sanders et al., 1975). Arbuscular mycorrhizal associations may

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also increase nitrogen accumulation in plant tissues probably as a result of hyphae accessing otherwise unavailable forms of soil N (Ibijbijen et al., 1996a). Biological nitrogen fixation in legume hosts may also be enhanced by AMF probably indirectly through relief of P stress (Ibijbijen et al., 1996b). In addition to growth benefits from improved nutrition there is evidence that AMF infected plants exhibit improved reproductive capacity (Xiaohong and Koide, 1994); improved plant adaptation to water stress (Subramanian et al., 1995; Ruiz-Lozano et al., 1996) and improved plant health through antagonistic and/or competitive effects on pests and pathogens (Gange and West, 1994). It has been demonstrated that AMF activity is adversely affected by conventional compared with organic management systems (Sattelmacher et al., 1991; Douds et al., 1995). Populations of AMF may be directly or indirectly affected by the previous management practices of high-input systems including the use of fertilisers, synthetic pest and disease control measures and soil tillage (reviewed by Johnson and Pfleger, 1992). A major difference between high-input and organically managed systems will be the level (and type) of fertiliser used. It is not possible to generalise about the effects of fertilisers on AMF although they are likely to be more deleterious when applied to less fertile soils (Johnson and Pfleger, 1992). The use of farmyard manure may favour AMF compared with inorganic fertilisers, by increasing soil organic matter levels, so creating the physico-chemical conditions which encourage AMF (Harinikumar and Bagyaraj, 1989). The relative balance of major nutrients has also been shown to affect AMF activity (Gryndler et al., 1990). High levels of available soil nutrients may also affect the species of AMF present as a result of differences in their sensitivity to high nutrient, notably P, availability (Davies et al., 1984). The objective of this study was to examine the functional differences between AMF populations (expressed as their effect on host plant growth response) isolated from grassland soils under long term organic or conventional management and tested under experimental conditions equivalent to soil management under an organic farming system. This would provide an indication of agronomic effects on AMF in such systems and of potential prob-

lems in converting from high-input to low-input management.

2. Materials and methods Several approaches were taken to compare the effectivity of AMF isolated from grassland soils under organic or conventional management. AMF spores were isolated from 24 field sites and were used to inoculate plants under controlled conditions, following which both plant growth and AMF development were assessed. 2.1. Field site details Six grassland farms under organic management and seven farms under high-input conventional management (>250 kg N/ha/year; 50–100 kg P2 O5 /ha/year) were selected to examine the effectiveness of the indigenous AMF spore populations present under the two contrasting soil management regimes (Table 1). Organically-managed farms varied with respect to time of conversion and type of fertiliser regime (Table 1). All organic farms selected had a history of low inputs before conversion, and no farm selected had any synthetic fertiliser input for at least 5 years before conversion. Soil pH, available P and K were measured and an approximate estimate of the percentage of Trifolium repens L. (white clover) ground cover was also made (Table 1). Spores of AMF were, in most cases, taken from two fields at each farm. Spores were taken from 24 fields to give 13 inoculum treatments from conventional systems (1 farm out of seven had only 1 field selected) and 11 treatments from organic systems (1 farm out of six only had 1 field selected). All fields selected had pasture established for at least 6 years and were grown for conservation with grazing afterwards. All fields shared a common soil series, Denbigh (USDA subgroup Typic Dystrochreptbrown earth clay loam), a major soil type in Wales and Western England. Soil spore density and plant root AMF colonisation (see below) were determined on material taken from each site. Table 1 gives further site information, including fertiliser regime, and for organically managed farms, time of conversion.

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Table 1 Soil pH, available P and K, Trifolium index, % AMF infection of the roots and the AMF soil spore density for selected grassland farms under (a) conventional high-input or (b) organic managementa Farm code

Soil pH Soil Pb Soil Kb

(a) Conventional management 1a 5.8 5.3 1b 5.6 10.8 2a 5.5 5.0 2b 6.0 7.7 3a 5.1 4.2 3b 5.0 4.6 4a 5.6 6.2 4b 6.1 4.4 5a 5.3 3.3 5b 5.5 5.3 6a 5.9 3.4 6b 5.5 5.6 7b 5.5 4.5 Means 5.6 5.4 (b) Organic management 8a 5.5 1.1 8b 5.7 1.1 9a 5.6 2.1 9b 6.0 1.1 10a 5.7 2.3 10b 5.5 2.5 11a 5.5 1.2 11b 5.0 1.3 12a 5.9 1.2 13a 6.4 4.6 13b 5.9 3.6 Means 5.7 2.0 ∗∗∗ Significanced ns

Trifolium indexc % AMF infection Spore density Fertiliser (N : P : K)/regime Farm converted

118 250 295 360 212 220 300 446 167 222 253 270 525 279.8

0 1 0 0 1 0 2 2 2 2 3 3 2 1.4

42.0 66.0 11.5 39.2 41.6 36.8 24.8 43.1 49.9 35.9 33.4 42.5 58.0 40.4

16.7 14.2 13.2 20.4 8.4 7.0 7.7 12.0 15.5 7.0 14.0 9.8 11.8 12.1

300 :80 : 100 300 : 80 : 100 350 : 60 : 50 350 : 60 : 50 275 : 50 : 100 + slurry 275 : 50 : 100 + slurry 375 : 75 : 75 375 : 75 : 75 350 : 60 : 50 350 : 60 : 50 300 : 125 : 125 250 : 30 : 00 250 : 30 : 00

82 119 88 135 109 120 69 117 115 400 230 144.0

3 3 2 2 3 3 0 0 3 3 3 2.3 ns

73.4 80.1 69.9 67.7 62.6 51.4 50.7 62.3 63.4 56.7 61.8 63.6

33.2 31.0 37.5 48.7 37.5 45.5 34.0 15.2 44.2 15.0 36.2 34.4

FYM/Slurry FYM/Slurry No inputs No inputs FYM FYM No inputs No inputs FYM/slurry FYM FYM

∗∗

∗∗∗

1972 1972 1985 1985 1990 1990 1984 1984 Always organic 1987 1987

∗∗∗

a Fertiliser

regime indicated (FYM: farm yard manure; no inputs excludes those from grazing animals). P and K (mg/kg). c Trifolium index (visual rating of ground cover of Trifolium repens where 0 = 0%; 1 = <25%; 2 = <50% and 3 = >50%); spore density (per g d.w. soil). d Significant differences between organic and conventional means indicated (ns: not significant; ∗∗ = p > 0.01; ∗∗∗ = p > 0.001). b Soil

2.2. AMF assessment of the field sites 2.2.1. Spore numbers Air-dried soil was mixed and sieved through a 4 mm sieve. Spores were extracted from 50 g sub-samples of soil as described above. After centrifuging, the remaining supernatant was washed into a cylinder and diluted to 25 ml. A 2 ml sub-sample was filtered through Whatman membrane filter paper (0.45 microns pore size) on which a grid had been drawn and spores counted under a dissecting microscope. Results were expressed as spores per g dry soil.

2.2.2. Root assessment 100 mg fresh weight, sub-samples of the root system were cleared and stained (Phillips and Hayman, 1970; Brundrett et al., 1996). Percentage of root length infected with AMF and total length of infected root was determined using a grid line intersect method (Giovannetti and Mosse, 1980). No attempt was made to distinguish plant species, the assessment being made on well-mixed samples.

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2.3. AMF inoculum

2.5. Host plants

Spores collected in 1993 from soils at the 24 assessed field sites were used as inoculum. At least 2.5 kg of soil was collected from each site from 18 random positions using a soil corer (diameter 6 cm; length 10 cm). Care was taken to avoid collecting from areas which might be associated with dung and urine (e.g. near gateways, camping positions) and areas of poor drainage. All samples were taken over a short period (approximately 2 weeks) during autumn. The mixed soil was wet sieved 5–6 times (smallest sieve size 53 microns) and the sucrose centrifugation method used to remove spores from the filtrate (Brundrett et al., 1996). In order to ensure an adequate representation of all AMF species present approximately 1000 spores were used to inoculate each pot for all mycorrhizal treatments. This approach discounted differences in field spore densities. Spores were mixed throughout a layer of soil (100–200 g) about halfway down each pot. Spore filtrate from each treatment was mixed and added to all pots to ensure that control and mycorrhizal treatments contained similar background populations of other soil micro-organisms.

(i) Trifolium repens L. cv. ’Menna’ (white clover) was chosen as a host plant because it is susceptible to mycorrhizal infection and its nitrogen requirements are satisfied by fixation, thus avoiding the need for nitrogen inputs, an experimental constraint given the objective of simulating organic conditions. Menna was selected because the cultivar is used for general grazing and is found in organic systems. To minimise effects resulting from variations in germination and establishment an excess number of seeds were planted in pots which were then thinned to three plants after 2 weeks, before an inoculation treatment growth effect would be expected. All plants were inoculated with Rhizobium trifolii (SP20 – obtained from IGER Aberystwyth). The rhizobium was grown on Petri dishes for 7 days and then washed off with distilled water. A diluted suspension (5 ml) was added to each of the pots at sowing and after 2–3 weeks growth, in order to ensure good nodulation. (ii) Lolium perenne L. cv. ‘Parcour’ (perennial ryegrass) is common to conventional and organic systems and was selected to address any experimental bias that may result from using Trifolium repens as a host plant. Trifolium species are more common in organic than high input conventional systems and may support a different mycorrhizal population which could be favoured by the use of Trifolium as a host plant. Again several seeds were sown and thinned to three plants per pot after 2 weeks. (iii) Allium ameloprasum L. (leek) was chosen as it is a coarse rooted species. These are generally more dependent on mycorrhizae for P supply (Baylis, 1975). It is known to give a good yield response to inoculation and is therefore, a potentially more sensitive indicator of any inoculum treatment. Seedlings were transplanted 1 week after emergence at a rate of one plant per pot.

2.4. Growing medium A standardised soil was used to evaluate plant response to AMF infection in all treatments. The soil used as a growing medium was collected from a local organic farm (OS Ref: SN 625889) at a single location to a depth of 50 cm and sieved to remove large stones but not to break down natural aggregates. The soil was of the same series as that for the sites under investigation. After air drying, the soil was irradiated (Isotron, Swindon – 20 kGy) in order to remove all AMF inoculum from the soil which was then subsequently re-introduced in treatment pots. Following irradiation soil available P was measured at 1.33 mg/kg, soil K at 60 mg/kg and pH at 6.4 (see below for extraction procedures). Pots (1 l) of the growing medium were placed on individual columns of sand (to promote free drainage) and wetted initially from below to establish capillary continuity. A layer of Typar (Du Pont), a water permeable geotextile, was placed between the pot and sand to prevent roots growing into the sand.

2.6. Experimental design AMF spores from the 24 field sites, were used on the three host plants as described. Two control treatments were also included; no AMF inoculum was added (testing response to AMF) and no inoculum was added but rock phosphate was supplied (at 100 mg P2 O5 /kg

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soil) to the growing media (testing plant responsiveness to P). Each treatment was replicated three times. All experiments were carried out under standard environmental conditions. Temperature was maintained in the range 20–25◦ C. During winter months photoperiod was maintained at 16 h by use of artificial lighting (Phillips SON-T AGRO 400W). Plants were watered by automatic overhead irrigation to maintain a moisture condition which did not limit plant growth. An outbreak of thrips was controlled by a predatory mite, Amblyseius cucumeris (Novaris BCM Ltd, Colchester, UK). The time to the final harvest and assessment of plant response to AMF treatment varied with host species (Allium 84 days; Trifolium 240 days and Lolium 108 days). Soil phosphorus was extracted with alkaline sodium bicarbonate and soil potassium with M ammonium nitrate, and determined using standard procedures (MAFF, 1986). Soil pH was determined in a 10 g soil/25 ml water mix (MAFF, 1986). Analysis of variance and correlation analysis were performed using Excel 4.0 (Microsoft Corporation, 1993). 3. Results 3.1. Site descriptions Conventionally managed farms differed from organically managed ones in having generally higher levels of available nutrients and a lower proportion of the pasture as Trifolium (Table 1). Individual farms varied with respect to the type and amount of inputs used. In organically managed farms this included the use of slurry, farmyard manure or ecreta alone (Table 1). The % AMF infection of the roots was approximately one-third greater and AMF spore density in the soil was approximately three times higher under organic than conventional, high-input management systems. Despite clear overall treatment differences there was considerable variation among individual farms. 3.2. AMF effectivity Mean yield response to inoculation by AMF spore populations isolated from fields under organic management was significantly greater than the yield

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response to inoculation with spores isolated from conventional, high-input systems for both Allium and Trifolium (Tables 2 and 3). Allium was highly responsive to all the AMF inoculum applied, increasing the dry shoot weight by up to a factor of 40 over that of uninoculated controls (Table 2). Mean values for shoot growth were 40% greater for the organic AMF inoculum compared with AMF from conventional management systems. Spores from eight out of 11 organic fields resulted in a host growth response equal to or greater than the median response of all plants. Root weight also tended to be greater in the organic compared with the conventional AMF treatment but this effect was not significant. All inoculated plants had good levels of % AMF root infection, with inoculum from organic systems tending to give slightly higher infection than inoculum from conventional systems although the overall means were not significantly different. The increase in size of the root system in organic treatments however, resulted in a significantly greater total AMF-infected root length compared with conventional treatments. Inoculation from any field site led to marked increases in total shoot P compared with the control or rock phosphate treatments (Table 2). The lack of a significant difference in total shoot P between organic and conventional treatments may have reflected the need to bulk samples (because of plant size) for P analyses, resulting in reduced replication levels. P concentration (data not shown) revealed no significant treatment differences. Plant performance (shoot and root weight in Table 4) and P uptake (total shoot P in Table 4) were related to mycorrhizal performance indicators (% AMF infection and infected root length in Table 4), although these associations appeared to be stronger for the organic treatment (Table 4). Root weight and total shoot P were positively correlated with % AMF infection as well as with infected root length in the organic treatment. In both organic and conventional treatments infected root length was a good indicator of both shoot and root weight as well as total shoot P. In a correlation analyses combining all data (organic and conventional means) there was an improvement in all these relationships. In addition shoot weight was also significantly correlated with % AMF infection. Field soil and AMF variables (field soil P, field soil pH, field spore density and field % AMF infection

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Table 2 Response of Allium infected with AMF from grassland soil under conventional or organic managementa Management Control Organicb Conventional Organic Rock phosphate Conventional Conventional Conventional Conventional Conventional Conventional Organic Conventional Conventional Conventional Organic Conventional Organic Organic Organic Organic Organic Conventional Organic Conventional Organic Organic means Conventional means Significancec

Farm code 11a 1b 13a 2b 5b 3a 4a 6a 3b 9a 7a 5a 4b 10b 2a 13b 8b 9b 11b 12a 1a 10a 7b 8a

Shoot weight (mg)

Root weight (mg)

Total shoot P

% AM infection

Infected root length (cm)

0.03 0.16 0.16 0.16 0.17 0.21 0.23 0.25 0.25 0.26 0.28 0.30 0.40 0.40 0.40 0.40 0.41 0.50 0.53 0.58 0.60 0.69 0.86 0.89 0.93 1.22 0.55 0.39

0.01 0.06 0.06 0.06 0.08 0.10 0.09 0.11 0.11 0.11 0.14 0.13 0.15 0.13 0.14 0.16 0.19 0.19 0.25 0.18 0.26 0.25 0.39 0.25 0.32 0.43 0.20 0.16 ns

0.04 0.46 0.46 0.37 0.13 0.05 0.69 0.74 0.85 0.66 0.87 0.74 1.04 0.88 1.06 0.98 1.27 1.60 1.72 1.80 1.32 2.07 1.72 1.96 2.26 3.30 1.48 0.97 ns

0 50.6 52.6 53.3 0 59.0 53.63 35.0 35.3 66.6 34.0 53.3 73.6 45.6 77.3 63.6 71.3 74.6 71.0 72.3 57.6 75.6 64.6 56.6 71.0 76.0 64.0 56.9 ns

0 391 361.3 181.3 0 809.3 52.2 201.6 398.6 528.0 253.3 505.0 691.0 464.3 505.3 579.3 786.0 1464.0 777.0 1047.0 1158.6 1472.3 1415.6 879.0 1945.3 2653.6 1009.83 670.14

∗

∗

a Response

measured as final shoot and root dry weight, % AMF infection of the roots, total infected root length, and total shoot P (mg). Response to AMF infection compared to non-mycorrhizal control treatment. Response to phosphorus in rock phosphate treatment. b Organic treatments are represented in italics. c Significant differences between organic and conventional treatment means indicated (∗ = p > 0.05; ns: not significant).

in Table 4) also, were correlated with plant performance. Field % AMF infection was a good indicator of harvest root weight and total shoot P in the organic treatment (Table 4). Levels of field soil P were negatively correlated with harvest shoot weight and total shoot P (significant only when data combined) (Table 4) in both organic and conventional treatments. Mycorrhizal infection was significantly correlated (for combined data) with field spore density (Table 4), and for the conventional management treatment only, also with field soil pH (Table 4). The Trifolium results reflected the main trends observed for Allium (Table 3). Total shoot weight (from combining cuts 1–3 taken at 84, 168 and 240 days) was significantly greater in the organic than in the conventional treatment but the overall increase was much

less (approximately 10% compared with 40% for Allium). AMF spores from seven out of 11 organic fields resulted in a host response equal or greater than the median response of all plants (compared with eight out 11 for Allium). Total shoot P was higher in the organic than the conventional treatment for all cuts but this was only significant for the second cut. Although % AMF infection was greater in organic treatments the total infected root length was not different from conventional treatments. Direct comparisons with Allium are difficult, however, because it is likely that the response of plants to AMF infection will be affected by differences in the timing of AMF colonisation and the host plant response to it. After 84 days, when Allium was harvested and was exhibiting a 40% yield response

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Table 3 Response of Trifolium infected with AMF from grassland soil under conventional or organic managementa Management

Control Conventional Conventional Conventional Rock phosphate Organicb Organic Conventional Organic Conventional Conventional Organic Conventional Conventional Conventional Conventional Conventional Organic Organic Organic Conventional Organic Organic Conventional Organic Organic Organic means Conventional means Significancec

Farm code

7a 6a 2a 8b 13b 4a 10a 4b 3a 9a 5a 5b 1b 7b 3b 12a 13a 11a 1a 11b 8a 2b 9b 10b

Shoot Weight Cut 1

Shoot Weight Cut 2

Shoot Weight Cut 3

Total shoot (mg)

Root weight (mg)

Total P-Cut1 (mg)

Total P-Cut2

Total P-Cut3

% AMF infection

Infected root length (cm)0

0.40 1.32 0.77 1.17 2.20 0.78 0.87 0.66 1.22 1.15 1.04 1.88 1.52 1.11 0.97 1.17 1.28 1.17 1.34 1.02 1.12 1.13 1.68 1.06 1.76 1.27 1.28 1.10 ns

2.08 2.74 2.85 2.59 1.97 3.11 2.93 3.14 3.53 3.52 3.27 3.39 3.77 3.47 3.64 3.33 3.60 3.48 3.68 3.69 3.66 3.82 4.22 3.24 3.87 3.56 3.57 3.29 ns

3.83 4.10 4.73 4.70 4.36 4.69 4.83 4.86 4.17 4.29 5.02 4.22 4.22 5.06 5.19 5.34 5.00 5.44 5.10 5.89 6.02 6.23 5.24 7.08 6.56 7.51 5.44 5.05 ns

6.31 8.16 8.35 8.46 8.53 8.58 8.63 8.66 8.92 8.96 9.33 9.49 9.51 9.64 9.80 9.84 9.88 10.09 10.12 10.60 10.80 11.18 11.14 11.38 12.19 12.34 10.30 9.44

0.82 0.86 0.69 1.04 0.92 0.72 0.64 0.76 0.64 0.67 0.91 0.74 0.81 1.18 0.87 0.94 1.12 0.96 0.94 0.89 1.03 0.99 0.87 0.88 1.02 0.95 0.85 0.90 ns

0.38 3.04 1.66 2.40 4.95 1.76 1.96 1.09 2.14 2.07 2.70 3.48 2.81 2.00 1.70 1.99 2.50 3.22 2.61 1.42 2.13 1.64 2.69 1.59 3.61 2.16 2.43 2.13 ns

1.66 5.07 4.13 4.53 4.43 4.20 4.40 5.65 5.12 6.16 5.40 4.41 6.03 4.86 5.46 5.66 5.22 5.92 5.15 4.06 6.04 4.97 5.06 5.35 5.03 5.16 4.86 5.35

2.87 7.59 6.86 7.29 9.59 7.04 8.21 8.01 8.97 8.58 8.03 5.70 4.85 5.57 6.75 8.01 7.50 7.62 7.14 9.13 9.03 8.72 9.17 10.97 10.17 7.89 8.16 7.62 ns

0 57.4 37.1 35.2 0 44.1 68.0 0 63.4 65.8 65.89 61.9 67.8 67.8 55.0 62.3 64.7 58.3 68.0 73.6 62.6 65.2 69.2 67.6 68.0 56.8 63.3 54.5 ns

0 699.0 691.1 1740.1 0 1364.0 744.4 1202.5 693.7 781.6 79.9 858.3 371.3 1066.7 1034.9 988.0 1302.3 718.9 786.4 1065.5 1080.6 801.5 1105.7 786.3 1452.9 995.3 962.4 978.8 ns

∗

∗

a Response

measured as shoot weight (cuts 1–3 and total weight), and root weight, total shoot P (for cuts 1–3), % AMF infection of the roots and total infected root length. Response to AMF infection compared to non-mycorrhizal control treatment. Response to phosphorus in rock phosphate treatment. b Organic treatments are represented in italics. c Significant differences between organic and conventional treatment means indicated (∗ = p > 0.05; ns: not significant).

to AMF inoculation, the mean shoot response of Trifolium to AMF infection was not different between organic and conventional treatment means (Cut 1 in Table 3). The most noticeable difference between the Trifolium and Allium findings at this time was the response of control non-mycorrhizal plants to added rock phosphate. This resulted in a much greater response to P than to any of the added AMF inoculum in Trifolium, whereas the response to P of Allium was relatively poor. This indicated that the AMF colonisation of Trifolium may have occurred later than in Allium. This is supported by the relatively reduced response to P of Trifolium

as the experiment progressed (Cut 2 and Cut 3 in Table 3). Although for combined data (organic and conventional treatments) % AMF infection was correlated with root weight, the strong relationship between measured mycorrhizal variables (% AMF infection and infected root length in Table 5) and plant response (shoot weight cuts 1-3, total shoot weight, root weight and total shoot P in Table 5), seen for Allium, were not repeated for Trifolium (Table 5). Similarly measured field variables were poor or inconsistent indicators of plant response (Table 5). Although field % AMF infection was a good indica-

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Table 4 Correlation coefficients between Allium (shoot and root weight, total shoot P) and mycorrhizal (% AMF infection of roots; infected root length) means at final harvest, for Allium previously inoculated with AMF from (a) organic and (b) conventionally managed grassland farms Additional correlation coefficients for (c) combined data are also shown. Correlation data between these final plant harvest means and source field site variables (% AMF infection; soil spore density; soil P; soil pH) are also given

(a) Organic means Field soil Pa Field % AMF infectiona Field spore densitya Field soil pHa % AMF infection Infected root length (b) Conventional means Field soil Pa Field % AMF infectiona Field spore desnitya Field soil pHa % AMF infection Infected root length (c) Combined means Field soil Pa Field % AMF infectiona Field spore densitya Field soil pHa % AMF infection Infected root length

Shoot weight

Root weight

Total shoot P

% AMF infection

Infected root length

−0.426 (0.191)b 0.512 (0.107) 0.185 (0.586) −0.102 (0.766) 0.589 (0.057) 0.868 (0.0005)∗∗∗

−0.486 (0.126) 0.619 (0.042)c∗ 0.074 (0.828) −0.202 (0.552) 0.644 (0.032)∗ 0.889 (0.0002)∗∗∗

−0.465 (0.150) 0.605 (0.049)∗ 0.282 (0.400) −0.001 (0.998) 0.768 (0.006)∗∗ 0.916 (0.0001)∗∗∗

−0.282 (0.400) 0.509 (0.109) 0.442 (0.173) 0.267 (0.426)

−0.380 (0.249) 0.742 (0.009)∗∗ 0.151 (0.657) −0.113 (0.740) 0.742 (0.009)∗∗

−0.392 (0.208) 0.188 (0.558) 0.223 (0.485) 0.142 (0.660) 0.499 (0.099) 0.910 (0)∗∗∗

−0.343 (0.275) 0.038 (0.907) 0.247 (0.439) 0.137 (0.671) 0.433 (0.159) 0.861 (0.0003)∗∗∗

−0.486 (0.109) 0.067 (0.837) −0.091 (0.779) −0.046 (0.888) 0.436 (0.156) 0.782 (0.003)∗∗

0.0005 (0.999) 0.052 (0.873) 0.384 (0.217) 0.635 (0.027)∗

−0.444 (0.036)∗ 0.398 (0.059) 0.323 (0.133) 0.046 (0.835) 0.546 (0.007)∗∗ 0.893 (0)∗∗∗

−0.403 (0.057) 0.320 (0.137) 0.234 (0.283) −0.01 (0.964) 0.526 (0.01)∗ 0.878 (0)∗∗∗

−0.525 (0.01)∗ 0.430 (0.04)∗ 0.382 (0.072) 0.042 (0.848) 0.585 (0.003)∗∗ 0.877 (0)∗∗∗

−0.275 (0.205) 0.326 (0.129) 0.426 (0.043)∗ 0.482 (0.02)∗

0.742 (0.009)∗∗ −0.111 (0.732) 0.204 (0.524) 0.399 (0.198) 0.308 (0.329) 0.587 (0.045)∗

0.587 (0.045)∗ −0.343 (0.109) 0.391 (0.065) 0.336 (0.118) 0.101 (0.648) 0.643 (0.0009)∗∗∗

0.643 (0.0009)∗∗∗

a Site

factors. values in parentheses. c∗∗∗ p > 0.001; ∗∗ p > 0.01; ∗ p > 0.05. Where probability value is below 0.0001 value is given as 0. b Probability

tor of final harvest % AMF infection in organic treatments, this was not the case in conventional treatments. The Lolium plants exhibited highly variable levels of AMF colonisation with mean levels below 20%.

There was also visual evidence that the plants were suffering from severe nutrient deficiency from week 8 onwards. This experiment was subsequently terminated and no data are shown.

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253

Table 5 Correlation coefficients between Trifolium (weight of shoot cuts 1–3; total shoot weight; final harvest root weight; total shoot P) and mycorrhizal (% AMF infection of roots; infected root length) means at final harvest, for Trifolium previously inoculated with AMF from (a) organic and (b) conventionally managed grassland farms. Additionally correlation coefficients for (c) combined data are also shown. Correlation data between these final harvest plant means and source field site variables (% AMF infection; soil spore density; soil P; soil pH) are also given

(a) Organic means Field % AMF infectiona Field spore desnitya Field soil Pa Field soil pHa % AM infection Infected root length (b) Conventional means Field % AMF infectiona Field spore densitya Field soil Pa Field soil pHa % AMF infection Infected root length (c) Combined means Field % AMF infectiona Field sporesa Field soil Pa Field soil pHa % AMF infection Infected root length

Shoot weight Cut 1

Shoot weight Cut 2

Shoot weight Cut 3

Total shoot weight

Root weight

Total shoot P

% AMF infection

Infected root length

0.154 (0.651)b 0.235 (0.470) −0.109 (0.749) 0.145 (0.669) 0.701 (0.838) −0.161 (0.637)

−0.074 (0.828) −0.120 (0.725) −0.343 (0.303) −0.233 (0.489) −0.399 (0.223) −0.217 (0.522)

−0.481 (0.139) 0.214 (0.527) −0.197 (0.563) −0.268 (0.426) −0.457 (0.158) 0.323 (0.333)

−0.349 (0.292) 0.199 (0.558) −0.272 (0.418) −0.229 (0.498) −0.183 (0.590) 0.350 (0.291)

−0.313 (0.349) −0.061 (0.858) −0.237 (0.483) −0.079 (0.818) −0.593 (0.055) 0.241 (0.478)

−0.031 (0.929) 0.403 (0.219) −0.285 (0.396) 0.188 (0.579) −0.175 (0.608) 0.177 (0.602)

0.784 (0.004)∗∗c 0.376 (0.255) −0.258 (0.444) 0.116 (0.734)

0.427 (0.190) 0.292 (0.382) −0.513 (0.107) −0.041 (0.905) −0.026 (0.939)

0.245 (0.419) 0.128 (0.677) −0.298 (0.322) −0.375 (0.207) 0.022 (0.942) −0.240 (0.429)

0.620 (0.024) 0.105 (0.734) 0.111 (0.719) −0.128 (0.689) 0.376 (0.206) −0.287 (0.341)

0.056) (0.855) 0.422 (0.151) 0.399 (0.177) 0.109 (0.724) −0.313 (0.298) 0.216 (0.478)

0.327 (0.276) 0.412 (0.162) 0.303 (0.315) −0.038 (0.903) −0.114 (0.710) 0.019 (0.949)

−0.148 (0.629) −0.224 (0.463) 0.064 (0.836) −0.501 (0.081) −0.102 (0.741) 0.563 (0.042)

0.066) (0.829) 0.284 (0.348) 0.171 (0.576) 0.262 (0.388) −0.049 (0.871) −0.008 (0.979)

0.343 (0.101) 0.369 (0.076) −0.351 (0.093) −0.006 (0.978) −0.115 (0.967) −0.041 (0.851)

0.509 (0.011)* 0.280 (0.185) −0.301 (0.153) −0.112 (0.602) −0.103 (0.632) 0.992 (0.645)

0.023 (0.915) 0.287 (0.174) −0.031 (0.885) −0.074 (0.730) −0.418 (0.042)∗ 0.248 (0.243)

0.264 (0.213) 0.398 (0.054) −0.206 (0.335) −0.094 (0.664) −0.353 (0.09) 0.147 (0.493)

−0.268 (0.206) −0.207 (0.333) 0.113 (0.598) −0.311 (0.139) −0.299 (0.156) 0.444 (0.03)*

0.084 (0.698) 0.255 (0.228) −0.054 (0.801) 0.232 (0.275) −0.129 (0.549) 0.060 (0.780)

−0.026 (0.939) 0.018 (0.953) −0.179 (0.558) −0.099 (0.748) 0.084 (0.786)

−0.543 (0.055) −0.288 (0.341) 0.190 (0.534) −0.238 (0.435) −0.650 (0.016)*

−0.650 (0.016)* 0.079 (0.714) −0.034 (0.874) 0.048 (0.826) 0.073 (0.735)

−0.198 (0.354) 0.015 (0.944) 0.009 (0.966) −0.149 (0.487) 0.377 (0.069)

0.377 (0.069)

a Site

factors. values in parentheses. c∗∗∗ p > 0.001; ∗∗ p > 0.01; ∗ p > 0.05. Where probability value is below 0.0001 value is given as 0. b Probability

4. Discussion The results indicate that AMF from organic systems were, on average, more effective in promoting plant

growth than those from conventional systems when tested under experimental conditions equivalent to organic soil management. Although others have demonstrated that AMF activity is adversely affected by con-

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ventional compared with organic management (Sattelmacher et al., 1991; Douds et al., 1995) very little quantitative information on the agronomic effects is available. Although no qualitative assessments were made of the AMF populations from the different treatments it is clear that they differed significantly in terms of their function or efficacy. If further insight into the functional diversity of the AMF under investigation is to be achieved future research will need to characterise these populations (e.g. with respect to species diversity) although this was not the principal objective of this study. Approximately 150 species of AMF have been identified, this based largely on morphological features and recent developments on AMF taxonomy are posted on the INVAM web site (International collection of arbuscular and Vesicular-Arbuscular Mycorrhizal fungi) (http://invam.caf.wvu.edu/). As more sensitive techniques are developed, including the use of genetic markers, this should facilitate the accurate characterisation of AMF into key functional groups (based on some measure of effectiveness, such as plant growth response). Although the overall finding suggested AMF from organically managed soils were more effective (measured in terms of host shoot response to inoculation), within this study, however, examples of highly effective inocula were obtained from conventional systems, and conversely, relatively ineffective AMF inocula were obtained from organic systems. Factors which influence the AMF populations from each field are complex and may be unique. The information provided by farmers about site management, often going back over tens of years, may not always have been accurate. Farms also vary with respect to the type and amount of inputs used. Each organic farm may have had a different type of management before conversion and converted at different times. The conventional farms differed with respect to the balance of nutrients applied and this may have affected AMF activity (Gryndler et al., 1990). These factors may have combined to produce variable levels of soil P regardless of management type. There is some evidence to suggest that field soil P levels were negatively correlated with experimental plant response to AMF. Differing sensitivities to P by AMF (Davis et al., 1984) may have affected the species of AMF present. It is not possible to separate the relative importance of such effects in

the present study. It will be important to improve understanding of the processes that critically affect AMF populations under intensive management, and in particular their recovery to full effectiveness following a switch to organic management. The use of spores as inoculum may have also affected these findings. Spores taken from the soil may not necessarily be representative of those infecting plant roots and populations may be affected by factors which vary seasonally such as drought (Sylvia and Williams, 1992). The use of infected roots as inoculum may partially address this issue and this will be explored in future studies. The host species present (the cultivar of grass or clover used) may also affect AMF populations. The plant’s ability to form mycorrhizal associations (and the extent to which the plant host benefits from mycorrhizal infection) may be a heritable trait. For example some organic farms had no clover present at all and it is possible that this may have affected the AMF species present. Plant breeding programmes may also impact on AMF populations. Under conditions of high fertility AMF are generally less effective (in terms of positive growth responses of the host plant). Plants selected by breeding programmes for growth under these conditions may also select genotypes and cultivars which respond poorly to AMF. Mankse (1990) showed that high yielding cultivars of wheat were much less responsive to AMF infection under low fertility conditions than land races. Although the mean difference in yield observed for Trifolium, following inoculation with AMF from organic or conventionally managed grasslands, were modest (10%), they are not ecologically or economically insignificant. Indeed the results from individual farms suggest that there is scope for further improvements in yield if more selective manipulation or control of the symbiosis is achieved. This is where future research must be focused. Understanding of such systems will also benefit from the characterisation of AMF species present before, during and after conversion to organic management.

Acknowledgements This work was sponsored by the United Kingdom Ministry of Agriculture, Fisheries and Food. Thanks

W.R. Eason et al. / Agriculture, Ecosystems and Environment 73 (1999) 245–255

are expressed to John Toler for his help with the care and maintenance of the experiment, and to anonymous referees for improvements to an earlier revision of this paper.

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