A field trial was performed in the 2011-2013 growing seasons in the Haouz plain at about 30 km at the West of Marrakech (31˚4'60" N and 7˚3'0" W, Morocco). Soil chemical properties (0 - 0.10 m layer) were as follows: pH (H₂O) 7.2; carbon (%) 1.54; nitrogen (%) 0.08, C/N 19.2; Total P (mg∙kg⁻¹) 502.9 and Olsen P (mg∙kg⁻¹) 22.1. The climate of the experimental site is semiarid Mediterranean, with a mean annual rainfall of 282 mm, mostly in Autumn/Winter (59%) and in Spring (22%). The dry season is from May to September. The mean air temperature is 17.9˚C in Autumn, 12.8˚C in Winter, 18.5˚C in Spring, and 24.7˚C in Summer. Soil, cropped in the previous growing season with durum wheat (Triticum durum Desf.), was plowed to a depth of 0.30 m in Summer and then shallowly harrowed to control weeds. No herbicide nor chemical fertilizers were applied. The experimental design had a randomized block design with one factor and four replication blocks. The factor was the direct AMF inoculation or not (control). Plots were 3.0 m × 3.0 m; each main plot was spaced 1.0 m out from the next. Inoculation with AMF included the application of the commercial AMF inoculum (Inoculum plus™). This spore inoculum including R. irregularis (2 × 10⁴ spores per gram of inoculum) and peat was mixed with a disinfected sand (120˚C, 20 min) to obtain a total spore density of 40 spores∙g⁻¹.

Durum wheat was sown in December 2011 and 2012 at a rate of 400 viable seeds∙m⁻² in rows 0.18 m apart. The experimental plot consisted of four rows 3 m long. Weeds were controlled by hand during the experiment. At sowing, five grams of the AMF inoculum (200 spores of R. irregularis) were placed in each planting rows at 0.20 m intervals.

After one and two growing seasons, at wheat tillering, the total number of wheat plant and spike per plot were counted. Then ten randomly chosen plants in the middle of the plot were harvested. The seeds from each plant were collected, counted and weighed to determine the dry weight of 1000 seeds (grain yield). The aerial parts of each plant were then oven dried at 70˚C during 2 weeks and weighed. After drying, shoot tissues were ground, ashed (500˚C), digested in 2 ml HCl 6N and 10 ml HNO₃ N for nitrogen and then analyzed by colorimetry for phosphorus [24] . For nitrogen determination (Kjeldahl method), they were digested in 15 ml H₂SO₄ (36 N) containing 50 g∙l⁻¹ of salicylic acid. Roots from five other randomly chosen plants in the middle of each plot were sampled and root subsamples of about 3 g each were taken. Each subsample was stained with 0.05% trypan blue in lactic acid according to Phillips and Hayman [25] ; root colonization by AMF was then measured with the grid intersect method according to Giovannetti and Mosse [26] .

Inoculation effectiveness (IE) of AMF inoculation on plant yield was calculated based according to the following formula:

with +M, inoculated plants; −M, non-inoculated plants.

After one and two growing seasons, soil cores (1 kg) were collected at 0- to 10-cm depth in each plot. About 10 soil samples were taken from each plot and pooled together. Soil samples were crushed and passed through a 2-mm sieve. Then hyphal length was measured by the filtration-grid-line method [27] .

Patterns of in situ catabolic potential (ISCP) were designed to determine the functions of soil microbial communities and the microbial functional diversity in soil treatments after one and two growing seasons [28] . With a micro-respirometry method carried out in 96-well microtiter plates [29] , 28 organic substrates (3 wells per substrate) were tested; Thirteen carbohydrates (D-mannose, D-mannitol, D-trehalose, L-arabinose, D-xylose, D- sucrose, D-galactose, meso-inositol, D-sorbitol, L-rhamnose, L-arabitol, mesoerythriol, D-Glucose), 8 carboxylic acids (citric acid, maleic acid, D,L-malic acid, oxalic acid, Na-glucomate, α-ketoglutaric acid, L-ascorbic acid) and 7 amino acids (L-asparagine, D,L-valine, L-methionine, L-glutamine, D,L-alanine, D,L-serine, N- acetyl-D-Glucosamine). Detection plates were prepared according to the MicroResp™ recommendations, and data were recorded by using a Tecan infinite M200 Plate Absorbance reader (Absorbance measured at 572 nm). For each well, the absolute respiratory activity was determined by subtracting the absorbance value at t0 to the value after 6 days of incubation at 27˚C in the dark (t6). The average basal respiration value was then subtracted to all the individual substrate respiration values. For each organic substrate, the substrate-specific respiratory activity was averaged and the value was finally divided by the sum of all the mean substrate specific respiratory activities (p_i value). The catabolic evenness (E) was calculated to determine the catabolic diversity of soil treatments. It represents the variability of catabolized substrates amongst the range of the targeted substrates and is calculated using the Simpson-Yule index with p_i = (respiration response to individual substrates)/ (total respiration activity induced by all substrates for a soil treatment) [30] . The catabolic evenness could be used to evaluate the ability of microbial communities to resist against environmental stress or disturbance [30] . Data were calculated for the individual responses to substrates but also for the average responses with carbohydrates, carboxylic acids and amino acids.

The mycorrhizal soil infectivity was estimated according to Kisa et al. [31] . Four dilutions of each soil sample were made by thoroughly mixing the original cultural soil substrate with the same soil but autoclaved (120˚C, 40 min). The levels of dilution (undisinfected soil/disinfected soil) were: 100%, 50%, 25% and 0% (v:v). Three replicates were prepared for each dilution. Seeds of durum wheat were surface sterilized with 10% sodium hypochlorite, washed with sterile distilled water (120˚C, 20 min) and pre-germinated for 2 days in Petri dishes on humid filter paper. One germinated seed was then transplanted into each of 100 mL pots filled with 100 g of different soil dilution. The pots were placed in a glasshouse under natural light (day length approximately 10 h, mean temperature 22˚C) and watered daily with deionized water. After one month of growth, seedlings were uprooted and their entire root systems were washed under tap water. For each pot, the extent of AM colonization was assessed as described above.

All the data were subjected to a two-way analysis of variance and comparisons among means were made using the Newman-Keuls test (P < 0.05). The percentages of the mycorrhizal colonization were transformed by arcsin(sqrt) before the statistical analysis. To evaluate the mycorrhizal soil infectivity, the relationships between the extent of AM colonization and soil dilutions were analyzed using mixed effects linear regression models. The p-values for model fits were obtained by ANOVA [32] . Computations were conducted with the R software [33] , using the “NLME” package [34] . In the regression models, the fixed effects were the soil origin and the level of dilution (undisinfected soil/disinfected soil: 1, 1/2, 1/4, 0).

After one growing year, AMF application did not impact the total biomass yield, the spike number per ha, the dry weight of 1000 seeds (grain yield) (Table 1). In contrast, a positive effect of the AMF inoculation was recorded after two years of cultivation on the wheat growth and on the grain yield as well as the wheat phosphorus uptake (Table 1). The inoculation effectiveness was of 22.5% for the total biomass yield per ha, 29.3% for the spike number per ha and 7.7% for the grain yield (Table 1).

The mycorrhizal colonization of wheat plants and the hyphal length were not significantly impacted by the AMF inoculation after one growing year whereas they significantly improved after two growing seasons (Table 1).

After two years of cultivation, the catabolic evenness of the soils amended with the AMF inoculum were significantly higher than those calculated for the not inoculated treatments. The highest standardized average respi-

⁽¹⁾Standard error. ⁽²⁾For each year, data in the same line followed by the same letter are not significantly different according to the Newman-Keul’s test (p < 0.05). ⁽³⁾nd = not determined.

ration SIRs (Substrate Induced Respirations) to amino-acids and carboxylic acids were recorded in the AMF inoculated treatments after 1 and 2 growing seasons except for the amino-acid group for the second growing year (Table 2). The AMF inoculation gave very different patterns of in situ catabolic potential (ISCP) (Figure 1). The substrates preferentially used in samples collected from the AMF treatments compared to the control were alanine, glutamine, asparagine, Na-glutamate, ascorbic acid, ketoglutaric acid, oxalic acid, gluconic acid, malic acid, maleic acid and citric acid for the first year of cultivation whereas for the secong growing season, the highest SIR responses were recorded in the AMF inoculated soil with ascorbic acid, ketoglutaric acid, malic acid, maleic acid and citric acid (Figure 1).

The regression model between the extent of AM colonization and soil dilutions for the four different treatments showed that the treatment and the percentage of soil dilution both had very strong effects (P < 0.0001). The differences between the control and the AMF inoculated treatment were only significant after two growing years (P < 0.001) (Figure 2).