We compare the effect of arbuscular mycorrhizal (AM) colonization and PO 4 À3 fertilization on nitrate assimilation, plant growth and proline content in lettuce plants growing under well-watered (À0.04 MPa) or drought (À0.17 MPa) conditions. We also tested how AM-colonization and PO 4 À3 fertilization influenced N uptake ( 15 N) and the percentage of N derived from the fertilizer (% NdfF) by plants under a concentration gradient of N in soil. Growth of mycorrhizal plants was comparable with that of P-fertilized plants only under well-watered conditions. Shoot nitrogen content, proline and nitrate reductase activity were greater in AM than in P-fertilized plants under drought. The addition of 100 mg g À1 P to the soil did not replace the AM effect under drought. Under well-watered conditions, AM plants showed similar (at 3 mmol N), greater (at 6 mmol N) or lesser (at 9 mmol N) %NdfF than P-fertilized plants. Comparing a control (without AM inoculation) to AM plants, differences in % NdfF ranged from 138% (3 mmol N) to 22.6% (6 mmol N) whereas no differences were found at 9 mmol N. In comparison with P fertilization, mycorrhizal effects on %NdfF were only evident at the lowest N levels, which indicated a regulatory mechanism for N uptake in AM plants affected by N availability in the soil. At the highest N level, P-fertilized plants showed the greatest %NdfF. In conclusion, AM symbiosis is important for N acquisition and N fertilizer utilization but this beneficial mycorrhizal effect on N nutrition is reduced under large quantities of N fertilizer.

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The load and diversity of plant growth-promoting rhizobacteria (PGPR) are used as biomarkers to evaluate the health and quality of the soil. In the present study, the diversity of PGPRs and the physicochemical properties of the soil were used as comparative biomarkers in two adjacent soils (a pine forest soil and an agricultural soil) of the same region in Mexico City in order to investigate the effects of land use change. Bacterial diversity and physicochemical properties differed between the two soils. In the pine forest soil, PGPR were distributed at similar proportions in the Proteobacteria (29.41%), Actinobacteria (29.41%) and Firmicutes (35.29%) phyla, whereas the remaining PGPR were in Bacteroidetes (5.88%). In the agricultural soil, most PGPR belonged to the Phylum Firmicutes (50%), with the remaining belonging to Proteobacteria (22.73%), Actinobacteria (18.18%) and Bacteroidetes (9.09%). Percentages of bacteria producing indole acetic acid (90.91%) and siderophores (40.91%) were higher in agricultural soil. A canonical correspondence analysis (CCA) was used to correlate PGPR with the physicochemical characteristics of the soils. The CCA revealed that differences between both soils and the physicochemical properties of the soils affected isolated bacterial species and their distribution. These results demonstrate that the PGPR are correlated with the physicochemical properties of the soil, exhibiting differences between an agricultural soil and a pine forest soil.

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