Various forms of organic and inorganic P fertilizers did not negatively affect soil- and root-inhabiting AM fungi in a maize-soybean rotation system.

Arbuscular mycorrhizal (AM) fungi are key components of most agricultural ecosystems. Therefore, understanding the impact of agricultural practices on their community structure is essential to improve nutrient mobilization and reduce plant stress in the field. The effects of five different organic or mineral sources of phosphorus (P) for a maize-soybean rotation system on AM fungal diversity in roots and soil were assessed over a 3-year period. Total DNA was extracted from root and soil samples collected at three different plant growth stages. An 18S rRNA gene fragment was amplified and taxa were detected and identified using denaturing gradient gel electrophoresis followed by sequencing. AM fungal biomass was estimated by fatty acid methyl ester analysis. Soil P fertility parameters were also monitored and analyzed for possible changes related with fertilization or growth stages. Seven AM fungal ribotypes were detected. Fertilization significantly modified soil P flux, but had barely any effect on AM fungi community structure or biomass. There was no difference in the AM fungal community between plant growth stages. Specific ribotypes could not be significantly associated to P treatment. Ribotypes were associated with root or soil samples with variable detection frequencies between seasons. AM fungal biomass remained stable throughout the growing seasons. This study demonstrated that roots and soil host distinct AM fungal communities and that these are very temporally stable. The influence of contrasting forms of P fertilizers was not significant over 3 years of crop rotation.

Long-term phosphorus fertilization impacts soil fungal and bacterial diversity but not AM fungal community in alfalfa.

Soil function may be affected by cropping practices impacting the soil microbial community. The effect of different phosphorus (P) fertilization rates (0, 20, or 40 kg P(2)O(5) ha(-1)) on soil microbial diversity was studied in 8-year-old alfalfa monocultures. The hypothesis that P fertilization modifies soil microbial community was tested using denaturing gradient gel electrophoresis and phospholipids fatty acid (PLFA) profiling to describe soil bacteria, fungi, and arbuscular mycorrhizal (AM) fungi diversity. Soil parameters related to fertility (soil phosphate flux, soluble P, moisture, phosphatase and dehydrogenase assays, and carbon and nitrogen content of the light fraction of soil organic matter) were also monitored and related to soil microbial ribotype profiles. Change in soil P fertility with the application of fertilizer had no effect on crop yield in 8 years, but on the year of this study was associated with shifts in the composition of fungal and bacterial communities without affecting their richness, as evidenced by the absence of effect on the average number of ribotypes detected. However, variation in soil P level created by a history of differential fertilization did not significantly influence AM fungi ribotype assemblages nor AM fungi biomass measured with the PLFA 16:1omega5. Fertilization increased P flux and soil soluble P level but reduced soil moisture and soil microbial activity, as revealed by dehydrogenase assay. Results suggest that soil P fertility management could influence soil processes involving soil microorganisms. Seasonal variations were also recorded in microbial activity, soil soluble P level as well as in the abundance of specific bacterial and fungal PLFA indicators of soil microbial biomass.