Propagules of Arbuscular mycorrhizae in P-deficient soils under different land uses, in semi-arid NE Brazil

The conversion of tropical dry forest into areas used for subsistence agriculture or livestock production is a common feature of the semi-arid region of NE Brazil. Our study looked into the effect of these land use changes on propagules of arbuscular mycorrhizal fungi (AMF) at ten sites distributed in the states of Paraiba and Pernambuco. Each site consisted of an area under native vegetation (Dry-Forest) adjacent to a cultivated area in the same slope position. Soil samples were taken at distance intervals of 20-30 m from two depths (0-7.5 and 7.5-15 cm) along a transect crossing the adjacent areas (10 sites x 2 land uses x 2 depths x 4 sampling points). Roots (< 2 mm) found in the soil samples (n = 160) were stained with trypan blue to assess the percentage of AMF colonization as well as the type of fungal structures. The AMF spores were separated from soil by wet sieving, incubated in iodonitrotetrazolium chloride (INT) solution and counted, those stained with INT were considered viable. Soil samples were analyzed for resin -extractable P and total organic carbon (TOC). For data analysis, the 10 areas under dry forest were separated in two sub-groups: Undisturbed-Dry-Forest (UDF, n = 6) and Disturbed-Dry-Forest (DDF, n = 4), owing to the different degradation degrees. Likewise, the 10 cultivated areas were also separated in two sub-groups: Preserued-Cultiuated (PC, n = 4) and Degraded-Cultiuated (DC, n = 6). Thus, four levels ofincreasing land use intensity or degradation were established, in agreement with the land use history, visual symptoms of vegetation or soil degradation, and soil erosion measured using the 137CS technique. Effects of increasing soil degradation on AMF propagules were observed in the 0-7.5 cm layer only, except for non-viable spores. The density of viable spores varied from 1.4 to 6.8 spores 150 g soil and of non-uiable spores from 91.4 to 226 spores/50g soil. The most consistent results in relation to the effect of selective logging (thinning) on propagules were found in the DDF group which had a larger number of viable spores, more intense root colonization and a lower proportion of samples with vesicules than the other groups. Soil samples had very low concentrations of resin-extractable P (< 6 mg kg(-1) soil). Root colonization was less intense in samples where the resin -extractable soil P was lower than 1.5 mg kg(-1) soil than in samples with higher P concentrations. It was further observed that root colonization and the number of viable spores were favored by higher TOC concentrations.