Arbuscular mycorrhiza and enzymatic activities in the rhizosphere of Trachypogon plumosus Ness. in three acid savanna soils

Phosphorus (P) is one of the most limiting macronutrients in savannas. Seasonality strongly affects nutrition processes in such ecosystems. We studied arbuscular mycorrhizae (AM), microbial and phosphatase activity in the rhizosphere of Trachypogon plumosus Ness. in three acid savanna soils differing in taxonomic order and P content. Microbial number and activity, P mineralization and AM dynamics were quantified in rhizospheric samples of T. plumosus during the dry and the rainy season at three sites in the Estación Experimental La Iguana, Guárico State, Venezuela. Soils were characterized and infective AM propagules enumerated using the most probable number method on Sorghum vulgare Pers. Acid phosphatase activity (APA), dehydrogenase activity (DHA) and number of bacteria and fungi were determined in rhizospheric samples. AM colonized root length percentage (%CRL) was also determined in T. plumosus. Our data showed that the three soil orders had a very low fertility. AM infective potential showed values similar to those reported for tropical savannas. Also, AM %CRL was high for a grass species. APA increased in the rainy season in all cases. However, DHA and microbial counts decreased during the rainy season suggesting that soil microorganisms do not mediate the increase in APA. AM colonization, seasonal changes in microbial activity and in APA seemed to be important processes for P availability and T. plumosus P-uptake in savanna ecosystems.