Seasonality of organic phosphorus mineralization in the rhizosphere of the native savanna grass, Trachypogon plumosus

Savanna ecosystems have low primary productivity, strong seasonality, and acid soils with low phosphorus (P) content. Organic P (Po) comprises around 50% of the total soil P and is plant-available only after mineralization. Rhizosphere processes mediated by plants, microorganisms and arbuscular mycorrhiza (AM) are important for plant P nutrition. We studied P transformation rates, Po-fractionation, acid phosphomonoesterase activity (APA), AM status, dehydrogenase activity (DHA), and bacterial and fungal plate counts in the rhizosphere of the native dominant grass Trachypogon plumosus. We collected samples from three acid savanna soils differing in order and P content (Entisol, Vertisol and Ultisol) at Estación Experimental La Iguana (Northeastern Venezuela) during the dry, rainy and transitional seasons over a 2-year-period. Less available Po fractions (moderately labile, moderately and highly resistant) seem to be involved in short-term P-cycling transformations as they significantly varied with season. During the rainy season plant P content (576–1160 mg P kg−1 dry weight) and APA (44–200 mg PNP kg−1 dry soil) were higher, while microbial number and activity (DHA) were lower. The higher P availability in the Entisol (6–9 mg P kg−1 dry soil) resulted in a better plant nutritional status and inhibited APA. T. plumosus seems to be highly dependent on AM symbiosis (45–71% AM colonized root length, 0.6–8 AM spores g−1 dry soil), especially during the rainy season. Po mineralization processes, mediated by biological associations in the rhizosphere, are crucial for understanding seasonal P-cycling and fertility in acid savanna soils.