Legume cover and P2O5 content in Montado systems: a case study of biodiverse pastures on cambisols

Abstract

The Montado, a dominant agro-silvo-pastoral system in Southern Portugal, features rainfed pastures primarily composed of legumes and grasses. Portuguese pasture area has expanded over the last century for economic, environmental, social, and cultural reasons. These pastures offer low-cost forage and improve long-term soil fertility through animal manure incorporation, enhancing soil organic matter, and leguminous N-fixation via Rhizobium. However, despite known benefits of phosphorus (P) in Portuguese soils, its specific role in Montado pasture production is understudied. This study evaluated the relationship between soil P content (as P2O5) and legume cover. Floristic inventories (Spring 2021) and soil sampling (0–0.20 m, 2022) were conducted at 24 points (12 under/12 outside tree canopies) in two holm oak (Quercus rotundifolia Lam.) Montado plots on Cambisols (600 mm annual precipitation, 0.6 NH – Normal Head, cattle grazing) at Mitra Experimental Farm (University of Évora). Results indicate a moderate negative correlation (R²=0.55) between soil P2O5 content and legume percentage under tree canopies. No significant correlation was found outside canopies. These findings contradict the general perception that higher soil P boosts legume cover. Potential explanations include the specific agricultural year and above-average soil P levels, which could favor other competing plant families over legumes. Furthermore, complex P retention dynamics in soil, linked to its acidic pH, might also contribute to this inverse relationship.