Phosphorus cycling in grazed native and fertilised pastures oversown with legumes on basaltic soils

Abstract

The mayority of the total amount of P taken up by the animals under grazing is recycled mainly through faeces. We studied the potential cycling of P through the faeces of calves grazing natural pastures (PN) and pastures oversown with legumes in Uruguay. The pastures oversown with lotus corniculatus (Lotus corniculatus L.) and white clover (Trifolium repens L.) were fertilized with 30 and 60 kg of P₂O₅ ha^-1 annually (P1 and P2 respectively). The evaluation was performed from june 1997 to july 1999. We found a close relationship between the P concentration in pastures and faeces (P_faeces = 1.28 P_pasture + 0.08, r² = 0.77, P = 0.001). The P concentration of faeces from the fertilized pasture was greater than that from natural pastures in all sampling times. The proportion of organic P was higher in PN than in P1 and P2. Higher correlation was found between inorganic and total P in faeces than between organic and total P. The inorganic P was mostly water soluble (61%). The highest P contents of faeces were observed at the end of the winter-early spring, and the lowest towards the end of the summer. It is concluded that the amount of P cycled were higher in fertilized pastures meanwhile in natural pastures this process was rather inefficient due to smaller amounts of P and higher proportion of organic P in the faeces.