Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India

Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on P-32 recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of P-32 was monitored through radioassay of leaves. Regardless of the species, P-32 uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of P-32 from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more P-32 than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of P-32 by trees. An grass species exerted a complementary effect on P-32 absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the P-32 uptake by grass species.