Nitrogen fixation by Trifolium repens and Lotus tenuis-based pastures in the Flooding Pampa, Argentina

An experiment was carried out to estimate N2 fixation of white clover and lotus-based pastures and to study the effects of legumes on two contrasting tall fescue genotypes managed with two continuous grazing intensities on a typical alkaline hydromorphic soil 'Natraquoll'. Binary mixtures of white clover (WC) or lotus (L) and tall fescue temperate (EP) or Mediterranean (MK) were employed. Grazing by steers was applied to maintain two herbage mass levels: 800-1 200 for high level of use (HLU) or 1 800-2 200 kg DM·ha-1 for low level of use (LLU). Measurement of legume cover and nitrogenase activity in sample areas with legumes of the experimental units were carried out ten times in 1986 and eight in 1987. Legume cover was always near 60 % in the samples and varied from 4 to 24 % in the paddocks. N2 fixation was estimated by weighing the nitrogenase activity of samples by legume cover. In 1986 it was: 59 kg N·ha-1 year -1 in WC/MK, 52 in WC/EP, 42 in L/MK, 27 in L/EP, 52 in HLU and 39 in LLU, the differences being not significant (P > 0.05). In 1987 the values were: 25 kg N·ha-1 year-1 in WC/MK, 14 in WC/EP, 27 in L/MK, 27 in L/EP, 25 in HLU and 21 in LLU. Mixture x use level interaction was significant (HLU > LLU within L/MK, P 0,05). En 1987 les valeurs obtenues ont été 25 kg N·ha-1·année pour WC/MK, 14 pour WC/EP, 27 pour L/MK, 27 pour L/EP, 25 pour HLU et 21 pour LLU. L'interaction entre mélange et niveau d'utilisation a été significative (HLU > LLU pour L/MK, p < 0,05). Une variation saisonnière de la fixation d'azote a été observée. Le contenu en légumineuses des prairies semble être le principal facteur déterminant de la fixation d'azote. (© Inra/Elsevier, Paris.