15N-Based estimation of nitrogen fixation by eight tropical forage-legumes at two levels of P:K supply

Abstract The effects of two fertilizer treatments on growth and symbiotic nitrogen fixation of eight pre-established tropical forage-legumes ( Centrosema acutifolium, C. macrocarpum, Zornia glabra, Pueraria phaseoloides, Desmodium ovalifolium, Stylosanthes macrocephala, S. guianensis and S. capitata were evaluated under a cutting regime by using the 15 N dilution technique. Rooting pattern and nodulation were also evaluated. The relative N accumulation curves of the legumes and control (savanna grasses) were studied in a separate experiment. The legumes were established in furrows separated by native savanna in an Oxisol of the Eastern Plains of Colombia to give a density of 5–6 plants m −2 . The two fertilizer treatments, (a) a basal fertilizer of micro- and macro-elements including 80 kg P ha −1 and 70 kg K ha −1 , and (b) the same fertilizer without P and K, were applied to the legumes at the start of the rainy season one year after establishment. With P:K fertilizer, all legumes derived at least 70% of their nitrogen from the symbiosis (% N dfa ), whereas without P:K both lower values and larger differences in the %N dfa (44–84 % N dfa ) between species were observed. The greatest effect of P:K on % N dfa was observed in D. ovalifolium (70 and 44 % N dfa with and without P:K respectively), but the effect on its yield was relatively small. In contrast, S. macrocephala responded with a large increase in yield (380% with P:K), although it was the only species in which P:K did not have a significant effect on % N dfa . Total shoot N derived from fixation with P:K fertilizer ranged from 25 kg N ha −1 for D. ovalifolium to 115 kg N ha −1 for P. phaseoloides , with three periods of regrowth over a total of 17 weeks. Without P:K fertilizer, 11–48 kg N ha −1 were derived from fixation. Legume ranking for total N derived from fixation mostly reflected yield differences. However, D. ovalifolium ranked clearly lower for total N from fixation than for yield due to its low % N dfa and low N concentration.