PHOSPHORUS REQUIREMENTS AND NITROGEN ACCUMULATION BY N-2-FIXING AND NON-N-2-FIXING LEGUMINOUS TREES GROWING IN LOW P SOILS

The variation in P uptake and use efficiency and N accumulation by Gli ricidia sepium (N-2-fixing tree), Senna siamea and S. spectabilis (leg uminous non-N-2-fixing trees) were examined in the field at Fashola (s avanna zone), southwestern Nigeria, using four P rates, 0, 20, 40 and 80 kg P ha(-1). Growth of G. sepium and S. spectabilis responded to P application at 24 weeks after planting (WAP) and average yield increas es of 58% and 145% were observed by the application of 40 kg P ha(-1) for the two species, respectively. Such a P response was not found in S. siamea at 24 WAP and for any of the species at 48 WAP. G. sepium ac cumulated more P (on average 162%) than S. siamea and S. spectabilis a t 24 WAP and had greater root length and a higher percentage of mycorr hizal infection. However, at 48 WAP S. siamea had 2.5 times more P tha n G. sepium. Differences in the physiological P use efficiency (PPUE) between G. sepium and the non-N-2-fixing trees were significant at the 0 P level, being higher for S. siamea (average, 0.61 g shoot mg(-1) P ) than for G. sepium (0.27 g shoot mg(-1) P). G. sepium had a consiste ntly lower atom % N-15 than S. spectabilis, while that of S. siamea fo r most of the time did not differ from that of G. sepium. The referenc e plant affected N-2 fixation extimates, with negative values and a hi gher variability (CV 60%) associated with S. siamea than with S. spect abilis (CV<20%). Consequently, S. spectabilis was selected as a better reference plant for measuring N-2 fixation in G. sepium. G. sepium fi xed on average 35% and 54% of its N at 24 and 48 WAP, respectively. Ex cept at the lowest P rate, percentage and amount of N fixed were not g enerally enhanced by P application.