Distribution and sorption of phosphate in a savanna soil under improved pastures in northern Nigeria

Land use patterns affect soil nutrient transformation and availability. The study determined the distribution of phosphorus (P) fractions and sorption in five pasture fields composed of Andropogon gayanus , Brachiaria decumbens , Chloris gayana , Digitaria smutsii , and Stylosanthes guianensis . The objectives were to characterize P fractions in improved pastures and to determine the effect of forage species on soil P lability. Total P (P-t) across the pastures was not significantly different. Organic P (P-o) accounted, on the average, for 64% of P-t . Resin-P, considered the plant-available P, ranged from 4 to 10 mg kg(-1) , suggesting acute P deficiency in the pastures. The sum of P fractions extracted by 0.5 M NaHCO3 , 0.1 M NaOH, and 1.0 M HCl, together with the resin-P, accounted for less than 35% of P-t . Factor analysis indicated that plant-available P approximated by resin-P was furnished by HCO3 -P-o mineralization and HCl-P. The highest concentrations of HCO3 -P-o and OH-P-o were maintained by Brachiaria decumbens . Grouping P-i and P-o fractions into labile and nonlabile fraction showed that Brachiaria decumbens maintained the greatest concentration of labile P as a proportion of its P-t . The pasture soils sorbed between 31 and 65% of added P from a standard concentration of 50 mmol kg(-1) . Phosphorus sorbed by soils from the pasture fields was in the order: Digitaria smutsii = Stylosanthes guianensis > Brachiaria decumbens = Chloris gayana > Andropogon gayanus , whereas resin recovery of sorbed P was greater in Brachiaria decumbens than other pastures. Between 82 and 92% of sorbed P was bound irreversibly. It was concluded that the relatively high concentration of labile P maintained by soil under Brachiaria decumbens was probably related to its capacity to sequester more carbon than the other pastures.