Effects of goat manure and inorganic phosphate addition on soil inorganic and microbial biomass phosphorus fractions under laboratory incubation conditions

Changes in inorganic phosphorus (P) and soil microbial biomass P following the addition of goat manure and different amounts of inorganic P to an Umbric Ferralsol were assessed over 12 weeks under laboratory incubation conditions using a sequential fractionation procedure. Triple superphosphate was added at rates equivalent to 0, 45, 90, 135 and 180 mg P kg-1, with or without goat manure at a rate of 10 g kg-1soil on a dry-weight basis and incubated moist. Resin P, 0.5 mol L-1 NaHCO(3) extractable inorganic P (NaHCO(3)-P(i)), 0.1 mol L-1 NaOH extractable inorganic P (NaOH P(i)) and soil microbial biomass P concentrations were determined on days 7, 14, 28, 56 and 84. The magnitudes of the inorganic P fractions extracted were: NaOH P(i) > Resin P > NaHCO(3) P(i). Thus, NaOH P(i) was the major sink for the applied P. The dynamics of the three labile P fractions (resin P, NaHCO(3) P(i) and microbial biomass P) varied considerably during the incubation period. The resin P fraction consistently declined with time in all treatments, whereas the NaHCO(3) P(i) fraction changed little with time in the control and goat manure amended soil, but increased rapidly with time when inorganic P was applied alone or in combination with goat manure. Microbial biomass P increased with time in all treatments, peaking on day 28 and declining thereafter. The co-application of inorganic P with goat manure produced up to twofold more microbial biomass P than either inorganic P or goat manure applied alone. Therefore, the combined application of manure with low rates of P fertilizers may be a cost effective strategy for increasing the efficiency of fertilizer P use through enhanced biological cycling of P in small-holder farms in South Africa.