Distribution of phosphorus in an above-to-below-ground profile in a Bornean tropical rain forest

Abstract

Abstract:Ecosystem pool of phosphorus (P) was determined as the sum of above-ground vegetation, roots, necromass and soils to 1 m deep in a tropical rain forest in Sabah, Malaysia. Relationships among soil P fractions, acid phosphatase activity and fine-root biomass across soil horizons were also determined to understand P availability. Ecosystem pools of P, and of simultaneously quantified nitrogen (N) and carbon (C) were 3.4, 12 and 370 Mg ha−1, respectively. Only 2.6% of the total ecosystem P was in the above-ground vegetation, contrasting to C (60%) and N (16%). Canopy foliage of dominant tree species showed an extremely high N to P ratio of 31.5, which implied the excessively short supply of P compared with ample N. Soil P primarily consisted of recalcitrant occluded fractions (78–91%) and only 4% was labile. Approximately three-quarters of labile soil P was an organic fraction (Po). The concentration of labile Po did not differ between soil horizons, while both phosphatase activity and fine-root density were the greatest in the topsoil (top 5 cm) and dramatically decreased with depth. This suggests that trees depend on the acquisition of P from the labile Po in the topsoil, despite a greater amount of labile P in the subsoil. Trees with a high foliar N/P ratio may invest N to acquire P from the topsoil by secreting phosphatase that consists of proteins, rather than investing C to extending roots to scavenge P in the subsoil.