Plant available phosphorus in homegarden and native forest soils under high rainfall in an equatorial humid tropics

Abstract

Though the affinity of iron oxides and hydroxides for phosphorus (P) is thought to limit growth and productivity of plants on iron-rich acidic soils in humid tropical climate, tall trees are found in tropical rainforests, and vigorous growth occurs in herbaceous vegetation in the humid tropical climate of the Andamans. Our study reports how high rainfall influences Bray and Kurtz P-1 extractable P, water filled pore space (WFPS), microbial biomass carbon (MB-C) and phosphorus (MB-P) in the gravelly-sandy-loamy soils in three major land use systems: moist-evergreen forest, semi-evergreen forest and homegarden, in the equatorial humid tropical climate of South Andaman island of India. In addition, an ex-situ experiment investigated how labile carbon (glucose) under different soil-water regimes [25% field capacity (FC) and 100% FC] affected the P in the soils under the land use systems. WFPS, across the land use systems, was more than two times higher during the high rainfall than dry spell (72–80 hr after the high rainfall). The extractable P, across the land use systems, increased about 5 to 6 fold during the high rainfall than dry spell. The increase was the highest in the homegarden and lowest in moist-evergreen forest. The extractable P was positively correlated with the WFPS. MB-C and MB-P were lower during the high rainfall than dry spell in all the land use systems. In the ex-situ experiment, the extractable P increased 1.4 to 1.7 fold more in water (100% FC) + glucose than water (100% FC) alone treatment. Water at 25% FC did not increase the extractable P in the soils under all the land use systems. After cessation of the high rainfall, WFPS declined quickly within 2-hr in the upper layer (0–5 cm) of the soils. These indicate that the high rainfall together with labile carbon increases the extractable P through anaerobiosis, whereas, quick decline in the WFPS (0–5 cm) following cessation of the high rainfall helps its uptake by plants and microflora.