TROPICAL TREE PLANTATION ROTATION EFFECTS ON FERTILITY, EARTHWORM ABUNDANCE, AND BIOMASS OF A HYDRUDAND SOIL IN HAWAI`I

Abstract

This study examined how second-rotation tree species affected: (1) soil fertility and soil organic C for the 0- to 15- and 15- to 30-cm depths of an Akaka silty clay loam (Acrudoxic Hydrudand) three and six years into the second rotation that followed an 18-year first rotation and, (2) earthworm abundance, and biomass. Four first-second rotation species pairings were used: Sydney blue gum [SBG, Eucalyptus saligna)] to SBG, albizia [ALB, Falcataria moluccana] to ALB, SBG to ALB and ALB to SBG. Rotation affected soil pH at the 0- to 15-cm depth and were primarily attributed to the legacy effects with lower pH for rotations first planted to ALB than SBG. The SBG→ALB rotation consistently had greater exchangeable Ca and Mg than the other rotations at both soil depths. By year six there was: (1) greater soil organic C concentration and organic C gain since the first rotation at the 0- to 15-cm depth for ALB→ALB than SBG→SBG but ALB→ALB did not differ from SBG→ALB or ALB→SBG, (2) soil total N buildup from N2-fixation by ALB in SBG→ALB and total N decline for SBG→SBG, and (3) no difference in soil total N difference between ALB→SBG and SBG→ALB or between ALB→ALB and SBG→ALB. We found that both Amynthas gracilis and Pontoscolex corethrurus earthworms had greater densities and biomass in the treatment ALB→ALB than in the other treatments and were both positively correlated with soil N.