Recovery of soil properties and functions in different rainforest restoration pathways

Abstract

Rainforests in the tropics and subtropics are declining rapidly, leading to significant changes in soil physical and chemical characteristics and biochemical cycles that connect vegetation and soil. Effects of such changes on soil carbon (C) and nitrogen (N) pools are still poorly understood and contradictory results have been reported in different studies. We studied changes in C and N dynamics associated with deforestation and reforestation in the Big Scrub region of subtropical eastern Australia, where over 99% of the former rainforest cover was converted by European settlers for pasture or agriculture. In this area, the most common reforestation pathways are tree-planting for ecological restoration purposes, autogenic regrowth dominated by the invasive tree species camphor laurel (Cinnamomum camphora), and management of this regrowth to accelerate the development of a more native tree community. Nineteen soil properties were measured at 25 sites, five within each of: remnant rainforest, pasture, ecological restoration plantings 10-15-year old, camphor laurel regrowth 20-40-year old, and rainforest regrowth 3-6 years after poisoning of camphor trees within older regrowth. Of eight components of N cycling measured, four differed significantly between site-types: nitrate-N, plant-available nitrate-N and nitrification rates were highest in rainforest and lowest in pasture, with revegetated sites showing intermediate levels; while plant-available ammonium-N showed a reverse pattern. Among revegetated sites, camphor-dominated regrowth showed slightly less regeneration of N dynamics than treated camphor and replanted sites. Among soil attributes related to the soil C cycle and soil microbial activity, there was little variation with deforestation or reforestation. Four of five other soil attributes (pH, bulk density, fine root biomass, and plant-available phosphate-P) showed significant variation among site-types, with specific patterns varying. All three reforestation pathways were able to restore soil properties to varying degrees, although the rate of recovery was lowest in untreated camphor regrowth.