Litter layer residence time in forest and coffee agroforestry systems in Sumberjaya, West Lampung

Forest conversion to coffee-based agroforestry leads to sudden disappearance of the litter layer and a decrease in the rate of litter fall, reducing food for ecosystem engineers such as earthworms. With time, however, a new litter layer is created potentially returning to forest-like conditions at the soil surface. This research quantified litter thickness, earthworm populations and soil macroporosity in response to land use change in the Sumberjaya benchmark area (West Lampung, Indonesia) by comparing: (a) remnant forest (control); (b) multistrata shaded coffee with fruit and timber trees, as well as nitrogen-fixing shade trees; (c) shaded coffee (nitrogen-fixing shade trees, but less than five tree species per plot); and (d) sun coffee ('monoculture') with coffee forming more than 80% of total stem basal area. Plots were selected with tree ages of 7-10 years in three slope classes: (a) flat (0-10 degrees); (b) medium (10-30 degrees); and (c) steep (> 30 degrees). The mean standing necromass was 6.1, 4.5, 3.8 and 3.0 Mg ha(-1) for forest, multistrata, shade coffee and sun coffee, respectively, without significant influences of slope. Fine, partly decomposed litter was 33-40% of total necromass, coarse leaf litter 14-16%, and twigs and branches comprised the remaining 43-52%. Soil organic carbon content (C-org) was highest in the forest. The largest annual litter input was found in the remnant forest (14 Mg ha(-1) year(-1)), followed by multistrata, shaded and monoculture coffee systems, i.e., 9.8, 6.6 and 4.0 Mg ha(-1) year(-1), respectively. The population density of earthworms in the forest was 50% lower than in multistrata coffee gardens (150 individuals m(-2)), but its biomass (31 g m(-2)) was twice that in the multistrata coffee gardens. The lowest population density of earthworms was found in the shade coffee system (150 individuals m(-2)) with a biomass of 7 g m(-2). A simple model suggests that the standing litter in the various land use systems is consistent with measured litter inputs and decay rates, but that the soil organic matter (SOM) content and macroporosity of the shade and multistrata systems are less than predicted. The recovery of a surface litter layer in sun coffee systems can provide protection from erosion with time, but will not be sufficient to restore macroporosity at the level of forest soils, leading to hydrologic alterations that favor overland flow. (c) 2006 Elsevier B.V. All rights reserved.