Subsidence and carbon dioxide emissions in a smallholder peatland mosaic in Sumatra, Indonesia.

Most attention in quantifying carbon dioxide (CO) emissions from tropical peatlands has been on large-scale plantations (industrial timber, oil palm ()), differing in drainage and land-use practices from those of smallholder farms. We measured subsidence and changes in bulk density and carbon organic content to calculate CO emissions over 2.5 years in a remnant logged-over forest and four dominant smallholder land-use types in Tanjung Jabung Barat District, Jambi Province, Sumatra, Indonesia: (1) simple rubber () agroforest (> 30 years), (2) mixed coconut () and coffee gardens () (> 40 years), (3) mixed betel nut () and coffee gardens (> 20 years), and (4) oil palm plantation (1 year). We quantified changes in microtopography for each site for greater accuracy of subsidence estimates and tested the effects of nitrogen and phosphorus application. All sites had a fibric type of peat with depths of 50 to > 100 cm. A recently established oil palm had the highest rate of peat subsidence and emission (4.7 cm year or 121 Mg CO ha year) while the remnant forest had the lowest (1.8 cm year or 40 Mg CO ha year). Other land-use types subsided by 2-3 cm year, emitting 70-85 Mg CO ha year. Fertilizer application did not have a consistent effect on inferred emissions. Additional emissions in the first years after drainage, despite groundwater tables of 40 cm, were of the order of belowground biomass of peat forest. Despite maintaining higher water tables, smallholder landscapes have CO emissions close to, but above, current IPCC defaults.