The Impact of Land Use on Peat Characteristics in the Highlands of Humbang Hasundutan, Indonesia

Peatlands are vital carbon reservoirs, but their ecological roles are increasingly being compromised by land use change. While tropical peatlands are often associated with lowlands, distinct highland peatlands also occur, they remain insufficiently explored. The Humbang Hasundutan peatlands formed on the southern flank of the Toba caldera following the ~74 ka super-eruption, where persistent waterlogging in cool, wet uplands enabled accumulation of predominantly woody peats. This study investigated the effects of recent land use changes on the chemical and biological properties of peat soils in Humbang Hasundutan (elevation 1350–1430 m.a.s.l.), comparing forests, open lands, and cultivated areas. Soil samples were collected from three sub-districts (Dolok Sanggul, Pollung, Lintong Nihuta) at two depths (10 cm and 40 cm) and analysed for carbon (C), nitrogen (N), pH, and microbial respiration. Results revealed the significant degradation in cultivated lands, with C content dropping to 10–15%, compared to 57.30% in forests. Nitrogen levels were highest in Dolok Sanggul (1.38% in cultivated land) and Pollung (1.32% in open land). C:N ratio varied from 66 in forests to 34 in cropping lands. Soil pH varied by land use, with cultivated areas showing elevated pH (5.09) due to mineral soil mixing, while natural forests retained acidic conditions (pH 3.9–4.4). Microbial respiration was highest in forests (5.49 mg CO2/day) but decreased in disturbed areas. These results stress the climate-mitigation value of intact highland peat forests and the urgency of tailored restoration via rewetting and native revegetation, alongside cautious agroecological management.