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Large scale oil palm monocultures are continuously replacing Indonesia's tropical rainforests. The impact of intensive nitrogen (N) fertilization and herbicide application on biogeochemical processes in these monocultures remains unknown. Thus, we aim to understand NPK and herbicide effects on soil carbon (C) cycle. We hypothesize that (1) increased microbial activity due to N fertilization leads to faster soil organic matter (SOM) decomposition and to increased CO 2 emissions from soil. Despite this, we expect (2) that herbicide application increases soil organic carbon (SOC) sequestration, as it counteracts the fertilizer's effects by hampering soil microbial activity, by deactivating the same synthesizing pathway as in plants, leading to decreased microbial respiration. We investigated the effects of fertilization (conventional and reduced NPK) and weed control (herbicide or mechanical weeding) on SOC decomposition in the rows between the palms (interrows) and around the palm stems (weeding circles). 14C labelled glucose was added to soil to analyze the response of microbial activity and SOC mineralization during 30-day incubation. Conventional fertilization caused microbial activation and destabilized SOC by priming, resulting in increased CO 2 efflux. Glyphosate application plus fertilization further increased microbial activity contradicting our hypothesis. In combination with high fertilizer amounts, glyphosate strongly increased microbial activity. NPK might occupy a greater share of sorption capacities; hence, glyphosate is still available and can act as an additional source of C and possibly N and P. Therefore, SOC decomposition was 1.5 accelerated compared to the control when glyphosate and fertilizer application were combined. In contrast, reduced SOC decomposition, i.e., a strong negative priming effect (−47.9 μg g −1), occurred under reduced NPK fertilization without herbicides suggesting positive effects on C accumulation and storage. Our results emphasize the risk of management intensification and the need for a low-impact management strategy to maintain soil fertility and the function as a C reservoir. Summary of research outcomes: Decomposition responses towards contrasting management practices. Faster decomposition and higher CO 2 efflux under conventional fertilization level and herbicide application. Retarded SOC decomposition and support of SOC accumulation under reduced fertilization and mechanical weeding. Effects most prominent in the WC. IRs mostly characterized by missing C input and C limitation. No effects of management extensification could be found in the IRs. [Display omitted] • High NPK fertilization in combination with glyphosate increases CO2 emissions from soil. • Glyphosate acts as additional nutrient source for microorganisms. • Negative priming effects under low NPK fertilization and mechanical weed control, implying retardation of SOC decomposition. • Harvesting paths are strictly nutrient and C limited. [ABSTRACT FROM AUTHOR]

• Flooding in riparian areas counters the effects of land use change on C storage. • C preservation due to oxygen-limited mineralization under anaerobic conditions. • δ13C values represent changing environmental conditions. Progressive conversion of tropical rainforests to agricultural monocultures in South East Asia increasingly affects landscape types such as riparian areas. The impacts of conversions on soil organic matter (SOM) vary with changing landforms. However, this was often not accounted for in previous studies where SOM in soils in riparian areas was combined with SOM from well-drained adjacent slopes. Because riparian areas have a high carbon (C) storage potential, our objectives were i) to assess their C stocks after conversion to rubber and oil palm plantations in Sumatra (Indonesia) and ii) to compare the impacts of land use conversion on C stocks between riparian and well-drained areas. Average soil C stock losses from the top 30 cm were about 14% and 4% following conversion of riparian forest to rubber and oil palm plantations, respectively, indicating a high resistance of C to mineralization. C losses from well-drained areas were twice as high as from riparian areas after the respective conversion. δ13C values from riparian areas showed clear heterogeneity down soil profiles that was explained i) by alternating oxic and anoxic conditions, leading to reduced SOM and litter decomposition in riparian areas and ii) by mineral sediments and organic matter deposition and accumulation by erosion from adjacent slopes covered by plantations. We conclude that riparian areas are more resilient in terms of soil C storage towards land-use change than well-drained areas because of sediment deposition and reduced oxygen availability. On this basis, we developed a conceptual model of the effects of land-use change and various ecotone characteristics on SOM mineralization in the top- and subsoil of riparian areas. [ABSTRACT FROM AUTHOR]