Management extensification in oil palm plantations reduces SOC decomposition

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 CO2 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 CO2 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.