Inorganic carbon: an overlooked pool in global soil carbon research

Soils are one of the major players in the global carbon (C) cycle and climate change by functioning as a sink or a source of atmospheric carbon dioxide (CO2). The largest terrestrial C reservoir in soils comprises two main pools: organic (SOC) and inorganic C (SIC), each having distinct fates and functions but with a large disparity in global research attention. This study quantified global soil C research trends and the proportional focus on SOC and SIC pools based on a bibliometric analysis. Research on soil C pools started in 1905 and has produced over 42,000 publications (> 1.6 million citations). Although the global C stocks down to 2 m depth are nearly the same for SOC and SIC, the research has dominantly examined SOC (> 96% of publications and citations) with a minimal share on SIC (< 4%). Approximately 39% of the soil C research was focused on climate change. Despite poor coverage and publications, the climate change-related research impact (citations per document) of SIC studies was higher than that of SOC. Machine learning, biochar, soil properties, and climate change were the recent top trend topics for SOC research (2018-2022), whereas soil acidification, organic C, climate change, and Holocene were recent trends for SIC. SOC research was contributed by 150 countries compared to 85 for SIC. As assessed by publications, soil C research was mainly concentrated in a few countries, with only 10 countries accounting for 75% of the research. China and the USA were the major producers (44%), collaborators (36%), and funders of soil C research. SIC is a long-lived soil C pool with a turnover rate of more than 1000 years in natural ecosystems but intensive agricultural practices have accelerated SIC losses, making SIC an important player in global C cycle and climate change. The lack of attention and investment towards SIC research could jeopardize the ongoing efforts to mitigate climate change impacts to meet the 1.5-2.0 oC targets under the Paris Climate Agreement of 2015. This study calls for expanding the research focus on SIC and including SIC fluxes in C budgets and models, without which the representation of the global C cycle is incomplete.