Managing soils to increase organic carbon storage presents a potential opportunity to mitigate and adapt to global change challenges, while providing numerous co‐benefits and ecosystem services. However, soils differ widely in their potential for carbon sequestration, and knowledge of biophysical limits to carbon accumulation may aid in informing priority regions. Consequently, there is great interest in assessing whether soils exhibit a maximum capacity for storing organic carbon, particularly within organo–mineral associations given the finite nature of reactive minerals in a soil. While the concept of soil carbon saturation has existed for over 25 years, recent studies have argued for and against its importance. Here, we summarize the conceptual understanding of soil carbon saturation at both micro‐ and macro‐scales, define key terminology, and address common concerns and misconceptions. We review methods used to quantify soil carbon saturation, highlighting the theory and potential caveats of each approach. Critically, we explore the utility of the principles of soil carbon saturation for informing carbon accumulation, vulnerability to loss, and representations in process‐based models. We highlight key knowledge gaps and propose next steps for furthering our mechanistic understanding of soil carbon saturation and its implications for soil management.

To better understand the impact of converting native forests to intensively managed plantations on soil carbon (C) and nitrogen (N) dynamics in subtropical China, we examined the seasonal patterns of water-soluble organic C (WSOC) and N (WSON) concentrations in soils in Chinese chestnut (Castanea mollissima Blume) (CF) and bamboo (Phyllostachys praecox C.D. Chu & C.S. Chou) plantation forests (BF) and adjacent native evergreen broadleaf forests (NF) in Ling-long Mountain, Zhejiang Province, China. The plantations were disturbed through surface soil removal and were fertilized and/or mulched, from which economic products (such as nuts and bamboo shoots) were annually harvested. We found that WSOC and WSON had large seasonal variations and were lower in the warmer than in the colder season. Average WSOC concentrations followed the order of BF (58.6) > NF (35.1) > CF (18.1 mg C kg^-1), a pattern mainly caused by mulching in BF in winter and the removal of surface soil in CF. Soil total C and N followed the order of BF > NF > CF. The extensive inorganic and organic fertilizer application in BF caused WSON concentrations to be 21 and 14 times higher than those in NF and CF, respectively. Conversion of native forests to plantations lowered soil WSOC:WSON and soil C:N ratios. The seasonal dynamics of WSOC:SOC (soil organic C) and WSON/TN ratios followed the same patterns of WSOC and WSON, respectively. The impacts of forest types on WSOC/SOC ratio, which is a measure of the quality of organic matter, were dependent on seasonal changes of management practices and/or tree growth. Nevertheless mean annual WSON/TN ratios of BF and CF were 2 and 12 times that of NF, indicating that a greater proportion of the total soil N pool became solubilized in the intensively managed plantations. We conclude that land-use conversion and associated management practices had a profound impact on WSOC, WSON, and total C and N concentrations in the studied forest soils in subtropical China.Key words: Forest management, water-soluble organic C, water-soluble organic N, WSOC/WSON ratio