The effects of elevated CO2 and temperature on soil organic carbon and total nitrogen contents and mineralization in the 0 to 50 cm paddy soil layer were masked by different land use history

•Land use history alleviated the effects of 5-year T-FACE on dynamics of SOC and TN in 0‒50 cm paddy soil profile.•The effect of elevated CO2 on δ13C was only found in 0‒10 cm soil layer.•Significant correlations of SOC, TN, δ13C, δ15N, decomposed C and mineralized N were found in 0‒50 cm soil profiles.•Investigation of original land use is needed prior to field observation experiment in paddy soils. Global warming can accelerate soil organic matter (SOM) decomposition resulting in faster carbon (C) loss and positive climate-C feedback. Previous studies on response of SOM decomposition to climate change mainly focus on plow soil layer. However, the effects of elevated CO2 and soil warming on soil organic carbon (SOC) and total nitrogen (TN) contents and mineralization are rarely studied in subsoil layer. In this study, soil samples were collected from the 0–50-cm paddy soil layer of the Tsukuba free-air CO2 enrichment experimental site with elevated CO2 (+200 ppm) and soil warming (+2 °C), Japan, after 5-year rice growth season. The amounts of SOC, TN, δ13C, and δ15N were analyzed. A 4-week anaerobic incubation experiment was conducted to measure C decomposition and N mineralization potentials. Due to the intrinsic variation in SOC and TN contents in soil layers and fields, the effects of elevated CO2 and soil warming on C decomposition and N mineralization potentials could not be determined here. However, the effect of elevated CO2 on δ13C was only found in 0‒10-cm soil layer. In the 0–50-cm soil profiles, significant correlations were observed among SOC and TN, δ13C and δ15N, decomposed C and mineralized N, and δ13C of decomposed C. The variables associated with soil C and N pools, and dynamics showed large spatial heterogeneity within paddy field, due to variation in the original land use history. Therefore, great caution should be exercised when evaluating the effects of elevated CO2 and temperature on SOM decomposition and sequestration in paddy soil profiles. [Display omitted]