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

Abstract

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.