Responses of various organic carbon pools to elevated temperatures in soils.

Soil organic carbon (SOC) and its composition may be vulnerable to the effects of microbial degradation and various environmental stresses. Hitherto, the responses of various SOC pools to warming have rarely been explored. In this study, an incubation experiment was performed with top soils (0-20 cm) from Alfisol and Ultisol at three temperatures (15, 30 and 45 °C). Warming significantly decreased the contents of SOC, particulate organic carbon (POC), mineral associated organic carbon (MAOC) and iron bound organic carbon (Fe-OC) to different degrees. However, the proportion of MAOC and Fe-OC to SOC increased by 3.6-13.3 % and 3.8-7.3 %, respectively, with rising temperature, suggesting that the temperature response of MAOC and especially Fe-OC mineralization is lower than other SOC pools. From the analysis of the Fe-OC structure by various spectroscopic techniques, it was found that elevated temperature increased the proportion of aromatic C but decreased that of aliphatic C to Fe-OC. Soil pH, identified as the most important environmental variable for controlling Fe-OC chemical structure by Mantel test, exhibited a significant negative correlation with aliphatic Fe-OC and positive correlation with aromatic Fe-OC. Synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy affirmed the higher binding strength of aromatic C with Fe oxides than aliphatic C in both soils. In addition, elevated temperature induced the increase and decrease of K-strategy bacteria and r-strategy bacteria, respectively, indicating warming slowed the bacterial growth, which could produce less necromass carbon for the association of Fe oxides and caused the decrease in Fe-OC. In summary, warming-induced changes in pH and microbial community structure can lead to a decrease in Fe-OC content, whereas the increased proportions confirmed that Fe-OC remains the most stable OC pool facing with short-term soil warming. These findings are helpful for better understanding the importance of soil minerals, especially Fe oxides, in the regulation of soil C sequestration under the context of climate change.