Soil Organic Carbon Mineralization as Affected by Cyclical Temperature Fluctuations in a Karst Region of Southwestern China

The diurnal fluctuation in soil temperature may influence soil organic carbon(SOC) mineralization, but there is no consensus on SOC mineralization response to the cyclical fluctuation in soil temperature. A 56-d incubation experiment was conducted to investigate the effects of constant and variable temperatures on SOC mineralization. Three soils were collected from the karst region in western Guizhou Province, southwestern China, including a limestone soil under forest, a limestone soil under crops and a yellow soil under crops. According to the World Reference Base(WRB) classification, the two limestone soils were classified as Haplic Luvisols and the yellow soil as a Dystric Luvisol. These soils were incubated at three constant temperatures(15, 20 and 25?C) and cyclically fluctuating temperatures(diurnal cycle between 15 and 25?C). The results showed that the 56-d cumulative SOC mineralized(C56)at the fluctuating temperatures was between those at constant 15 and 25?C, suggesting that the cumulative SOC mineralization was restricted by temperature range. The SOC mineralization responses to the fluctuating temperatures were different among the three soils, especially in contrast to those at constant 20?C. Compared with constant 20?C, significant(P < 0.05) decreases and increases in C56 value were found in the limestone soil under forest and yellow soil under crops at the fluctuating temperatures, respectively. At the fluctuating temperatures, the forest soil with lower temperature coefficient Q10(the relative change in SOC mineralization rate as a result of increasing the temperature by 10?C) had a significantly(P < 0.05) lower SOC mineralization intensity than the two cropland soils. These indicated that differences in temperature pattern(constant or fluctuating) could significantly influence SOC mineralization, and SOC mineralization responses to the fluctuating temperatures might be affected by soil characteristics.Moreover, the warmer temperatures might improve the ability of soil microbes to decompose the recalcitrant SOC fraction, and cyclical fluctuations in temperature could influence SOC mineralization through changing the labile SOC pool size and the mineralization rate of the recalcitrant SOC in soils.