Abstract:

Aims Winter soil respiration plays a crucial role in terrestrial carbon cycle, which could lose carbon gained in the growing season. With global warming, the average near-surface air temperatures will rise by 0.3 to 4.8 °C. Winter is expected to be warmer obviously than other seasons. Thus, the elevated temperature can significantly affect soil respiration. The coastal wetland has shallow underground water level and is affected by the fresh water and salt water. Elevated temperature can cause the increase of soil salinity, and as a result high salinity can limit soil respiration. Our objectives were to determine the diurnal and seasonal dynamics of soil respiration in a coastal wetland during the non-growing season, and to explore the responses of soil respiration to environmental factors, especially soil temperature and salinity.
Methods A manipulative warming experiment was conducted in a costal wetland in the Yellow River Delta using the infrared heaters. A complete random block design with two treatments, including control and warming, and each treatment was replicated each treatment four times. Soil respiration was measured twice a month during the non-growing season by a LI-8100 soil CO₂ efflux system. The measurements were taken every 2 h for 24 h at clear days. During each soil respiration measurement, soil environmental parameters were determined simultaneously, including soil temperature, moisture and salinity.
Important findings The diurnal variation of soil respiration in the warming plots was closely coupled with that in the control plots, and both exhibited single-peak curves. The daily soil respiration in the warming was higher than that in the control from November 2014 to January 2015. Contrarily, from March to April 2015. During the non-growing seasons, there were no significant differences in the daily mean soil respiration between the two treatments. However, soil temperature and soil salt content in the warming plots were significantly higher than those in the control plots. The non-growing season was divided into the no salt restriction period (November 2014 to middle February 2015) and salt restriction period (middle February 2015 to April 2015). During non-growing season, soil respiration in the warming had no significant difference compared with that in control. During the no salt restriction period, soil respiration in the warming was 22.9% (p < 0.01) greater than the control when soil temperature at 10 cm depth in warming was elevated by 4.0 °C compared with that in control. However, experimental warming decreased temperature sensitivity of soil respiration (Q₁₀). During salt restriction period, soil warming decreased soil respiration by 20.7% compared with the control although with higher temperature (3.3 °C), which may be attributed to the increased soil salt content (Soil electric conductivity increased from 4.4 ds·m^-1 to 5.3 ds·m^-1). The high water content can limit soil respiration in some extent. In addition, the Q₁₀ value in the warming had no significant difference compared with that in control during this period. Therefore, soil warming can not only increase soil respiration by elevating soil temperature, but also decrease soil respiration by increasing soil salt content due to evaporation, which consequently regulating the soil carbon balance of coastal wetlands.

Key words: elevated temperature, soil respiration, non-growing season, soil salt content, coastal wetland, Yellow River Delta