Effect of straw mulching on soil respiration and its' temperature sensitivity under different crop rotation systems

Soil respiration is the second largest source of terrestrial carbon (C) flux between the atmosphere and the terrestrial ecosystems. It is critical for regulating global soil C dynamics. As soil temperature and soil moisture would exert stronger effects on soil respiration in the future, a thorough understanding of the response of soil microbes to temperature change can provide a novel method of studying the effects of drought on soil respiration and of predicting drought-induced changes in future terrestrial C cycle. Temperature sensitivity of soil respiration can explain the relationship between soil respiration and soil temperature. The objective of this study was to explore the effects of straw mulch on the linkages between the changes in soil respiration and temperature. The study aimed to lay the basis of C cycle process in agro-ecosystems in Yangling, Shaanxi province. To that end, a 2-year field experiment (October 2012 to September 2014) was conducted to study the linkages under different crop rotation systems. It included two treatments of no straw (NS) and straw mulch (SM) in winter wheat-summer maize rotation and winter wheat-summer soybean rotation systems. Soil respiration rate, temperature, and moisture were analyzed under different crop rotation systems. In addition, the Q₁₀ (with Q₁₀ value as the multiplier in determining soil respiration rate after temperature increase of 10 ℃) was used to determine the effect of soil temperature change on soil respiration. It was noted that SM significantly (P < 0.05) increased soil respiration rate during crop growth period. Mean soil respiration rate and cumulative soil respiration during crop growth period significantly increased under straw mulch (P < 0.01). The order of mean soil respiration rate under various crops was as follows: maize (3.401-4.810 μmol·m^-2·s^-1) > soybean (3.390-3.762 μmol·m^-2·s^-1) > wheat (2.673-3.141 μmol·m^-2·s^-1). Then the order of cumulative soil respiration among different rotations was as follows: wheat-maize34.68-40.81 t(CO₂)·hm^-2 > wheat-soybean30.04-33.86 t(CO₂)·hm^-2. In addition, soil temperature varied significantly (P < 0.05) among different crops. Particularly, soil temperature under wheat-maize rotation system was higher than that under wheat-soybean rotation system during the growth stage of wheat. Soil temperature at 5 cm soil depth in maize field was higher than that in soybean field during the summer of 2014. It was noted that SM treatment was a major regulator of soil temperature — significantly increasing it in winter and then significantly decreasing it in spring and summer. Moreover, mean soil moisture content in the 0-30 cm soil layer was significantly higher under SM treatment than under NS treatment during the dry season. Further, mean soil moisture content in the 0-30 cm layer during wheat growth period varied significantly among different crop rotation systems, which was associated with root characteristics under crop rotation. The mean soil moisture content for the 0-30 cm soil layer during maize growth period was significantly higher than that during soybean growth period. Soil temperature at the 5-10 cm soil layer was positively correlated with soil respiration. However, soil moisture at the 0-30 cm soil layer was not significantly correlated with soil respiration. Changes in soil temperature at 5 and 10 cm soil depths were respectively 64.6%-67.3% and 51.5%-59.6% explained by the variance in soil respiration. In this study, Q₁₀ varied within 1.70-2.01 across different crop rotation systems and was significantly higher under wheat-maize than wheat-soybean rotation system. In addition, Q₁₀ was significantly higher under SM treatment than under NS treatment. Therefore, SM treatment was more advantageous in terms of the ability to effectively reduce temperature sensitivity of soil respiration and to accurately predict soil moisture and soil heat conditions.