Soil respiration and its influencing factors in rice-rape rotation fields during rape growing season

Soil is a major biosphere for carbon (C) reserve, containing globally twice C as much as the atmosphere and three times as much as vegetation. After photosynthesis, soil respiration remains the second largest carbon flux in the ecosystem, accounting for 60%~90% of total ecosystem respiration. Small changes in soil respiration across large areas have been reported to produce significant effects on CO2 atmospheric concentrations. This has led to potential positive feedbacks between increasing temperature and enhanced soil respiration that in turn accelerate global warming. Therefore, soil respiration has become a critical research field in global carbon cycle. Soil respiration in specific ecosystems has been characterized in terms of magnitude, and temporal and spatial variability. Detail on soil respiration and the controlling factors have been critical for constraining ecosystem C budget and understanding soil response to global climate change. In this paper a rice-rape rotation field in Southwest China was used to study the spatial and temporal variations in soil respiration during rape growth season in November 2009 through April 2010. Soil respiration rates were measured on monthly basis using the closed chamber technique and three measurement positions (on-plant, inter-plants and inter-rows) selected. The results showed that diurnal patterns of soil respiration followed uni-humped curves. Maximum soil respiration rates appeared at 15:00 and minimum at 7:00. Soil respiration rates showed significant seasonal changes: initially declined and then increased during the experimental period. The order of daily average soil respiration rates was March 27, 2010 (293.25 mg·m^-2·h^-1) > April 17, 2010 (275.22 mg·m^-2·h^-1) > February 28, 2010 (186.25 mg·m^-2·h^-1) > January 28, 2010 (164.44 mg·m^-2·h^-1) > November 22, 2009 (140.25 mg·m^-2·h^-1) > December 20, 2009 (102.07 mg·m^-2·h^-1). There were significant spatial variations in soil respiration patterns at the plant scale. Higher soil respiration rates tended to occur near rape plants during growth season. On-plant soil respiration rates were highest (336.71 mg·m^-2·h^-1), followed by inter-plant (248.48 mg·m^-2·h^-1) and then inter-row (141.77 mg·m^-2·h^-1). Soil respiration was calculated as the sum of root and microbial respiration. The contribution of each group required thorough understanding in order to evaluate the implications of environmental changes for soil carbon cycling and sequestration. In this study, direct microbial respiration rates were observed in inter-row plots without roots. Therefore root respiration was the difference between soil respiration rates of planted and unplanted soils. Average contribution of root respiration to soil respiration was 51.03%, with a range of 25.78%~72.61%. Soil respiration rate was affected by several environmental factors. It was exponentially correlated with soil temperature, linearly correlated with root biomass, and positively correlated with soil microbial biomass carbon. Soil respiration was also readily affected by oxidized carbon and particulate organic carbon.