Abstract:  The temperature sensitivity of soil respiration (Q10) is often used in exponential models to predict or interpolate soil respiration (RS). However, beyond temperature, Q10 can be influenced by factors such as soil water content, substrate availability and microbial activity, so that Q10 may vary with seasonally fluctuating conditions and processes, and differences between Q10 derived from different time scales may exist. In order to understand the variation of RS and Q10 on croplands soils, RS was measured continuously for half an hour by a multichannel automatic soil CO2 efflux system (Li-8150, USA) in a summer maize-winter wheat double cropping system from June 2018 to June 2019 in Beijing suburb. Summer maize growth season was divided into three stages (2018-06-22—2018-07-15, 2018-07-16—2018-08-15, 2018-08-16—2018-09-17), and winter wheat growth season were decomposed into five stages (2018-10-01—2018-11-22, 2018-11-23—2019-02-25, 2018-02-26—2019-04-10, 2019-04-11—2019-05-13, 2019-05-14—2019-06-17) according to the phenology. Seasonal variation of RS and Q10 values at different crop growth stages were studied, respectively, and their responses to the influence factors including soil temperature at 5cm depth, soil water content, leaf area index and aboveground biomass were analyzed in this article. Moreover, diurnal dynamics of soil respiration rate at different growth state were calculated. The main results are showed as follow: (1) the diurnal variation of the soil respiration rates in different growth stages appeared as a single-peak curve as well as the soil temperature. But soil temperature often peaked later than the half-hourly soil respiration rates at the wheat growth season. The lag time of soil temperature for each growth stages were 4.5, 5.0, 5.0, 2.5, 0.5h. The relationships between diurnal RS and soil temperature showed a clockwise nearly elliptic curve. (2) Q10 exhibited a strong seasonal variation. At seedling, jointing to tasseling, and flowering to mature stage of summer maize, Q10 values were 2.27, 6.13 and 1.28 respectively. During the whole growth period of summer maize, soil water content ranged from 19.52% to 45.43%, and an quadric curve downwards of soil moisture could explain 50% of variation for RS (P<0.05), with the threshold value being 27.84%, which came to about 83.83% of field capacity. Exponential models of soil temperature could only explain 3% of variation for RS (P>0.05), and the Q10 value obtaining from whole growth stage of summer maize was 1.29. (3) Q10 values at different growth stages of winter wheat were 4.17, 8.41, 6.57, 2.53, 1.92, respectively, negatively correlated with soil temperature (P<0.05). At the scale of whole winter wheat season, Q10 value reached to 2.50, and soil temperature was a major factor influencing RS, which explained 88% of the variation for RS (P<0.01). On one-year scale, soil temperature and soil water content explained 54% (P<0.01), 28% (P<0.05) of the variation for RS, and Q10 was 1.72. In consideration of the difference of Q10 at each stage, we found that RS may decoupled from soil temperature at higher soil water content which were closed to field capacity, influencing the applicability of Q10 value. Additionally, time scale should be ascertained with caution, for the Q10 values obtained from inappropriate scales may underestimate or overestimate the future soil respiration rates under the conditions of global warming.

Key words:  Soil respiration, Temperature sensitively, Q10, Summer maize-winter wheat