Study area
The study area (91°21′E, 29°41′N, 3688 m above sea level) is located at the Lhasa Agro-ecosystem Research Station, Tibet Autonomous Region in China. Mean annual air temperature is 7.9 °C and mean annual precipitation is around 425 mm, with more than 90 % concentrated in the period from June to September (He et al. 2011). The annual air temperature was 8.5 °C and annual precipitation was 642.4 mm in 2014. That is, it is a warmer and wetter year (2014).
Experimental design
The experimental soils have been used for crop planting since 1970s. Infrared heaters were used to increase temperature during the whole study period from May 26 to September 14 in 2014. There were three warming treatments with three replicates: the control (CK), low (1000 W) and high (2000 W) warming treatments with a total of nine 2 m × 2 m experimental plots. A 165 cm × 15 cm infrared heater (Kalglo Electronics Inc., Bethlehem, PA, USA) was suspended approximately 1.7 m above the ground in the center of each 2 m × 2 m plot. There was approximately 6–7 m distance between plots.
The highland barley was sown in May 26, 2014 and harvested in September 14, 2014. There was approximately 0.25 m between seeding rows and the seeding was 18.75 g m−2. There were no highland barley outside the 2 m × 2 m plot and there were no other vegetation types within each 2 m × 2 m plot. There were approximately 1150 plants per 2 m × 2 m plot.
Soil temperature and soil moisture monitor
Soil temperature (T
s
) and SM sensors were set a depth of 0.05 m in the center of each plot. For each plot, the two sensors were connected to a data logger (HOBO weather station, Onset Computer, Bourne, MA, USA). The measurements were taken every minute, and the data was processed to provide an average every 5 min.
Soil respiration measurements
A CO2 flux system (LI-8100, LI-COR Biosciences, Lincoln, NE, USA) with a 20 cm diameter opaque survey chamber was used to measure R
s
(Fu et al. 2014). A polyvinyl chloride (PVC) collar (diameter, 20 cm; height, 5 cm) was inserted about 2–3 cm into the soil in the center of each plot (the soil temperature and soil moisture sensors were just beneath the PVC collar) in May 2014. The PVC collar was left the same place during the whole study period. The soil temperatures and moistures were consistent with those beneath the chambers. We started to measure R
s
in May 2014. The opaque survey chamber was manually mounted on the PVC collar in each plot for the measurement of R
s
. Daily cycles of R
s
measurements were generated from 8:00 to 8:00 on June 5–6 (approximately 3–4 days after sprouting), July 26–27 (approximately flag leaf stage), August 6–7 (approximately blooming stage), August 26–27 (approximately waxy ripeness stage) and September 6–7 (from yellow ripeness stage to full ripeness stage). The measuring interval was 2 and 3 h during the daytime (8:00–20:00) and nighttime (20:00–8:00), respectively.
Statistical analysis
A repeated-measures ANOVA with experimental warming as the between subject factor and measuring date and time as the within subject factors was conducted for R
s
. Duncan multiple comparisons were performed among the three warming treatments.
Exponential regression analyses were conducted between R
s
and T
s
, whereas linear regression analyses were conducted between R
s
and SM for each treatment. For each treatment, a stepwise multiple regression analysis was used to analyze the relationships between R
s
and T
s
and SM, before which natural-logarithm transformations were made for R
s
and SM.
We analyzed the sensitivity of R
s
to soil temperature for each treatment using all measurement data according to
$$R_{s} = ae^{{bT_{s} }} ,$$
(1)
where a is the intercept of R
s
when T
s
is 0 °C (i.e. the R
s
value when T
s
is 0 °C), and b reflects the temperature sensitivity of R
s
(Shen et al. 2015). The b values were used to calculate the respiration quotient (Q
10)
$$Q_{10} = \frac{{R_{t + 10} }}{{R_{t} }} = e^{10b}$$
(2)
where t is a given reference soil temperature, R
t+10 and R
t
are the R
s
values when soil temperature is t + 10 and t °C, respectively.
To decrease the disturbance of SM on Q
10 of R
s
, we also chose data to compare the Q
10 of R
s
among the three treatments by the two following rules: (1) there were no correlations between R
s
and T
s
when SM <0.17 m3 m−3 for the high warming treatment. The R
s
was obviously suppressed when SM <0.17 m3 m−3 for the low warming treatment. Therefore, only measuring data when SM >0.17 m3 m−3 were used; and (2) measuring date and time was consistent among the three treatments. That is, 26 groups of measured R
s
, T
s
and SM were used for each treatment.
All the statistical analyses were performed using the SPSS software (version 16.0; SPSS Inc., Chicago, IL, USA).