Soil water regulates the control of photosynthesis on diel hysteresis between soil respiration and temperature in a desert shrub land

No consensus has been reached on the causes of diel hysteresis between soil respiration (Rs) and temperature. 10 Explanations for the occurrence of hysteresis have involved both biological and physical mechanisms. The specifics of these explanations, however, tend to vary with the particular ecosystem or biome being investigated. This study examined the seasonal variation in diel hysteresis and its controlling factors in a desert-shrub ecosystem in northwest (NW) China. The study was based on continuous measurements of Rs, air temperature (Ta), temperature at the soil surface and below (Tsurf and Ts), volumetric soil water content (SWC), and photosynthesis over an entire year in 2013. Trends in diel Rs were observed to 15 vary with SWC over the growing season. Diel variations in Rs were more closely associated with Tsurf than with photosynthesis as SWC increased, leading to Rs being in phase with Tsurf, particularly when SWC > 0.08 m 3 m -3 . However, as SWC decreased below 0.08 m 3 m -3 (ratio of SWC to soil porosity = 0.26), diel variations in Rs were more closely related to variations in photosynthesis, leading to a pronounced diel hysteresis and asynchronicity between Rs and Tsurf. It was shown that SWC was responsible for regulating the relative control between photosynthesis and temperature on diel Rs, resulting in 20 seasonal variation in hysteresis. Our findings highlight the importance of biological mechanisms and the role of SWC in regulating diel hysteresis between Rs and temperature. We recommend further studies to explore the actual mechanisms involved in explaining changes in the relative contribution of autotrophic and heterotrophic respiration to total Rs. These studies may help elucidate the role of SWC in affecting seasonal variation in diel hysteresis.

Over decades of research, two main processes have been reported to relate to diel hysteresis between Rs and Ts.One is associated with the physical processes of heat and gas transport in soils (Vargas and Allen, 2008;Phillips et al., 2011;Zhang et al., 2015).Generally, soil CO2 fluxes are measured at the soil surface, and are related to temperatures in the soil.Transport of CO2-gas to the soil surface takes time to occur, which may cause delays to appear in observed respiration rates, causing hysteretic loops to form between Rs and Ts (Zhang et al., 2015).The other is associated with the biological process of photosynthate supply (Tang et al., 2005;Kuzyakov and Gavrichkova, 2010;Vargas et al., 2011;Wang et al., 2014).Beyond the control of temperature, soil CO2 fluxes have been associated with plant photosynthesis.Photosynthesis usually peaks at midday (e.g., 11:00-13:00), providing substrate for belowground roots and rhizosphere-microbe respiration, but oscillates out of phase with Ts, usually peaking in the afternoon (e.g., 14:00-16:00).Such influences of current photosynthesis could lead to the formation of hysteretic loops in the relationship between Rs and Ts.These studies highlight the need to consider the inherent role of photosynthesis for a more accurate interpretation of Rs (Tang et al., 2005;Kuzyakov and Gavrichkova, 2010;Vargas et al., 2011).Physical and biological processes that relate to substrates and production-transport of carbon (C) in plants and soils are not mutually exclusive and both likely play crucial roles in affecting diel variation in Rs (Stoy et al., 2007;Phillips et al., 2011;Zhang et al., 2015;Song et al., 2015a, b).
Diel hysteresis between Rs and Ts has been shown to vary seasonally with soil water content (SWC; Tang et al., 2005;Riveros-Iregui et al., 2007;Carbone et al., 2008;Vargas and Allen, 2008;Ruehr et al., 2009;Wang et al., 2014).However, the influences of SWC on diel hysteresis are not uniform.Based on the Millington-Quirk model, high SWC blocks CO2-gas and thermal diffusion (Millington and Quirk, 1961), resulting in large hysteresis loops (Riveros-Iregui et al., 2007;Zhang et al., 2015).In contrast, other studies have reported that low SWC and high water vapor pressure deficits (VPD) can promote partial stomata closure, which leads to higher photosynthesis in the morning (e.g., 9:00-10:00) and supressed photosynthesis in mid-afternoon, leading to pronounced hysteresis during dry periods (Tang et al., 2005;Vargas and Allen, 2008;Carbone et al., 2008;Wang et al., 2014).Clearly to understand the causes of diel hysteresis, the role of SWC needs to be closely evaluated.
Drylands cover a quarter of the earth's land surface and play an important role in the global C cycle (Safriel and Adeel, 2005;Austin, 2011;Poulter et al., 2014).Many studies in forest ecosystems are based on the application of physical soil CO2 and heat transport models and evaluate the influences of SWC on CO2-gas and thermal diffusion (Riveros-Iregui et al., 2007;Phillips et al., 2011;Zhang et al., 2015).In general, many of these studies conclude that diel hysteresis is the result of physical processes alone.Few studies have evaluated the causes of diel hysteresis in drylands.Currently, it is not clear to what degree physical and biological processes control hysteresis in drylands.
Drylands are characterized with low productivity.As weak organic C-storage pools (West et al., 1994;Lange, 2003), drylands are noted for their large contribution of autotrophic production of CO2.The autotrophic component of Rs occurs as a direct consequence of root respiration, which is firmly coupled (within several hours) to recent photosynthesis (Liu et al., 2006;Baldocchi et al., 2006;Högberg and Read, 2006;Bahn et al., 2009;Kuzyakov and Gavrichkova, 2010).Consequently, photosynthesis may govern the level of variation in asynchronicity between Rs and Ts in drylands.In drylands, especially in desert ecosystems characterized by sandy soils with high soil porosity, the influence of SWC on gas diffusion is likely nominal.As a rule, most of the available water is used directly in sustaining biological activity in drylands (Noy-Meir, 1973).
Under drought conditions, stomata closure in plants at midday reduces water losses, resulting in a corresponding suppression of photosynthesis (Jia et al, 2014).Such changes in diel patterns of photosynthesis likely result in modifications of patterns in Rs, leading to hysteresis between Rs and Ts.Soil water content likely regulates photosynthesis and, in so doing, causes hysteresis between Rs and Ts to vary over the growing season.
In this study, we hypothesize that: (1) photosynthesis has a high degree of control in the formation of hysteretic loops between Rs and Ts; and (2) SWC regulates this control and its variation over the growing season.The main objectives of this research were to: (1) assess biological controls on diel hysteresis between Rs and Ts; (2) explore the causes that lead to variation in seasonal variation in diel hysteresis; and (3) understand SWC's role in influencing hysteresis.To undertake this work, we measured Rs, SWC, Ts, and photosynthesis in a dominant desert-shrub on a continuous basis for 2013.

Site description
The study was conducted at Yanchi Research Station of Beijing Forestry University, Ningxia, northwest China (37°42'31" N, 107°13'37" E, 1550 m a.s.l).The station is located at the southern edge of the Mu Us desert in the transition between the arid and semi-arid climatic zones.Based on 51 years of data  from the Meteorological Station at Yanchi, the mean annual air temperature at the station was 8.1 o C and the mean annual total precipitation was 292 mm (ranging between 250 to 350 mm), 63% of which fell in late summer (i.e., July-September; Wang et al., 2014;Jia et al., 2014).Annual potential evaporation was on average 5.5 kg m -2 d -1 (Gong et al., 2016).The soil at the research station was of a sandy type, with a bulk density of 1.6 g cm -3 .The total soil porosity within 0-2 and 5-25 cm depths was 50% and 38%, respectively.Soil organic matter, soil nitrogen, and pH were 0.21-2.14g kg -1 , 0.08-2.10g kg -1 , and 7.76-9.08,respectively (Wang et al., 2014;Jia et al, 2014).The vegetation was regenerated from aerial seeding applied in 1998 and is currently dominated by a semishrub species cover of Artemisia ordosica, averaging about 50-cm tall with a canopy size of about 80 cm × 60 cm (for additional site description, consult Jia et al. 2014and Wang et al. 2014, 2015).

Soil respiration and photosynthesis measurement
Two permanent polyvinyl chloride soil collars were initially installed on a small fixed sand dune in March, 2012.Collar dimensions were 20.3 cm in diameter and 10 cm in height, with 7 cm inserted into the soil.One collar was set on bare land with an opaque chamber (LI-8100-104, Nebraska, USA) and the other over an Artemisia ordosica plant (~10 cm tall) with a transparent chamber (LI-8100-104C).Soil respiration (µmol CO2 m -2 s -1 ) was directly estimated from CO2-flux measurements obtained with the opaque-chamber system.Photosynthetic rates (µmol CO2 m -2 s -1 ) of the selected plants were determined as the difference in CO2 fluxes obtained with the transparent and opaque chambers.
Continuous measurements of CO2 fluxes (µmol CO2 m -2 s -1 ) were made in situ with a Li-8100 CO2-gas analyzer and a LI-8150 multiplexer (LI-COR, Nebraska, USA) connected to each chamber.Instrument maintenance was carried out biweekly during the growing season, including removing plant-regrowth in the opaque-chamber installation, and cleaning to avoid blackout conditions associated with the transparent chamber.Measurement time for each chamber was 3 minutes and 15 seconds, including a 30-second pre-purge, 45-second post-purge, and 2-minute measurement period.

Measurements of temperatures, soil water content and other environmental factors
Hourly soil temperature (Ts, o C) and volumetric soil water content (SWC, m 3 m -3 ) at a 10-cm depth were measured simultaneously about 10 cm from the chambers using a LI-8150-203 temperature and ECH2O soil-moisture sensor (LI-COR, Nebraska, USA; see Wang et al., 2014).Other environmental variables were recorded every half hour using sensors mounted on a 6-m tall eddy-covariance tower approximately 800 m from our soil CO2-flux measurement site.Air temperature (Ta, o C) was measured with a thermohygrometer (HMP155A, Vaisala, Finland).Soil-surface temperature (Tsurf, o C) was measured with an infrared-emission sensor (Model SI-111, Campbell Scientific Inc., USA).Incident photosynthetically active radiation (PAR) was measured with a light-quantum sensor (PAR-LITE, Kipp and Zonen, the Netherlands) and precipitation (PPT, mm), with three tipping-bucket rain gages (Model TE525MM, Campbell Scientific Inc., USA) placed 50 m from the tower (see Jia et al., 2014).

Data processing and statistical analysis
In this study, CO2-flux measurements were screened by means of limit checking, i.e., hourly CO2-flux data < -30 or > 15 μmol CO2 m -2 s -1 were considered to be anomalous as a result of, for instance, gas leakage or plant damage by insects, and removed from the dataset (Wang et al., 2014(Wang et al., , 2015)).After limit checking, hourly CO2 fluxes greater than three times the standard deviation from the calculated mean of 5 days' worth of flux data were likewise removed.Quality control and instrument failure together resulted in 5% loss of hourly fluxes for all chambers, 4% for temperatures, and 8% for SWC (Fig. 1).Differences in mean annual Ts and SWC between the two chambers were 0.01 o C and 0.003 m 3 m -3 , respectively.
The Q10-function (e.g., Eq. 1) was used here to describe the response of Rs to temperature.Earlier studies have shown strong correlation between basal rate of Rs and photosynthesis (Irvine et al., 2005;Sampson et al., 2007).Response of Rs to changes in photosynthesis was, in turn, characterized as a linear function (Eq.2).Interaction between photosynthesis and temperature on Rs was conveyed through Eq. 3. The instantaneous relative importance (RI) of photosynthesis and temperature on Rs over the growing season was calculated with a correlation-based ratio (see Eq. 4).The importance of photosynthesis on Rs increases with a corresponding increase in RI: (1) (2) (3) where R10 is the respiration at 10 o C, Q10 is the temperature sensitivity of respiration, T is temperature, P is photosynthesis (µmol CO2 m -2 s -1 ), a, b, and c are regression coefficients, and ρp and ρt are the correlation coefficients between photosynthesis and Rs and temperature and Rs, respectively.
Pearson correlation analysis was used to calculate the correlation coefficient between temperature or photosynthesis and Rs.Cross-correlation analysis was used to estimate hysteresis in the relationship between temperature and Rs and photosynthesis-and Rs.We used root mean squared error (RMSE) and the coefficient of determination (R 2 ) as criteria in evaluating function performance.To evaluate seasonal variation in diel hysteresis, the mean monthly daily cycles of Rs, Ta, Tsurf, Ts, and photosynthesis were generated by averaging their hourly means at a given hour over a particular month (Table 1).Exponential and linear regression was used to evaluate the influence of SWC on the control of photosynthesis on temperature-Rs hysteresis.Likewise, influences of SWC on diel hysteresis was examined during a wet month with high rainfall and adequate SWC (July, PPT = 117.9mm) and a dry month with low rainfall and inadequate SWC (August, PPT = 10.9 mm; Wang et al., 2014).In order to evaluate the influence of photosynthesis on diel hysteresis in the temperature-Rs relationship, we compared the time lag (in hours) between measured and modeled Rs by means of Eq.'s 1 through 3 with a one-day moving window and a one-day time step over the growing season (April to October).Modeled Rs was calculated using the fitted parameters of each function and the measured hourly Tsurf and photosynthesis for each day.All statistical analyses were performed in MATLAB, with a significance level of 0.05 (R2010b, Mathworks Inc., Natick, MA, USA).
Diel patterns of monthly mean Rs were similar to those of Tsurf during the wet month and similar to those of photosynthesis during the dry month (Fig. 2g, h).During the wet month (July), monthly mean diel Rs was out of phase with photosynthesis, but in phase with Tsurf (Fig. 2g).Soil respiration peaked at 16:00 PM, exhibiting similar timing to Tsurf (i.e., 15:00 PM), but four hours later than photosynthesis (peaking at 12:00 PM; Fig. 2g).During the dry month (August), diel Rs was generally in phase with photosynthesis, but out of phase with Tsurf (Fig. 2h).Both photosynthesis and Rs plateaued between 10:00 AM-16:00 PM, whereas Tsurf peaked at 15:00 PM (Fig. 2h).

Control of photosynthesis and temperature on diel soil respiration
Among temperatures at the three levels, Tsurf correlated the strongest with Rs, due to the high R 2 's with monthly mean diel Rs (Table 1).Over the growing season, monthly mean diel Rs correlated fairly well with photosynthesis (Table 1).The response of Rs to temperature and photosynthesis was shown to be affected by SWC (Table 2, Fig. 3).During the wet month, Tsurf alone explained 97% of the variation in diel Rs (via Eq. 1), whereas photosynthesis explained 67% of the variation (Table 2, Fig. 3a).However, during the dry month, photosynthesis explained 88% of the variation in diel Rs (via Eq. 2), whereas Tsurf explained 76% of the variation (Fig. 3b, Table 2).Irrespective of dry or wet periods, Tsurf and photosynthesis together explained over 90% of the diel variation in Rs (via Eq. 3; see Fig 3 and Table 2).On the whole, RI varied as a function of SWC, decreasing whenever SWC increased (Fig. 4).

Effects of soil water content and photosynthesis on diel hysteresis in temperature-Rs relationship
During the wet month, hysteresis was not observed to occur in the monthly mean Tsurf -Rs relationship, whereas two-hour lags were found to occur in the photosynthesis-Rs relationship (Table 1; Fig. 3a).During the dry month, the opposite was observed, where one-hour lags were found to occur in the Tsurf -Rs relationship (Table 1, Fig. 3b).Over the growing season, Tsurf lagged behind Rs by about 0-4 hours (Fig. 5b), and Rs lagged behind photosynthesis by about the same amount (Fig. 5c).
This led to time lags between measured and modeled Rs regardless of the variable, Tsurf or photosynthesis, resulting in about 26% of the days of the growing season (accounting for 184 days, in total) having no time lag (Fig. 5e, f).However, taking into account both Tsurf and photosynthesis as input variables in the definition of Rs (via Eq. 3), time lags between measured and modeled Rs were mostly eliminated (Fig. 5a, d), with 84% of the days of the growing season displaying no time lag.

Degree of control of photosynthesis on diel hysteresis
In our study, we found that the diurnal pattern in temperature (Ta, Tsurf, and Ts) lagged behind Rs by 0-4 hours, which resulted in a counterclockwise loop in the relationship between Rs and temperature.Although the magnitude of hysteresis between Rs and temperature differed among the three temperature measurements, their seasonal variation was generally uniform.Among the temperature measurements, Tsurf was more closely related to diel Rs, resulting in weaker hysteresis.Magnitude of hysteresis between Rs and temperature was comparable to those in other plant systems, e.g., 3.5-5 h in a boreal aspen stand (Gaumont-Guay et al., 2006) and 0-5 h in a Chinese pine plantation (Jia et al., 2013).However, the direction of hysteresis was unlike that reported by Phillips et al. (2011), who had reported Rs lagging behind soil temperature.
In general, transfer of heat (downward) and gases (upward) through the soil complex by simple diffusion would take time to occur.Increased SWC would serve to impede this transfer (Millington and Quirk, 1961).If physical processes alone controlled hysteresis, you would expect Rs to lag behind Tsurf and hysteresis to increase with increasing SWC.However, such rationalization is not supported by our observations, which show Tsurf to lag behind Rs and hysteresis to decrease with increasing SWC.As a result, physical processes alone cannot account for the observed patterns in hysteresis between Rs and temperature.Combining photosynthesis and Tsurf as explanatory variables of Rs (via Eq. 3), we found 84% of the days over the growing season had no observable lag between measured-and modeled-Rs, relative to 27% of the days when Tsurf alone was used (associated with to Eq. 2), suggesting that photosynthesis has an important role in governing hysteresis in desert shrubland.Along with other studies, including those of Tang et al. (2005), Vargas and Allen (2008), Carbone et al. (2008), Kuzyakov and Gavrichkova (2010), and Wang et al. (2014), our findings provide increasing evidence of the role of photosynthesis in regulating diel hysteresis between Rs and temperature.

Photosynthesis control of soil respiration and diel hysteresis
The 0-4 h lag between Rs and photosynthesis observed are consistent with those observed in earlier studies, e.g., 0-4 h lag between ecosystem-level photosynthesis and Rs in a coastal wetland ecosystem (Han et al., 2014) and 0-3 h lag between plant photosynthesis and Rs in a steppe ecosystem (Yan et al., 2011).Short time lags suggest rapid response between recent photosynthesis and Rs (Kuzyakov and Gavrichova, 2010).This response is significantly faster than suggested in earlier studies, when approached from an isotopic or canopy/soil flux-based methodology (Howarth et al., 1994;Mikan et al., 2000;Jonson et al., 2002;Högberg et al., 2008;Kuzyakov and Gavrichova, 2010;Mencuccini and Hölttä , 2010;Kayler et al., 2010;Han et al., 2014).
According to the "goodness-of-fit" of Eq. 3 to the field data, the time lag between diel photosynthesis and Rs was likely caused by variations in temperature, regardless of SWC.Photosynthesis provide substrates to roots and rhizosphere microbes (Tang et al., 2005;Kuzyakov and Gavrichkova, 2010;Vargas et al., 2011;Han et al., 2014).Temperature directly drives enzymatic kinetics of respiratory metabolism in organisms ( Van't Hoff, 1898;Lloyd and Taylor, 1994).Photosynthesis is directly driven by radiation (specifically, photosynthetically active radiation).Temperature is also driven by radiation, but through heating of the surface and subsequent air and soil layers.Thus, diel patterns in temperature continuously lagged behind those of photosynthesis by a few hours (as indicated in Fig. 2).The interactions between photosynthesis and temperature led Rs to lag behind photosynthesis, but temperature lagged behind Rs (Fig. 2).This sequence of events may explain the difference in the direction of hysteresis observed here, in contrast to that reported in Phillips et al. (2011).Such explanation is different from the explanations for forest ecosystems, where the transport of photosynthates and influence of turgor and osmotic pressure may be responsible for the specific coupling observed between current photosynthesis and Rs (Steinmann et al., 2004;Högberg et al., 2008;Hölttä et al., 2006Hölttä et al., , 2009;;Mencuccini and Hölttä , 2010).Variations in coupling dynamics may occur because of differences in vegetation height among ecosystems (Kuzyakov and Gavrichova, 2010;Mencuccini and Hölttä , 2010).Unlike forest ecosystems, low-statured vegetation in shrub systems (~0.5 m), may elicit a few minutes of delay in the transportation of photosynthates and influence of turgor and osmotic pressure (Kuzyakov and Gavrichkova, 2010).Such small time lags cannot be easily identified in hourly measurements, resulting in an apparent temperature-dominated control of photosynthesis and Rs.

Influences of soil water content on seasonal variation in diel hysteresis
Diel Rs varied consistently with Tsurf, with no observable signs of hysteresis, when SWC > 0.08 m 3 m -3 .However, as SWC decreased from this value, diel Rs varied more closely with photosynthesis, leading to increased diel hysteresis between Rs and Tsurf.These results suggest that SWC played a more important role in regulating the relative control of photosynthesis and temperature on diel Rs over the growing season, supporting our second hypothesis.
A possible explanation for SWC regulating hysteresis might be associated with changes in substrate supply.During the wet period with SWC > 0.08 m 3 m -3 , increases in SWC ameliorates diffusion of soil C substrates and its access to soil microbes (Curiel Yuste et al., 2003;Jarvis et al., 2007).Amount of substrate to roots and rhizosphere microbes is also expected to be high as a result of high current photosynthesis (Baldocchi et al., 2006).As a result, diel Rs is not limited by C substrates provided by current photosynthesis and soil organic matter.Consequences of diel Rs may vary repeatedly in synchrony with diel temperature, with no indication of hysteresis when SWC > 0.08 m 3 m -3 (Fig. 6a).By contrast, during dry and hot phases, with SWC < 0.08 m 3 m -3 , inadequate soil water limits diffusion of soil C substrates and its access to soil microbes (Jassal et al., 2008) and also suppresses photosynthesis (supported by Fig. 2g, h).As a result, Rs may be limited by C substrates under dry conditions.It has been reported current photosynthesis can account for about 65-70% of total Rs over the growing season (Ekblad and Högberg et al., 2001;Högberg et al., 2001).Thus, diel Rs may vary more closely to photosynthesis during dry and hot phases over the growing season (Fig. 2h), resulting in increased hysteresis with decreasing SWC below 0.08 m 3 m -3 (Fig. 6b).
The 0.08 m 3 m -3 SWC threshold of this study was consistent with an earlier study by Wang et al. (2014) that reported that seasonal Rs decoupled from soil temperature as SWC fell below 0.08 m 3 m -3 .Earlier studies have reported similar responses of Rs to temperature (Palmroth et al., 2005;Jassal et al., 2008).For example, Rs in an 18-year-old temperate Douglas-fir stand decoupled from Ts when SWC fell below 0.11 m 3 m -3 .Our results suggest that the decoupling of Rs from temperature for low SWC was due to a shift in control from temperature to photosynthesis.Our work provides urgently needed new knowledge concerning causes/mechanisms involved in defining variation in diel hysteresis in desert shrubland.
Based on our work, we suggest that photosynthesis should be considered in simulations of diel Rs in drylands, especially when SWC falls below 0.08 m 3 m -3 .

Conclusions
Soil water content regulated the relative control between photosynthesis and temperature on diel Rs by changing the relative contribution of autotrophic and heterotrophic respiration to total Rs, causing seasonal variation in diel hysteresis between Rs and temperature.Hysteresis was not observed between Rs and Tsurf, when SWC > 0.08 m 3 m -3 , but the lag-hours increased as SWC decreased below this SWC threshold.Incorporating photosynthesis into Rs-temperature-based models reduces diel hysteresis and increases the overall level of goodness-of-fit.Our findings highlight the importance of biological mechanisms in diel hysteresis between Rs and temperature and the importance of SWC in plant photosynthesis-soil respiration dynamics in dryland ecosystems.Each hourly value (y-axis) for each day (x-axis) is shown as a value of 1 through 0; 1 denotes the peak value for a given day and 0, the daily minimum value.

Figure 2 .
Figure 2. Mean monthly diel cycle of soil water content (SWC), incident photosynthetically active radiation (PAR), temperature [i.e., air (Ta), soil-surface(Tsurf), and soil temperatures (Ts)], soil respiration (Rs), and photosynthesis (P) at an Artemisia ordosica-dominated site during a wet and dry month.Each point is the monthly mean for a particular time of day.Bars represent standard errors.

Figure 3 .
Figure 3. Diel variation of measured soil respiration (Rs) and modeled Rs by using temperature and photosynthesis as input variables in the calculation of Rs for both a wet and dry month (i.e., July and August, respectively); Rs-T function (Eq.1), Rs-P function (Eq .2),and Rs-T-P function (Eq.3).

Figure 4 .
Figure 4. Relationship between soil water content (SWC) and the relative importance (RI) of soil-surface temperature and photosynthesis at an Artemisia ordosica-dominated site as a function of soil respiration (Rs).

Figure 5 .
Figure 5.Time lags between measured and modeled soil respiration by means of soil-surface temperature and photosynthesis over the growing season; Rs-T function (Eq.1), Rs-P function (Eq.2), and Rs-P-T function (Eq.3).

Figure 6 .
Figure 6.Time lags between soil respiration (Rs) and soil-surface temperature (Tsurf), Rs, and photosynthesis at an Artemisia ordosicadominated site with respect to soil water content (SWC).Time lags were bin-averaged using SWC-intervals of 0.004 m 3 m -3

Table 1 .
Analysis of mean monthly diel cycles of soil respiration (Rs), air temperature (Ta), soil-surface temperature (Tsurf), soil temperature at a 10-cm depth (Ts), and photosynthesis (P) in a dominant desert-shrub ecosystem, including correlation coefficients and time lags times in Rs vs. Ta, Tsurf, Ts, and P cycles.Statistically significant Pearson's correlation coefficients (r; p < 0.05) are denoted in bold.

Table 2 .
Regressions based on the Q10, linear, and Q10-linear functions of soil respiration (Rs) for both a wet (July) and dry month (August) in 2013.Variables Tsurf ( o C) refers to the soil-surface temperature; P photosynthesis in the dominant shrub layer; R 2 the coefficient of determination; and RMSE the root mean squared error.