Does soil moisture overrule temperature dependency of soil respiration in Mediterranean riparian forests ?

Introduction Conclusions References


Introduction
Soil is the largest pool of terrestrial organic carbon in the biosphere, storing around 2344 Pg C in the top 3 m (Jobbágy and Jackson, 2000).Soil respiration (SR) is the main source of carbon efflux from ecosystems to the atmosphere, accounting for 60-90 % of the total ecosystem respiration (Schimel et al., 2001;Raich et al., 2002).Thus, SR plays an important role in the global carbon balance (Schimel et al., 2001;Raich et al., 2002), and even small changes of SR may induce positive feedbacks to climate change (Schlesinger and Andrews, 2000).Therefore, information of how SR interacts with environmental conditions, such as the response of specific components of soil respiration to temperature and moisture changes, will be a key part of the improvement of process-based models.
On large scales, such as in ecosystems and biomes, net primary production (NPP) may be the most important factor controlling SR (Wardle, 2002).NPP provides the inputs to the soil from aboveground litter and also belowground organic detritus (Raich and Potter, 1995).Moreover, root respiration is strongly dependent on the translocation of photosynthates from the aboveground part of the plant (Curiel-Yuste et al., 2004).At the smaller scale, SR has been found to be very sensitive to soil temperature and soil moisture (Fang and Moncrieff, 2001).Soil temperature has been recognized as the most important environmental factor controlling SR because it affects the respiratory enzymes of both roots and soil microbial biomass (Xu et al., 2011).In general, SR increases exponentially with increases of soil temperature (Epron et al., 1999;Lloyd and Taylor, 1994;Mielnick and Dugas, 2000).In contrast to the positive relationship C. T. Chang et al.: Soil moisture overrules temperature dependence of soil respiration between SR and soil temperature, both very high and very low soil moisture have been shown to diminish the temperature response of SR (Londo et al., 1999;Welsch and Hornberger, 2004) due to the potential oxygen limitations under high soil moisture (Skopp et al., 1990) and due to metabolic drought stress under very low soil moisture (Orchard and Cook, 1983).Soil moisture also affects plant composition and productivity (Häring et al., 2013) and thus controls the quantity and quality of both soil organic matter (SOM) and root exudate supply (Rustad et al., 2000).
Numerous studies have reported the effects of temperature and moisture on SR.However, studies about the combined effects of both factors are relatively few, and the information of how soil moisture affects the relationship between soil temperature and SR is scarce (Bowden et al., 1998;Davidson et al., 2006;Curiel-Yuste et al., 2007).In Mediterranean and semiarid ecosystems, SR is highly sensitive to soil moisture, and the temperature-driven increases in SR are likely dampened by low soil moisture (Conant et al., 2004;Raich and Potter, 1995;Rey et al., 2002).It is still unclear under which circumstances or environmental conditions would the primary control factor of SR switch from temperature to soil moisture.
SR can be divided into autotrophic and heterotrophic respiration by different biological sources (Hanson et al., 2000).Autotrophic respiration, also known as root respiration, is mainly dependent on NPP and tree physiology such as photosynthesis substrate supply (Heinemeyer et al., 2007;Hogberg et al., 2001).Heterotrophic respiration is the sum of microbial decomposition of SOM (Fang et al., 2005;Knorr et al., 2005).In theory, due to the different origins of autotrophic and heterotrophic respiration, they may have different sensitivities toward environmental factors and respond differently to seasonality (Epron et al., 2001;Kuzyakov and Larionova, 2006;Yan et al., 2010).
Riparian areas are characterized with high soil moisture and sustained water table.(McGlynn and Seibert, 2003).In these ecosystems, tree species composition and tree growth are strongly influenced by the topographic position concomitant with the changes in the soil water content.Thus, this may indirectly affect SR through litter input and nutrient availability.Because of the retardation of microbial decomposition with the frequent saturation of soil water, riparian areas tend to accumulate more SOM than hillslope areas do (Sjögersten et al., 2006).
The main objectives of this study were (1) to investigate the seasonal variations and relationships between SR and both soil temperature and moisture in a Mediterranean riparian forest along a groundwater level gradient; (2) to determine soil moisture thresholds at which SR is controlled by soil moisture rather than by temperature, even in such nonwater-stressed environments; (3) to compare SR responses under different tree species present in a Mediterranean riparian forest (Alnus glutinosa, Populus nigra and Fraxinus excelsior).With these aims, we carried out measurements of SR under different tree species along a groundwater level gradient in a riparian forest in NE Spain.The results of our study may help to better the understanding of the interactions between different components of SR with soil temperature and moisture as well as the role of different tree species.It also provides relevant information for SR model parameterization.

Site description
The experiment was conducted in a riparian forest growing along the Font de Regàs stream, a headwater tributary of the Tordera River, in Montseny Natural Park (north of Barcelona; 41 • 50 N, 2 • 30 E, altitudinal range 300-1200 m a.s.l.).The forest community of our study site consists of black alder (Alnus glutinosa L.), black locust (Robinia pseudoacacia L.), common ash (Fraxinus excelsior L.) and black poplar (Populus nigra L.).As result of water and nutrient availability, A. glutinosa and P. nigra trees are mostly distributed near the river, whereas F. excelsior trees are located further away on the upper site, near the hill.R. pseudoacacia trees are scattered over the study area and were not monitored.Mean annual temperature is 12 • C with maximum and minimum average temperatures of 14 and 10 • C, respectively.The mean annual precipitation is 872 mm .The riparian soil is sandy loam with low rock content (< 13 %), weakly acidic (pH of 6.7) and has an average bulk density of 1.09 g cm −3 .

Experimental design
We divided the groundwater gradient (riparian-hillslope transect) into four levels according to the distance from the riverside and by tree species composition (Fig. 1).The distances of level 1 to level 4 (L1 to L4) from the river centre were 2.7, 4.4, 6.8 and 11.8 m, respectively.The three target tree species, A. glutinosa, P. nigra and F. excelsior were located at levels L1, L2 and L3, respectively.To examine the interaction effects on SR of tree species, soil moisture and temperature, we set three riparian-hillslope transects to measure the variation of total SR (sum of soil autotrophic and heterotrophic respiration, hereafter referred to collectively as total SR, SR tot ) from different tree species.Soil chambers were placed 1.5 m from the stem of the target tree species.Moreover, we also set two transects to measure the topographic effects on soil heterotrophic respiration (SR H ). Due to the difficulty of trenching next to the riverbank, chambers for SR H were set only at levels L2, L3 and L4.To separate root respiration from SR H , we inserted a PVC tube (diameter: 65 cm; height: 40 cm) into the soil 5 months before starting the measurements.To avoid constraints on groundwater table fluctuations by the PVC tube, we cut two opposite windows into the PVC tube and covered by 65 µm mesh to prevent root growth through the windows.
Stainless steel rings were inserted permanently into the soil, down to 3 cm depth as the base of the soil chambers, and kept free from seedlings throughout the experiment duration.The distances of each soil chamber from the riverside varied slightly due to the tree distribution.

Field measurement
SR and soil temperatures were measured seasonally from summer 2011 to autumn 2012.These measurements were conducted continuously for 1 week within each season.A heavy rainfall event took place in winter 2012, resulting in elevated water levels of the river that washed away most of the litter layer within 3 m from the river bank.
CO 2 concentration was measured in situ with an automatic changeover open system.The system consists of an infrared gas analyser (IRGA, LiCor 6262, LiCor, Inc., Lincoln, NE, USA), a data logger (CR10, Campbell Scientific Inc., UT, USA), 12 pairs of channels, 12 soil chambers, 12 pairs of rotameters, 6 pumps and 2 flowmeters.Each pair of channels consists of two tubes connected to a soil chamber, one attached on the top of chamber (reference CO 2 concentration) and another attached at the base for calculating the increment of CO 2 concentration provided by SR.Soil chambers were placed at the beginning of each field campaign, and CO 2 concentrations were analysed and recorded sequentially over 1 min intervals at each chamber.Air was continuously forced through all chambers by pumps.Only one chamber was connected at a time to the IRGA to analyse the CO 2 concentration of the respective chamber, while air from the others was exhausted to the atmosphere until their own turn.The sequence was programmed every 4 cycles of differential IRGA measurements from 12 chambers, and an additional cycle of absolute IRGA measurement, which was then used to calculate the actual absolute ambient air concentration of CO 2 in ppm.The CO 2 concentration of the ambient air was determined as the difference between the scrubbed sample, which flows through soda lime and Mg(ClO 4 ) 2 , and the ambient air sample.
Soil chambers were protected by placing a 50 cm × 50 cm green fine mesh on top to avoid possible heating by direct sunlight during the measurements.Soil temperature of 5 cm depth was continuously measured with Pt100 temperature sensors and recorded in parallel with the CO 2 concentration analysis.Thirty centimetre integral soil moisture (cm 3 cm −3 , SM 30 ) in each level were determined and recorded halfhourly with a moisture reflectometer (CS616, Campbell Scientific).Additionally, we also measured 5 cm integral soil moisture (SM 5 ) next to each soil chamber once per day during each measuring field campaign with impedance probes (ThetaProbe soil moisture sensor, MI2x, Delta-T Devices, Cambridge, UK).A grid of 28 wells (PVC tubes of 35 mm in diameter) was installed to monitor groundwater table oscillation.Wells were distributed along the study site and at different distances from the stream: 2.7, 4.4, 6.8, 11.8 m (n = 7).Groundwater levels were monitored manually every 2 weeks using a sounding device with acoustic and light signal (Eijkelkamp Agrisearch Equipment).In autumn of 2012, after concluding the measurements, litter layer and soil samples (15 cm depth) inside each chamber were collected.Litter layer samples were weighted after oven drying at 65-70 • C for 24 h.Soil samples were first oven dried at 105 • C and then analysed to determine their organic carbon and nitrogen content by using the Walkley-Black and Kjeldahl methods, respectively.

Statistical analysis
Statistical analyses were performed with PASW statistics 18 (SPSS Inc., 2009, Chicago, IL, USA).The missing data of soil temperatures were estimated from air temperature values based on a regression analyses between air and soil temperatures.SR, soil temperature and soil moisture data were analysed using ANOVA to examine whether seasonal SR rates were different between levels and tree species.Data used to test the significance in ANOVA were based on daily means.Least significant difference (LSD) was used to detect differences between levels and tree species for each season.We used regression analysis to examine the relationship between SR and soil temperature.An univariate exponential equation was fitted (van 't Hoff, 1898): where SR is soil respiration rate (µmol C m −2 s −1 ), T is soil temperature ( • C), a and b are fitted parameters.The apparent Q 10 was calculated as A Q 10 value for the whole measurement period was computed for each topographic position and tree species on the basis of daily average SR rate and soil temperature.In addition, we estimated specific Q 10 values for summer of 2011 and 2012.Data collected were fitted to the exponential equation.In order to understand the interaction between soil temperature and soil moisture and the effect of soil moisture on regulating SR, we applied recursive partitioning analysis to search for the threshold of soil moisture.As models based on partitioning can only handle linear models, the Eq (1) was transformed by linearizing with logarithms: Logarithmic transformed SR values were used as the dependent variable.Once the soil moisture thresholds were obtained, linear and nonlinear regression analyses were used to determine the relationship between SR, soil temperature and soil moisture at each soil moisture interval.The recursive partitioning analysis was conducted in the R statistical environmental using the party package (Zeileis et al., 2008).

Seasonal variation of groundwater level, soil moisture, soil nitrogen and carbon content
Seasonal variation of air temperature and precipitation was remarkable.The precipitation in 2011 was significantly higher than in 2012, especially in summer.Summer precipitation in 2011 was 4 times higher (183 mm) than in 2012 (39 mm).SM 30 was significantly higher at L1 (Fig. 2).In summer 2012, due to a remarkable drought, SM 30 at L1 only showed a small decrease with respect to summer 2011; while at the other levels (L2, L3 and L4) SM 30 was markedly decreased.Groundwater levels showed no seasonal variation but were significantly different between them.Soil near the river contained less organic carbon and nitrogen, but a higher C : N ratio, with a C : N ratio of 12.13 (Table 1).Soil C : N ratio decreased from the riverside going uphill, whereas the dry weight of litter layer increased from the riverside going uphill.The largest amount of dry weight of litter layer was found under F. excelsior, and coincided with the highest soil organic carbon (SOC) and soil nitrogen concentrations between all levels.

Seasonal variation of SR H along hillslope transect
SR H rates ranged from 0.17 µmol C m −2 s −1 (in winter, L4) to 1.69 µmol C m −2 s −1 (in summer, L2, Fig. 3a-d).SR H decreased significantly from riparian zone (L2) to hill zone (L4), especially in summer.SR H measured from different levels were significantly different in all seasons (P < 0.05).SR H at L2 had a higher variability during the whole experiment.Minimum soil temperature coincided with maximum SM 5 in winter while maximum soil temperature was recorded in summer when SM 5 was lowest.SR H varied markedly during the year following the change of soil temperature from summer 2011 to spring 2012, and the changes of SM 5 for summer and autumn 2012.As expected, SR H was lower during winter when soil temperatures were the lowest of the year, and SR H was higher during the growing season.

Tree species and topographic effects on SR tot and SM 30
The observed variation of SR tot for the three tree species followed the change of soil temperature over the year (Fig. 3e-h).SR tot of P. nigra was the highest one, especially during summer, and SR tot of A. glutinosa was the lowest one throughout the year.There were no significant differences of soil temperatures between tree species locations.SM 5 did not differ between tree species locations but there was a tendency

Drought and rain pulse effects on SR
The low precipitation of summer 2012 caused a significant reduction of around 50 % of SM5, 14-35 % of SM30 and at the same time a reduction of SR between 21 and 49 %.
The Q 10 values ranged from 0.97-1.40 in summer 2011 and 0.63-1.14 in summer 2012 (Table 2).A rainfall event (13.5 mm) during the measurement period of summer 2012 caused a significant increase of soil moisture and SR rates at all levels (L1 to L4).The SM 5 increased around 21-74 % after the rainfall event even though it only caused a 0-38 % increase of the SM 30 (Table 3).This rainfall event caused a sharp increase of SR from 0.41-0.99µmol C m −2 s −1 to 0.59-1.66µmol C m −2 s −1 , which corresponds to an increase of SR around 34 to 68 %.

The switch of primary control factor of SR
We identified three SM 5 intervals for each SR H and SR tot (Table 4), which suggests the existence of thresholds in soil moisture effects.SR was positively related (P < 0.001) to soil temperature when SM 5 was higher than 23 % for SR H or higher than 27 % for SR tot .The lower thresholds for SR H and SR tot were 20 and 17 % of SM 5 respectively.Under the lower bound value, SR H showed a significantly positive relationship with SM 5 (Fig. 4, linear regression with r 2 of 0.89, 0.92 and 0.91 for L2, L3 and L4) while SR tot showed a weak positive relationship with SM 5 (Fig. 5, linear regression with r 2 of 0.56, 0.11 and 0.10 for L1, L2 and L3).The exponential model based on soil temperature accounts for 68 to 84 % of the variation in both SR H and SR tot rates at the higher SM 5 interval values.The fitted Q 10 values in high SM 5 intervals ranged from 1.49 to 2.14.Generally the Q 10 values of SR H were lower than the Q 10 of SR tot .

Effect of groundwater level and soil moisture on SR
In the studies of Martin and Bolstad (2005) and Pacific et al. (2008), it was indicated that the amount and availability of soil water varies depending on landscape position and topography.Both studies also show that small differences in micro-topography appear to be important in driving soil moisture conditions.This is in accordance with our results; the overall seasonal trends of soil moisture were similar, but differences in the relative magnitude of soil moisture still can be found between different levels.
In our study site, the SR H was significantly higher at L2 and decreased with the distance from the river.At the same time, SR tot of A. glutinosa at L1 was significantly lower than the other two species found at L2 and L3.This result could be explained by limitations of SR imposed by groundwater Figure 5. Differentiation of soil temperature (ST) and soil moisture (SM) as primary controlling factors for SR tot .At SM 5 ≥ 27 %, there are positive correlations of SR tot with soil temperature under all tree species (a).At 17 % ≤ SM 5 < 27 %, there are positive but slightly weaker correlations of SR tot with soil temperature under all tree species (b).At SM 5 < 17 %, there is no relationship between SR and ST as the inset figure shows (c); it switches from ST to SM 5 as the controlling factor with positive correlations between SR tot and SM 5 for all tree species.Campaigns with SM 5 < 17 % were all from spring and summer 2012.SM 5 (5 cm integral soil moisture).level in two different ways.First, when the groundwater level is low, the drought stresses soil microbial and root respiration activity; second, when groundwater level is high and close to topsoil surface, it limits soil aeration and likely reduces the effective respiring soil volume.Pacific et al. (2008) showed that the soil CO 2 concentrations were significantly higher in the riparian zone as a result of higher soil moisture.In contrast, Zanchi et al. (2011) found lower SR in plots after drainage, and suggested that the low C and N content in the topsoil near the river, where most of the soil CO 2 respiration is produced, could partially explain that low SR.The discrepancy of these two studies could be associated with the different drainage regimes as the poorly drained plots imply the anaerobic inhibition of SR.In our study, however, SR H was measured at L2, L3 and L4 under well-drained conditions, and SR H decreased concomitantly with the decrease in the availability of soil water.Nonetheless, SR tot of A. glutinosa was measured at L1, where the soils sometimes experienced flooding or poor draining conditions, and the root respiration may be inhibited by the high groundwater level.
Additionally, landscape position and topography not only altered the availability of soil water but also affected the annual range of soil moisture.This was shown in Zanchi et al. (2011), studying riparian SR in Amazonia.They indicated that riparian soil is very sensitive to the changes of water-flooding regimes.The high groundwater table in riparian zones implies intermittent anaerobic conditions and the inhibition of diffusion during water saturation.These differences in soil moisture caused by site topography may result in differences in SR even though the soil temperatures were similar among all sites.The different behaviours of SR H and SR tot from L1 to L4 from our results indicate a different contribution of SR H to SR tot .As the root system of A. glutinosa may constantly experience a saturated water regime, the relative contribution from root respiration may be much lower than the one of the other two species.

Rain pulse and drought effects on SR
The Mediterranean climate is characterized by summer droughts that particularly affect the top soil layers; therefore, rainfall events during these dry periods can trigger abrupt increases in SR that last for days (Bowling et al., 2011;Cisneros-Dozal et al., 2007;Lee et al., 2004;Unger et al., 2010).Lee et al. (2004) simulated precipitation and found that hardwood forest floors were very sensitive to changes in moisture in the upper soil layers.Moreover, Wang et al. (2012) noted that the response of litterfall respiration is very sensitive to rainfall, and the increase in soil moisture by rainfall primarily enhanced the litterfall respiration but decreased mineral SR.Similar results were published by Casals et al. (2011), who reported that SR after a precipitation pulse was mostly derived from SR H with a contribution up to 70 % of SR tot .Hence, our findings seem to be consistent with these previous studies.

Confounding effects of temperature and moisture on SR
This study aimed at assessing the importance of soil moisture on soil respiration and determining the threshold of soil moisture at which soil moisture overrules temperature in controlling SR.The response of SR to soil moisture has been widely studied and described by various types of functions such as linear or logarithmic functions, depending on the soil type, climate or vegetation type (Comstedt et al., 2010;Epron et al., 1999;Orchard and Cook, 1983).In our study, the seasonal courses of SR H and SR tot generally followed the seasonal cycle of temperature, but were moderated by soil moisture.Such a relationship is in agreement with other previous studies (Davidson et al., 1998;Martin and Bolstad, 2005;Wang et al., 2013).The positive linear relationship between SR and soil moisture in low soil moisture conditions found in our work agrees with many previous studies where low soil moisture constrains SR (Almagro et al., 2009;Davidson et al., 1998;Keith et al., 1997;Rey et al., 2002;Wang et al., 2013;Xu and Qi, 2001).In our study, the low soil moisture and warmer temperatures actually reduced SR rates, resulting in lower Q 10 values at the lower soil moisture.A similar decline of Q 10 with decreasing soil moisture was reported by Conant et al. (2004), Curiel-Yuste et al. (2003) and Wen et al. (2006).Low soil water content not only reduces the contact between the substrate, enzymes and microbes, it also decreases the substrate supply due to the increased drying-out of litter and topsoil layer (Davidson et al., 2006).Another possible reason for the observed lower Q 10 is that the reduction of photosynthesis decreases the translocation of photosynthates to the rhizosphere (Hogberg et al., 2001;Nordgren et al., 2003).
In a Norway spruce stand, Gärdenäs (2000) found that litter moisture explained most of the variation of SR, whereas mineral soil moisture, air and litter temperature had no ef-fects on SR.Our results showed that the seasonal variations of SR H and SR tot were mainly controlled by soil temperature, with a secondary influence of soil moisture (SM 5 ).Using the recursive partitioning method, we have identified clear thresholds for SM 5 effects on the temperature sensitivity of SR.Soil moisture thresholds at which SR temperature sensitivity is reduced have been found in several studies from different ecosystems (Fang and Moncrieff, 2001;Gaumont-Guay et al., 2006;Jassal et al., 2008;Lellei-Kovács et al., 2011;Palmroth et al., 2005;Wang et al., 2013).However, the threshold values in soil moisture seem to be site specific as the factors limiting water uptake by plants and microbes may differ by ecosystem.Even in the same climate region, different soil moisture thresholds have been found in previous studies.For example, Almagro et al. ( 2009) investigated how soil moisture modulated the sensitivity of soil respiration in different ecosystems in the Mediterranean region and found that the threshold value of soil moisture was 10 %.Above this soil moisture values, Q 10 ranged from 1.86 to 2.20 and decreased to 0.44 to 0.63 when soil moisture was lower than 10 %.However, Rey et al. (2002) found in a Mediterranean oak forest that soil temperature accounted for 85 % of the variation of SR when soil moisture was above 20 % with a Q 10 value of 2.34.Furthermore, Xu and Qi (2001) found that with soil moisture higher than 14 %, the Q 10 value was 1.8 and decreased to 1.4 when soil moisture was lower than 14 %.

Other factors affecting SR
In addition to soil moisture threshold values, we also found variations of SR H and SR tot between location and tree species in each soil moisture interval.For example, when SM 5 was lower than 20 %, SR H measured at L4 was always lower than SR H measured at L2 and L3.When SM 5 was lower than 17 %, SR tot of P. nigra was significantly higher than for the other two species, suggesting that there are still other factors affecting SR H and SR tot variations.Several explanations for this result are plausible.First, spatial variability in vegetation can affect SR due to differences in root respiration and the quantity and quality of detritus (Raich and Tufekcioglu, 2000).These biophysical gradients across landscape positions can lead to strong spatial heterogeneity in SR.Tree species in our study site exhibit different litterfall temporal patterns and may also contribute to the seasonal variation of the availability of SOC and nutrients to the microbial community and roots.Second, the vitality of tree species in responses to soil water regime could generate different root respiration rates.Additional data of daily sap flow of the studied trees from our study site (data not shown) confirmed the differences in tree transpiration and growth activity.For example, the water use efficiency of P. nigra was highest, followed by F. excelsior and A. glutinosa.Additionally, the mean diameter at breast height (DBH) of P. nigra is larger than mean DBH of the other tree species.P. nigra may be more efficient in taking up water and nutrients compared to the other two tree species.

Conclusions
This research demonstrates how soil moisture constrains the relationship between SR and soil temperature.We present critical threshold values of soil moisture where SR dependency on soil moisture overrules soil temperature dependency.Our results also reveal the importance of soil moisture as a predictor of SR even in a non-water-stressed environment such as riparian forests.Our findings provide support for modelling approaches that include soil temperature and soil moisture by making available parameters to predict SR rates.This study also has implications for a better understanding of global change impacts on the carbon cycle since soil water availability will likely become an increasingly crucial factor for some regions that are expected to suffer more frequent and severe droughts under climate change.

Figure 1 .
Figure 1.Sketch of levels along a gradient of soil water availability with tree species distribution and SR H chamber positions.

Figure 3 .
Figure 3. Seasonal variation of soil respiration, soil temperature and soil moisture.(a-d) of soil heterotrophic respiration: (a) SRH along groundwater level gradient.(b) 5 cm soil temperature.(c) 5 cm integral soil moisture (SM 5 ).(d) 30 cm integral soil moisture (SM 30 ). (e-h) Data of total soil respiration rates (SRtot) of three tree species.(e) SR tot under different tree species.(f) 5 cm soil temperature.(g) 5 cm integral soil moisture (SM 5 ).(h) 30 cm integral soil moisture (SM 30 ) .All values are mean SD.Data points marked with indicate significant differences between species at P < 0.05 (For details, please see to Appendices A and B).

Figure 4 .
Figure 4. Differentiation of soil temperature (ST) and soil moisture (SM) as primary controlling factors for SR H .At SM 5 ≥ 23 %, there are positive correlations of SR H with soil temperature in all levels (a).At 20 % ≤ SM 5 < 23 %, there is a transition with no clear relationship of either SM or ST with SR H (b). At SM 5 < 20 %, there is no relationship between SR and ST as the inset figure shows (c); it switches from ST to SM 5 as the controlling factor with positive correlations between SR H and SM 5 for all levels.Campaigns with SM 5 < 20 % were all from spring and summer 2012.SM 5 (5 cm integral soil moisture).

Table 1 .
Soil carbon and nitrogen content and dry weight of litter L and F organic horizons from soil respiration chambers.

Table 3 .
Comparison of soil respiration rates (SR) and soil moisture (SM) after a rainfall event of 13.5 mm in summer 2012.Heterotrophic SR (SR H ). Total SR (SR tot ).Five centimetre integral soil moisture (SM 5 ).Thirty centimetre integral soil moisture (SM 30 ).Data were averaged for the 2 days before and 2 days after the rainfall event.