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Impact of temperature and moisture on heterotrophic soil respiration along a moist tropical forest gradient in Australia.

Introduction

Tropical forests maintain the largest terrestrial biodiversity and are vulnerable to changes in climatic conditions (Foster 2001; Williams et al. 2003). Furthermore, they store the largest amount of carbon (C) of any biome, with >40% of world's terrestrial C stored in tropical ecosystems (Malhi et al. 1999; Raich et al. 2006; Schwendenmann and Pendall 2008). Disturbances to these vulnerable ecosystems will therefore likely lead to considerable changes in C02 exchanges with the atmosphere (Meir et al. 2008; Willis and Bhagwat 2009). The impact of warming on C fluxes in tropical rainforests has been explored in a range of studies (Kiese and Butterbach-Bahl 2002; Sotta et al. 2004; Saatchi et al. 2007; Cox et al. 2013). To estimate the impact of changing temperature and rainfall patterns on the C cycle of tropical rainforests in situ, several studies along altitudinal gradients have also been conducted (Girardin et al. 2010; Malhi et al. 2010; Moser et al. 2011). Most studies report an increase in soil C stocks with increased elevation (Raich et al. 2006; Wilcke et al. 2008; Dieleman et al. 2013), with soil C stocks at high elevations demonstrated to be higher than aboveground biomass C stocks in lowland rainforests (Malhi et al. 1999; Zimmermann et al. 2010). However, the fate of soil organic C (SOC) under projected climate change scenarios is still not well understood. Zimmermann et al. (2009) and Girardin et al. (2010) measured soil respiration along a tropical forest altitudinal gradient in Peru and found that total soil respiration (Rs) did not change along a temperature gradient of 14[degrees]C, but that relative biomass C allocation changed from above to below the ground, leading to different contributions of autotrophic and heterotrophic sources to [R.sub.s].

For analysing the effect of changed climatic conditions on soil C cycling, translocation of soil cores along altitudinal gradients was shown to be a successful approach in different ecosystems. Conant et al. (2008) translocated soil mesocosms across three semi-arid ecosystems in the San Francisco Mountains, USA, and found that heterotrophic soil respiration ([R.sub.sh]) rates were highest at the wettest site, whereas temperature was negatively correlated with [R.sub.sh]. In this semi-arid system, Rsh was controlled mainly by the size of soil C pools and by moisture. Hart (2006) replaced soil cores reciprocally between two forest sites along an altitudinal gradient in Arizona, USA, where there was a difference in mean annual temperature of 2.7[degrees]C and mean annual rainfall of 228 mm between the sites, and measured an increase in [R.sub.sh] rates of 190% when soil cores were translocated downslope. The [R.sub.sh] rates correlated as well with temperature as they did with moisture. Zimmermann et al. (2009) translocated soil cores along a tropical forest gradient in Peru and conducted a temperature-sensitivity analysis, which revealed that soil C-stabilisation processes mainly drove changes in temperature sensitivities. However, the effect of soil moisture on temperature sensitivity remained unclear.

The impact of changing climatic conditions on soil organic matter (SOM) decomposition of tropical soils has also been investigated in various laboratory incubation experiments (Bekku et al. 2003; Schwendenmann and Pendall 2008), but these incubation studies did not consider diurnal or seasonal climate patterns. Although the relationship between [R.sub.sh] and temperature has been explored in many studies across various ecosystems (Conant et al. 2011), the combined effect of temperature and soil moisture on SOM is not well explored. Soil moisture is known to limit soil respiration through limited access of microbial decomposers at low moisture levels and lack of oxygen at saturation levels of water (Chambers et al. 2004; Suseela et al. 2012). However, the predictive capacity of current model approaches for the respiration-moisture relation remains quite limited (Moyano et al. 2013).

In order to analyse the temperature and moisture effect on heterotrophic soil C effluxes, we translocated soil cores from three different moist tropical forest sites with different climatic conditions. The [R.sub.sh] rates were then measured regularly for 1 year, and model functions considering temperature and soil moisture were fitted to the measured [R.sub.sh] rates. The hypotheses we tested were: (i) soil moisture is the more dominant driver of [R.sub.sh] than temperature in moist tropical forests; (ii) translocation of soil cores from moist sites to sites with much lower rainfall will significantly alter the average [R.sub.sh] rates; and (iii) translocation of soil cores among different sites will not affect the relation between [R.sub.sh] and soil temperature.

Materials and methods

Study sites

Soil translocation was initiated in February 2010 in the Wet Tropics Bioregion of Far North Tropical Queensland, Australia. Soil cores from three tropical forest sites at different elevations with different climatic conditions were excavated and translocated to the other sites, with some cores re-emplaced at their original location to act as controls. The low- and high-elevation sites were at the base and top of the Mt Bellendcn Ker Range and the mid-elevation site was at Robson Creek, further to the east.

The highest site was on top of Mt Bellenden Ker (site BT, 17[degrees]15'55"S, 145[degrees]51'14"E) with elevation 1540m a.s.l., mean annual temperature 14.2[degrees]C and mean annual rainfall 8100 mm. The forest type was a simple microphyll vine-fern thicket, on a Leptosol (according to WRB 2006) formed on granite. The mid-elevation site was Robson Creek (site RC, 17[degrees]07'00"S, 145[degrees]37'50"E) at 700 m a.s.l. with mean annual temperature 20.4[degrees]C and mean annual rainfall 1770 mm. The soil type was a Gleysol developed on metamorphic sediments in a simple notophyll vine forest. The lowest site was at the base of Mt Bellenden Ker (site BB, 17[degrees]16'11"S, 145[degrees]54'01"E) at 100m a.s.l. Mean annual temperature is 23.4[degrees]C and mean annual rainfall 4630 mm. The forest growing on this Ferralsol overlying the same granite bedrock as at BT was a complex mesophyll vine forest. A further two sites in open wooded savannah were chosen for installation of excavated soil cores from the three sites described above to expose the soil cores to a larger range of climatic conditions. These sites were Davies Creek at 670m a.s.l. (site DC, 17[degrees]01'17"S, 145[degrees]35'05"E) with mean annual temperature 22.2[degrees]C and mean annual rainfall 1260 mm, and Smithfield at 40 m a.s.l. (site JC, 16[degrees]48'59"S, 145[degrees]40'56"E) with mean annual temperature 25.1[degrees]C and mean annual rainfall 1990 mm. Climatic parameters were taken from the Bureau of Meteorology (www.bom.gov.au/climate), and further details regarding some of the sites can be found in Butterbach-Bahl et al. (2004), Graham (2006) and McJannet et al. (2007).

Translocation setup

Soil cores were taken according to the methodology of Zimmermann et al. (2009, 2010). At each of the three sampling sites, leaf surface litter was removed and 15 plastic tubes of 10 cm diameter and 30 cm length were hammered into the ground. The core length was selected because 30 cm represented the maximum soil depth at the BT site. If the soil cores were compacted >5% (1.5 cm), the cores were discharged and newly taken. The tubes were then dug out and transported back to the laboratory, where a soil moisture sensor of 10 cm length (Vegetronix, Riverton, UT, USA) was pushed from the surface into the soil core, with the sensor cable led out of the core through a small hole, which was sealed with silicone after installation. To hinder root growth into the cores and to stop the loss of soil material from the base, a 63-pm nylon mesh was installed across the bottom of each core. Three cores from each sampling site were then installed in the ground at each of the five field sites by drilling holes with a 12-cm-wide auger. A layer of glass marbles was put on the soil core surfaces to protect the soils from splash erosion, a partial plastic cap was put on the top of the tubes to reduce the rainfall input, and a removable chicken wire to stop leaf litter input. The cap diameter was chosen to reduce the rainfall input by 20%, which is the average transpiratory loss of forests in the study region (McJannet et al. 2007). Because roots were excluded from the cores, there was no transpiratory loss from the cores. Although this 20% might be too small for soil moisture values at the highest elevation where cloud interception could contribute up to 66% of monthly water input (McJannet et al. 2007), and too low for drier sites, the reduction in rainfall input was necessary to avoid waterlogging within the cores and was based on best data available.

Dataloggers (XR5-SE; Pace Scientific, Mooresville, NC, USA) were installed at all five sites for recording rainfall (rainfall sensor RS-100; Pace Scientific) under the canopy, and soil temperature (temperature probe PT956; Pace Scientific) and soil moisture (soil moisture sensor Echo EC10; Degagon, Pullman, WA, USA) at 10 and 30 cm soil depths at half-hourly intervals. Additional temperature sensors (ibutton; Maxim, Sunnyvale, CA, USA) were installed in the shade at 10 cm above the soil surface.

Soil analyses

To quantify the soil C stocks within the top 30 cm at the three sampling sites, three soil profiles at each site were sampled in 5-cm intervals with metal tubes of 5 cm length and diameter. All soil samples were then dried at 60[degrees]C, crushed and sieved to 2 mm to remove stones and roots >2 mm. The soil samples were then ground and the C concentrations measured with an elemental analyser (EA 4010; Costech, Valencia, CA, USA). Fine soil densities, corrected for stone and root volumes, were determined, and the C stocks calculated. All values are presented with standard errors. The pH was measured in composite soil samples from the layers 0-10, 10-20 and 20-30 cm, by using an electrode (PHM210; Radiometer Analytical, Villeurbanne, France) in a mixture of dry soil and 0.01 m Ca[Cl.sub.2] at a ratio of 1:10. Soil texture values for composite samples were determined using a laser diffraction analyser (Mastersizer 2000; Malvern, Worcestershire, UK).

Soil respiration measurements

Effluxes of [R.sub.sh] from the soil cores were measured every 3 weeks from April 2010 to May 2011 in the field with an LI-8100 (LICOR, Lincoln, NE, USA) portable infrared gas analyser. With this equipment, the C[O.sub.2] increases in a closed airflow system between the soil cores and a 10-cm survey chamber. This increase was measured over 150s and the C[O.sub.2] flux rate calculated based on exponential best fit equations (Kutzbach et al. 2007). On each measurement occasion, soil temperature at 10 cm depth ([T.sub.s]) next to the cores and volumetric soil water content (VWC) from the surface to a depth of 10 cm within the cores were also recorded.

To check for decreases in [R.sub.sh] rates over time due to lower C availability within the decomposing soil cores, [R.sub.sh] rates of control cores at the start and at the end of the translocation experiment were compared. For this, periods with very similar temperature and moisture conditions were selected and [R.sub.sh] rates of the corresponding periods evaluated.

Respiration models

To evaluate the impact of temperature and moisture on the measured [R.sub.sh] rates, we used the following equations, to calculate the temperature effect alone:

[R.sub.sh] = [R.sub.10] x [Q.sup.(Ts-10)/10.sub.10] (1)

to calculate the soil moisture effect alone:

[R.sub.sh] = a + b x VWC - c x [VWC.sup.2] (2)

and to calculate the combined effect of temperature and soil moisture:

[R.sub.sh] = ([R.sub.10] x [Q.sup.(Ts-10)/10.sub.10]) x (a + b x VWC - c x [VWC.sup.2] - c x [VWC.sup.2]) (3)

where [R.sub.sh] is measured heterotrophic respiration rate ([micro]mol C[O.sub.2] [m.sup.-2] [s.sup.-1]); [T.sub.s] is soil temperature ([degrees]C) at 10 cm depth; VWC is volumetric water content (%) at 10 cm depth; and [R.sub.10], [Q.sub.10], a, b and c are fitted parameters. Furthermore, [Q.sub.10] gives the temperature sensitivity of the respiration as a factor by which the respiration accelerates with an increase in temperature of 10[degrees]C, and [R.sub.10] the fitted [R.sub.sh] rate at 10[degrees]C. This temperature sensitivity function was successfully used for annual soil respiration data of different ecosystems (Janssens and Pilegaard 2003; Reichstein et al. 2005; Zimmermann and Bird 2012), and the soil moisture term describes the site-specific limitation of [R.sub.sh] at very high (saturation) or low (drought) water contents (Chambers et al. 2004; Zimmermann et al. 2009). Davidson et al. (2000) assumed that a quadratic function relating soil moisture to microbial soil respiration might be the best option, and Moyano et al. (2013) confirmed the usefulness of this approach in a recent review.

Statistical analyses

Data were checked for normal distribution with a Shapiro-Wilk test, and differences among measurements tested for significance with an analysis of variance (ANOVA) for normally distributed data, and a Kruskal-Wallis one-way ANOVA for non-normally distributed data. Measured respiration rates were fitted to the model Eqns 1-3 by means of least-square regression using SIGMAPLOT 12.0 (SYSTAT Software Inc., San Jose, CA, USA). For this, data were first tested for variance homogeneity. Regression fits were then calculated individually for soil cores from the same origin installed at each site, and then for all pooled respiration measurements of soil cores from the same origin.

The model performances were then compared by using Akaike's information criterion (AIC); the model fit with the lowest AIC value was considered to be the best one (Burnham and Anderson 2004). AIC values were calculated only for model fits with significant correlations between measured and modelled data (P < 0.05). These model predictions were tested for normal distribution of errors, in which case the criterion was calculated as:

AIC = n x log([[sigma].sup.2]) + 2 x K (4)

where [[sigma].sup.2] = (residual sum of squares)/n; n is the sample size; and K the number of estimated parameters where the variance was also counted as an estimated parameter (Burnham and Anderson 2004).

Results

Site characteristics

Climatic parameters for the period March 2010-April 2011 coincided well with long-term averages provided by the Bureau of Meteorology (Table 1). Annual rainfall was highest at site BT, with 8019 mm (under canopy), followed by BB, JC, RC and DC with 1260 mm (Fig. 1). Measured rainfall was highest during February 2011, when category 5 Tropical Cyclone Yasi crossed the Queensland coast 75 km south of Bellenden Kcr. Monthly rainfall data were also available from the Bureau of Meteorology for BT and BB, and revealed a very high correlation between values measured at open vegetation sites and under the canopy (correlation coefficients [R.sup.2] > 0.95) with 60.6% and 36.2% of the total rainfall reaching the soil surface at BT and BB, respectively. However, VWC did not follow the same trend as rainfall, mostly because of the different soil textures and densities at each site. At BT, the topsoil was very moist throughout the year, whereas the soil layers at 20-30 cm depth drained much more quickly due to coarse fragments in the B/C layer of the Leptosol. VWCs at RC and BB were more similar for 10 and 30 cm depths. Average annual soil temperature at 10 cm depth was highest at site JC at 40 m a.s.l. (25.3[degrees]C) and decreased by ~0.56[degrees]C per 100 m in altitude to 15.9[degrees]C at BT at 1540 m a.s.l.

Soil C stocks in the top 30 cm were highest at BT (10.66 [+ or -] 0.15 kg C [m.sup.-2]) and decreased linearly with decreasing altitude to 6.13 [+ or -]0.06 kg C [m.sup.-2] at BB (C stock=0.0031 x altitude + 6.002, [R.sup.2] = 0.94, P < 0.01) (Table 1), which equates to a difference in soil C stock of 471 g C per [degrees]C. Average pH was lowest at BT (3.83 [+ or -] 0.13) and slightly higher at RC (4.33 [+ or -] 0.09) and BB (4.20 [+ or -] 0.02).

Soil respiration rates

The [R.sub.sh] rates for all translocated soil cores measured during the experiment are shown in Fig. 2. At each site, the three cores from the same origin were grouped together per measurement event, and the average values are presented with standard errors. [R.sub.sh] rates were not measured for the first 2 months after installation of the cores, because severed dead roots in the cores might have enhanced [R.sub.sh] rates (Kuzyakov 2006). After this initial stabilisation phase, [R.sub.sh] rates did not correlate with time since installation (Pearson product moment correlations not significant between [R.sub.sh] rates and days since installation of the cores; P > 0.05 in all cases), indicating no bias in the results from dead roots or depletion of SOM in the cores. [R.sub.sh] rates did not reveal any declining trend over the experiment, although C availability decreased over time due to the experimental setup hindering any new C input from reaching the cores. [R.sub.sh] rates of control cores from BT were not significantly different in April 2010 and January 2011 ([+ or -] = 0.73, 2-tailed f-test), when Ts values were similar (15.6 [degrees] C in April 2010 and 15.8 [degrees] C in January 2011). The same was true for control cores from RC when comparing [R.sub.sh] measurements in April 2010 and January 2011 ([+ or -] = 0.41), with Ts of 20.6[degrees]C and 20.8[degrees]C, respectively, and [R.sub.sh] rates of control cores from BB ([+ or -] = 0.76), with Ts of 23.5[degrees]C in April 2010 and 23.4[degrees]C in January 2011.

Cores installed at DC were only partly measured in 2011, because several cores became inhabited by termites, which led to respiration rates being an order of magnitude higher than for non-inhabited cores. [R.sub.sh] rates of all soil cores installed at BB were not considered after February 2011, this site being disturbed by a tropical cyclone. The translocation of the soils to new sites had a significant impact on average [R.sub.sh] rates measured during the study period. Figure 3 presents boxplots of the measured [R.sub.sh] rates. Translocation of the soil cores among sites allowed measurement of [R.sub.sh] rates across a temperature range of ~15[degrees]C, whereas the in situ temperature ranges during the [R.sub.sh] measurements over the year were 7.2[degrees]C for BT, 5.8[degrees]C for RC, and 8.5[degrees]C for BB. Soil cores from BT were only warmed up by the translocation, whereas soil cores from RC and BB were warmed and cooled compared with their native in situ [T.sub.s]. Furthermore, soil cores from BT received less rain by up to 6840 mm when installed at DC, whereas soil cores from RC received more rain at three of the five installation sites. Cores from BB received 3407 mm more rain at BT and up to 3370 mm less rain at the other sites.

Rates of [R.sub.sh] of soil cores from the coolest and wettest site, BT, were increased by ~150% when installed at RC and BB compared with their site of origin, and by ~300% when installed at sites DC and JC. [R.sub.sh] rates of soil cores from RC revealed a different pattern with decreased rates when installed at BT (-56%), at BB (-42%) and at DC (-4%), but increased rates when installed at JC (+18%). The lower [R.sub.sh] rates at BT and BB were significantly different from the [R.sub.sh] rates of the control cores installed at RC, whereas cores installed at DC and JC were not significantly different from the control cores. Soil cores taken at BB delivered significantly lower [R.sub.sh] rates when installed at BT (-50%), but slightly higher rates at RC (+18%), DC (+17%) and JC (+45%).

Modelled temperature and moisture dependence of [R.sub.sh]

Data analysis revealed variance homogeneity of [R.sub.sh] rates across all temperature ranges considered and allowed the calculation of regression models according to Eqns 1-3. The calculated parameters of these different model functions are given in Tables 2 and 3.

Soil cores from BT revealed a significant correlation of Rsh rate with [T.sub.s] at DC only. The [R.sub.sh] rates of cores from RC showed significant correlations with [T.sub.s] at RC, BB and DC, whereas the explained variance by temperature was rather low in RC and DC ([R.sup.2] of 0.47 and 0.46, respectively). However, comparing model fits of measured [R.sub.sh] rates of cores from RC revealed best results for soil cores reinstalled at their origin site RC, because AIC was lowest for this site (Table 2). As at BT, the soil cores from BB correlated significantly with Ts at DC only.

Use of Eqn 2 to relate [R.sub.sh] rates to VWC at the different host sites did not result in improved prediction models compared with Eqn 1. Soil cores from the wettest site, BT, showed a significant relation between [R.sub.sh] and VWC when installed at RC ([R.sup.2] of 0.70). However, [R.sub.sh] rates from all other soil cores installed at the different sites did not show significant relationships with VWC (Table 3).

Use of the combined Ts and VWC equation gave highly significant model fits for [R.sub.sh] rates of soil cores from BT installed at RC and BB, and for soil cores from RC in DC, but not for [R.sub.sh] rates from any other soil cores (Table 4).

Comparison of model fits considering AIC values was possible in only two cases. [R.sub.sh] rates of soil cores from BT installed at RC were better projected with the moisture alone (AIC = -9.65) than with the combined Eqn 3 (AIC = -5.67), and rates from cores originating from RC installed in DC were more significantly predicted with the combined Eqn 3 (AIC 3.84) than with the temperature alone Eqn 1 (AIC = 7.41).

Consideration of [R.sub.sh] rates from all host sites in one equation led to significant model fits in most cases (Table 5). [R.sub.sh] rates could be correlated significantly to Ts for all three soils and delivered [Q.sub.10] values of 2.63 for the soil cores from BT, 2.06 for soils from RC, and 2.00 for the soil cores from BB. The only significant fit between [R.sub.sh] rates and VWC was found for the soil cores originating from BT, where the explained variation was rather poor ([R.sup.2] = 0.16) and the shape of the regression fit did not show an optimal maximal moisture content for [R.sub.sh] but rather a decline in [R.sub.sh] rates at medium soil-moisture contents (Fig. 4). The combined [T.sub.s] and VWC approach of Eqn 3 resulted in highly significant regression fits for all three soils, with correlation coefficients 0.60-0.39, comparable to the temperature-alone fit of Eqn 1. Furthermore, temperature sensitivity values, Qw, were also very close to those obtained by Eqn 1. However, AIC values for Eqn 1 were always lower for the temperature-alone Eqn 1 than for the combined temperature and VWC Eqn 3, even though standard errors of estimates were almost identical (Table 5).

As a further analysis, we related average [R.sub.sh] rates as measured over the entire study year to the site-specific factors mean annual temperature, mean annual VWC and total rainfall (Table 6). The regression equations used were the same as for soil temperature and soil moisture, and a linear approach was chosen for total rainfall. Correlation coefficients between mean annual [R.sub.sh] rates and mean annual temperature were 0.45-0.98, between mean annual [R.sub.sh] rates and mean annual VWC 0.24-0.72, between mean annual [R.sub.sh] rates and rainfall 0.62-0.75, and between mean annual [R.sub.sh] rates and a combination of mean annual temperature and rainfall 0.83-1. Reduction of data to five points weakened the confidence in the modelled parameters, resulting in P values slightly greater than 0.05 in most cases (Table 6). By this approach, mean annual [R.sub.sh] rates for soil cores from RC could best be explained by total rainfall ([R.sup.2] = 0.81, P = 0.04), and mean annual [R.sub.sh] rates for soil cores from BB by the combination of mean annual temperature and rainfall ([R.sup.2] = 1.0, P < 0.01). However, the fit of measured data to mean annual temperature and rainfall in BB resulted in only one function parameter being significant. The only case where the AIC was better than in any approach before was the projection of [R.sub.sh] rates from RC by considering mean annual rainfall alone (AIC = 3.07), which also reduced the standard error of estimation from 1.12 to 0.66.

Discussion

Average soil respiration rates

Total soil respiration consists of autotrophic respiration ([R.sub.sa]) from plant sources and [R.sub.sh] from microbes decomposing accumulated SOM (Hanson et al. 2000; Bond-Lamberty and Thomson 2010). Most studies of soil C[O.sub.2] effluxes are based on Rs, and studies measuring [R.sub.sh] are mainly conducted as laboratory incubation experiments (von Liitzow and Kogel-Knabner 2009). The field method we used allows direct measurement of [R.sub.sh] in the field, because no living roots remained in the cores, and no enhanced C[O.sub.2] effluxes (potentially resulting from severed dead roots priming initial [R.sub.sh] rates) were observed after 2 months. This has the advantage that the soils experience 'real' environmental conditions, including episodic rainfall and diurnal temperature variation. The [R.sub.sh] rates of the soil cores taken and installed at the same sites represent the [R.sub.sh] components of the corresponding site, and they ranged from 0.91 to 4.22 [micro]mol C[O.sub.2] [m.sup.-2][s.sup.-1]. These values are in the upper range of reported [R.sub.sh] rates for tropical forests. Li et al. (2004) reported soil respiration rates that excluded root and litter in a tropical forest in Puerto Rico, with climatic conditions similar to our study gradient, of 0.46 [micro]mol C[O.sub.2] [m.sup.-2][s.sup.-1], and Zimmermann et al. (2010) reported measured [R.sub.sh] rates of 1.1-2.7 [micro]mol C[O.sub.2] [m.sup.-2][s.sup.-1] along a 3000-m altitudinal tropical forest gradient in Peru.

Climate dependence of heterotrophic respiration rates

According to theory (Lloyd and Taylor 1994; Davidson and Janssens 2006), [R.sub.sh] rates should increase at higher temperatures, as long as moisture or oxygen are not limiting. Soil cores originating from BT increased at all sites with higher temperatures and drier conditions, indicating no significant moisture limitation at any site. This is especially surprising, because total annual rainfall dropped to as low as 15% (in DC) relative to its native site. [R.sub.sh] rates of soil cores from RC, however, did not increase significantly at the warmer host sites, and even decreased at site BB, which was 3[degrees]C warmer but experienced 2.6 times the rainfall of RC. Soil cores from BB showed increased [R.sub.sh] rates at the warmest site, JC, but the lower rain input at RC and DC seemed not to have any major impact on [R.sub.sh] rates, if considered separately. These patterns were also confirmed by the regression models considering rain and mean annual temperature, because RC had the lowest AIC value for the model with total annual rainfall, and BT and BB for the model based on [T.sub.s] alone.

Although soil moisture is known to affect respiration rates (Davidson et al. 2000; Sotta et al. 2004; Meir et al. 2008), the soil moisture term in the combined functions analysed here did not show any significant improvement in fitting measured [R.sub.sh] rates, as indicted by the AIC values. The slightly better correlation coefficients between measured and modelled data accounting for soil temperature and moisture were not large enough to justify the two additionally estimated parameters for the moisture term in Eqn 3. Soil moisture alone was in no case a significant predictor for the measured [R.sub.sh] rates. A reason for this might be that the range of VWC along the translocation gradient was still not large enough to observe any effect of drought or saturation, although the soil cores were placed along sites with a difference in mean annual rainfall of ~6650 mm [year.sup.-1]. Davidson et al. (2000) found for an Amazonian forest soil (~1800 mm rain [year.sup.-1]) that [R.sub.s] rates were suppressed by high soil moisture only very close to saturation (matrix potential-0.005 MPa), and by low water contents at a matrix potential of ~-10MPa, which corresponds to a VWC of <5%. However, Sotta et al. (2004) found a cubic relation between VWC and [R.sub.s] ([R.sup.2] = 0.40) at values of 42- 45% VWC for a tropical forest soil with a mean annual rainfall of 2200 mm, and Hashimoto et al. (2004) reported an increase in [R.sub.s] at 10-40% VWC in 10 cm soil depth of a tropical monsoon forest in Thailand. Although our translocated soil cores covered a range of 19-37% VWC, soil moisture was not a significant predictor for [R.sub.sh]. There are two possible conclusions for this observation: (i) VWC was not a driving factor of [R.sub.sh] in our moist study forests, or (ii) the suggested cubic VWC function was not an adequate model to relate [R.sub.sh] rates to VWC measurements.

However, the decomposition of SOM depends not only on temperature and moisture, but also on substrate quality and microbial access to SOM (Davidson and Janssens 2006; Conant et al. 2011). Therefore, comparison of temperature sensitivity [Q.sub.10] values among soils with different C inputs and different textural protection mechanisms is somewhat arbitrary. Furthermore, calculated [Q.sub.10] values are highly dependent not only on the applied temperature function (Fang and Moncrieff 2001; Tuomi et al. 2008) but also on the considered temperature range, as our results showed. Consideration of the full translocation range of temperatures resulted in a [Q.sub.10] of 2.07 for the RC soil cores, but [Q.sub.10] was 2.75 when calculated for the native Ts only. Rates of [R.sub.sh] for soil cores from BT and BB did not result in significant model fits considering the on-site measurements only. Anyway, the [Q.sub.10] values calculated in this study for the SOM substrate respired in the first year were well within the range of other reported [Q.sub.10] values of 1.0-5.6 for tropical soils (Bekku et al. 2003; Bahn et al. 2010).

Impact of future climatic conditions on respiration rates

Suppiah et al. (2007) reported climate predictions for the study region based on 23 global climate models and emission scenarios A2 and A1B of the 1PCC Report Emission Scenarios (1PCC SRES 2000). Their analysis revealed that temperature will increase by ~2.6[degrees]C by 2080 (compared with 2010) in the Wet Tropics Bioregion of Far North Tropical Queensland. Annual rainfall was predicted to decrease by ~23% in the dry season and 1% in the wet season by 2080. The greatest reduction in annual rainfall as achieved by translocation was by bringing soil cores from BT to DC, which reduced the rainfall amount by 84%, and the smallest reduction was by bringing soil cores from RC to DC, which reduced the incoming water amount by 29%. Therefore, the translocation covered the entire range in rainfall and temperature as predicted by Suppiah et al. (2007) for the year 2080.

Assuming that Ts will increase at the same rate as air temperature, [R.sub.sh] would increase over the next 70 years by 29% in BT compared with the respiration rates in 2010-11, 21% in RC, and 19% in BB. The greater increases in [R.sub.sh] rates for the sites with the larger soil C stocks could lead to a total net C loss at the three study sites, as was also observed for soils along altitudinal gradients in South America (Girardin et al. 2010; Zimmermann et al. 2010). However, it must considered that these predictions ignore future potential depletion of C stocks with ongoing decomposition (Conant et al. 2008), changes in C input rates and substrate qualities as caused by potential shifts in tree species (Feeley and Silman 2010), or transformations in the microbial communities of the SOM decomposers (Bradford et al. 2008). Estimated future [R.sub.sh] rates as calculated here were based solely on temperature changes.

Advantage of translocation soil cores over laboratorial incubation studies

The translocation of relative large monoliths along the altitudinal gradient was successful for determining the impact of changes in temperatures and rainfall. Incubations of soil samples in the laboratory under controlled climatic conditions are normally conducted on much smaller soil cores (Bekku et al. 2003; Chen et al. 2010), on which the impact of cutting roots, disturbed soil structure and missing C input is larger, and [R.sub.sh] rates decline over the experiment duration. Zimmermann and Bird (2012) conducted an incubation study at the same sites with smaller tubes (5 cm diameter, 20 cm length) and reported repeated incubation rates that declined by 50% after 6 months. This effect is well known and is a major problem in incubation studies conducted in the laboratory taking several weeks to months (Schaufler et al. 2010). The larger soil cores translocated here did not show declining [R.sub.sh] rates due to lower C availability, indicating that C substrate supply to microbes was not limiting over the experiment period. Therefore, translocated soil monoliths with a large volume are well suited to study changes in [R.sub.sh] rates under natural conditions, even if the studied temperature range cannot be adjusted as well as in the laboratory, and temperature and moisture effects must be considered simultaneously.

Conclusions

Tropical rain forests are substantial global C stores and sensitive to changes in climatic conditions. Our results showed that warming would have the largest impact on tropical forest soils with high C stocks, because the temperature sensitivity of [R.sub.sh] rates increased with increasing elevations. From a global perspective, the amount of C stored in soils is more moisture-dependent than it is temperature-driven (Scharlemann et al. 2014). However, this is based on steady-state conditions. Rapid changes in climatic conditions as simulated here can be better estimated with temperature functions alone, because moisture was hardly a limiting factor in the moist tropical forest sites studied. An exception to this, however, might be extreme events with long drought periods that could change [R.sub.sh] rates substantially.

http://dx.doi.org/10.1071/SR14217

Received 25 July 2014, accepted 12 December 2014, published online 2 April 2015

Acknowledgements

This study was financed by an Australian Research Council Federation Fellowship to MIB. We thank the Department of Environment and Resource Management, Queensland Government Australia, for access to the national park sites, and Broadcast Australia for access and transport to Mt Bellenden Ker. A special thanks to Spiro Buhagiar and Ian McConnell for their support on transportation.

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M. Zimmermann (A,B,C), K. Davies (A), V. T. V. Pena de Zimmermann (A), and M. I. Bird (A)

(A) Centre for Tropical Environmental and Sustainability Science and School of Earth and Environmental Sciences, James Cook University, Cairns, Qld 4870, Australia.

(B) Institute of Soil Research, University of Natural Resources and Life Sciences, Peter Jordan St. 82, 1190 Vienna, Austria.

(C) Corresponding author. Email: Michael.zimmermann@boku.ac.at

Table 1. Site properties, soil carbon stocks (kg [m.sup.-2]),
soil texture (%) and pH values for the three soil sampling sites

Mean annual temperature (MAT) and rainfall were taken from long-term
records from the Bureau of Meteorology, Australia

Depth (cm)   C stock              Clay   Silt   Sand   pH

Bellenden Ker, top: 1540m a.s.l., MAT 14.2[degrees]C,
total rain 8100mm

0-10         5.02 [+ or -] 0.37   5.4    11.3   83.3   3.66
10-20        3.67 [+ or -] 0.29   6.1    13.9   80.1   3.60
20-30        1.97 [+ or -] 0.23   38.3   46.4   15.3   3.99

Robson Creek: 700 m a.s., l, MAT 20.4[degrees]C. total rain 1770mm

0-10         3.97 [+ or -] 0.33   17.9   26.1   55.9   4.51
10-20        2.63 [+ or -] 0.32   24.8   39.7   35.5   4.27
20-30        1.89 [+ or -] 0.14   16.6   30.7   52.7   4.21

Bellenden Ker, bottom: 100m a.s.l., MAT 23.4[degrees]C, total
rain 4630mm

0-10         2.81 [+ or -] 0.21   12.8   26.6   60.6   4.19
10-20        1.90 [+ or -] 0.07   19.1   32.7   48.1   4.18
20-30        1.42 [+ or -] 0.05   24.0   33.1   42.9   4.23

Table 2. Parameter fits, correlation coefficients and error
probabilities for regressions between heterotrophic soil
respiration and soil temperature according to the equation
[R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.(Ts - 10)/10] for soil
cores from the sites Bellenden Ker, top (BT), Robson Creek (RC) and
Bellenden Ker, bottom (BB) installed at the sites BT, RC, BB, DC
(Davies Creek) and JC (Smithfield)

SEE, Standard error of estimation; AIC, Akaike information criterion
for significant model fits. * P<0.05; ** P<0.01

Core     Installation   n    [R.sub.10]   [Q.sub.10]   [R.sup.2]
origin       site

BT            BT        10   1.26 **      0.56         0.16
              RC        14   3.08 **      0.77 **      0.06
              BB        7    5.85         0.49 *       0.47
              DC        7    1.61 *       2.26 *       0.66
              JC        11   1.94         1.74 *       0.17

RC            BT        7    2.16 *       0.74         0.06
              RC        13   1.62 *       2.75 *       0.47
              BB        8    0.66 *       3.06 *       0.79
              DC        10   1.41         2.62 *       0.46
              JC        9    3.04 *       1.43 **      0.28

BB            BT        9    2.11 *       0.35         0.26
              RC        13   1.69         1.82         0.13
              BB        10   0.93         2.21         0.27
              DC        11   1.34 *       2.00 **      0.40
              JC        11   2.79 *       1.22 **      0.04

Core     Installation     P      SEE     AIC
origin       site

BT            BT        0.25    0.34
              RC        0.39    0.44
              BB        0.09    0.39
              DC        0.03    0.62    2.05
              JC        0.21    0.99

RC            BT        0.60    0.59
              RC        <0.01   0.76    1.96
              BB        <0.01   0.17    6.07
              DC        0.03    0.96    4.69
              JC        0.15    0.66

BB            BT        0.17    0.47
              RC        0.22    0.78
              BB        0.13    0.66
              DC        0.04    0.58    -0.13
              JC        0.54    0.70

Table 3. Parameter fits, correlation coefficients and error
probabilities for regressions between heterotrophic soil
respiration and volumetric soil water content (VWC) according to
the equation [J.sub.sh] = a + b x VWC - c x [VWC.sup.2] for soil
cores from the sites Bellenden Ker, top (BT), Robson Creek (RC) and
Bellenden Ker, bottom (BB) installed at the sites BT, RC, BB, DC
(Davies Creek) and JC (Smithfield)

SEE, Standard error of estimation; AIC, Akaike information
criterion for significant model fits. * P<0.05

Core     Installation   n        a           b          c
origin       site

BT            BT        10      -9.38       0.66     -0.01
              RC        14     -29.97 *     2.10 *   -0.03 *
              BB        7      153.62      -9.42      0.15
              DC        7       72.79      -4.95      0.09
              JC        11     133.11      -7.35      0.10

RC            BT        7      -47.28       3.19     -0.05
              RC        13     -53.92       3.71     -0.06
              BB        8     -244.67      13.64     -0.19
              DC        10     211.50 *   -13.08 *    0.21 *
              JC        9     -574.68      33.44     -0.48

BB            BT        9       12.97       0.72     -0.01
              RC        13   -1756.18      98.35     -1.37
              BB        10     485.77     -26.77      0.37
              DC        11      -9.58       0.87     -0.01
              JC        11      -3.70       0.60     -0.01

Core     Installation   [R.sup.2]    P     SEE
origin       site

BT            BT          0.05      0.84   0.39
              RC          0.70      0.01   0.26
              BB          0.48      0.27   0.43
              DC          0.70      0.09   0.65
              JC          0.01      0.97   1.15

RC            BT          0.47      0.28   0.50
              RC          0.17      0.39   0.99
              BB          0.30      0.42   0.34
              DC          0.53      0.07   0.96
              JC          0.56      0.09   0.56

BB            BT          0.13      0.72   0.60
              RC          0.26      0.23   0.76
              BB          0.10      0.68   0.79
              DC          0.13      0.57   0.75
              JC          0.23      0.36   0.67

Table 4. Parameter fits, correlation coefficients and error
probabilities for regressions between heterotrophic soil
respiration and soil temperature together with volumetric soil
water content (VWC) according to the equation [R.sub.sh] =
([R.sub.10] x [Q.sub.10.sup.(Ts - 10)/10)]) x (a + b x VWC - c x
[VWC.sup.2]) for soil cores from the sites Bellenden Ker, top (BT),
Robson Creek (RC) and Bellenden Ker, bottom (BB) installed at the
sites BT, RC, BB, DC (Davies Creek) and JC (Smithfield)

SEE, Standard error of estimation; AIC, Akaike information
criterion for significant model fits. * P<0.05; ** P<0.01;
*** P<0.001

Core     Installation   n   [R.sub.10]  [Q.sub.10]      a
origin       site

BT            BT        10    9.28      0.20        -5.76
              RC        14   11.55      0.94 *      -2.78
              BB        7     5.08 **   1.56 *       6.56 ***
              DC        7     5.66      1.71         3.67
              JC        11   47.08 ***  2.06         4.13

RC            BT        7     7.13      1.03        -6.71
              RC        13    2.34      2.57 *      -3.24
              BB        8    25.03 ***  3.68 ***     3.67 *
              DC        10   18.10 ***  3.08 *       2.33
              JC        9    65.55 ***  1.14        -6.55

BB            BT        9     8.81 ***  0.29        -6.26
              RC        13  151.46 ***  1.57        -4.85
              BB        10   13.10 ***  2.35 **      4.87
              DC        11    1.73      2.23 *      -3.84
              JC        11    2.58      1.49         4.83 **

Core     Installation     b      c     [R.sup.2]   P    SEE    AIC
origin       site

BT            BT         0.38  -0.006    0.58     0.28  0.30
              RC         0.19  -0.003    0.70     0.02  0.29  -5.67
              BB        -0.25   0.004    1.00     0.01  0.10  -5.81
              DC        -0.23   0.004    0.83     0.30  0.68
              JC        -0.23   0.003    0.24     0.75  1.16

RC            BT         0.45  -0.007    0.47     0.78  0.70
              RC         0.25  -0.004    0.48     0.21  0.88
              BB        -0.21   0.003    0.85     0.13  0.20
              DC        -0.15   0.002    0.89     0.01  0.55   3.84
              JC         0.38  -0.005    0.58     0.38  0.66

BB            BT         0.36  -0.005    0.45     0.68  0.61
              RC         0.27  -0.004    0.28     0.56  0.83
              BB        -0.26   0.003    0.37     0.60  0.78
              DC         0.29  -0.005    0.53     0.21  0.63
              JC        -0.19   0.002    0.40     0.48  0.68

Table 5. Parameter fits, correlation coefficients and error
probabilities for regressions between pooled heterotrophic soil
respiration rates ([R.sub.sh]) and soil temperatures together with
volumetric soil water contents (VWC) according to the equations for
soil cores from the sites Bellcnden Ker, top (BT), Robson Creek
(RC) and Bellcnden Ker, bottom (BB) installed at all other sites

SEE, Standard error of estimation; AIC, Akaike information
criterion for significant model fits. * P<0.05; ** P<0.01;
*** P<0.001

Core     [R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.((Ts - 10)/10)
origin
         n    [R.sub.10]   [Q.sub.10]   [R.sup.2]       P

BT       49   0.99 ***     2.63 ***     0.50        <0.01
RC       47   1.71 ***     2.07 ***     0.37        <0.01
BB       54   1.29 ***     2.00 ***     0.40        <0.01

Core     [R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.((Ts - 10)/10)
origin
          SEE        AIC

BT       0.96      3.54
RC       1.12      9.65
BB       0.80     -5.43

Core          [R.sub.sh] = a + 6 x VWC + c x [VWC.sup.2]
origin
         n        a            b            c       [R.sup.2]

BT       49   19.67 **     -1.14 *       0.019 *    0.16
RC       47   41.57         2.88        -0.045      0.05
BB       54   -0.53         0.29        -0.006      0.04

Core        [R.sub.sh] = a + 6 x VWC + c x [VWC.sup.2]
origin
           P         SEE       AIC

BT       0.02     1.26        16.46
RC       0.29     1.38
BB       0.33     0.97

Core    [R.sub.sh] - [R.sub.10] x [Q.sub.10.sup.((Ts - 10)/10)]
origin              x (a + b x VWC + c x [VWC.sup.2])

         n    [R.sub.10]      010           a           b

BT       49   0.91         2.62 ***      4.35       -0.20
RC       47   1.86         2.08         -5.28 **     0.41
BB       54   0.78         1.87 ***     -4.4Q        0.41

Core     [R.sub.sh] - [R.sub.10] x [Q.sub.10.sup.((Ts - 10)/10)]
origin               x (a + b x VWC + c x [VWC.sup.2])

           c      [R.sup.2]     P     SEE     AIC

BT        0.003   0.60        <0.01   0.89    4.77
RC       -0.007   0.41        <0.01   1.11   14.27
BB       -9.007   0.39        <0.01   0.80   -1.97

Table 6. Parameter fits, correlation coefficients and error
probabilities for regressions between heterotrophic soil
respiration ([R.sub.sh]) and mean annual temperature (MAT), mean
annual soil moisture (VWC), mean annual rainfall (rain) and a
combination of MAT and rain as given by the equations for soil
cores originating from Bellenden Ker, top (BT), Robson Creek (RC)
and Bellenden Ker, bottom (BB) installed at the different sites

SEE, Standard errors of estimation; AIC, Akaike information
criterion for significant model fits. * P<0.05; ** P<0.01

                        [R.sub.sh] = [R.sub.10] x
                       [Q.sub.10.sup.((MAT - 10/10)]

Core     n   [R.sub.10]   [Q.sub.10]   [R.sup.2]       P
origin

BT       5      0.80        2.93 **          0.70   0.08
RC       5      1.67        1.91             0.45   0.22
BB       5      0.93 **     2.47 **          0.98  <0.01

         [R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.((MAT - 10/10)]

Core        SEE       AIC
origin

BT       0.82
RC       1.13
BB       0.16         -3.03

Core         [R.sub.sh] = a + b x VWC + c x [VWC.sup.2]
origin
         n         a           h              c    [R.sup.2]

BT       5     67.16       -4.59             0.079  0.72
RC       5   1176.25      -69.19             1.019  0.72
BB       5    202.98      -11.68             0.170  0.24

Core         [R.sub.sh] = a + b x VWC + c x [VWC.sup.2]
origin
             P        SEE     AIC

BT       0.28
RC       0.28
BB       0.76

Core         [R.sub.sh] = a + b x rain
origin
         n         a           h       [R.sup.2]       P

BT       5      4.20 **    -0.0004           0.75   0.06
RC       5      4.99 **    -0.0004 *         0.81   0.04
BB       5      3 54       -0.0003           0.62   0.11

Core     [R.sub.sh] = a + b x rain
origin
            SEE       AIC

BT       0.746
RC       0.658         3.07
BB       0.630

Core             [R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.
origin             ((MAT - 10/10)] x (a + b x rain)

         n   [R.sub.10]   [Q.sub.10]          a        b

BT       5      0.0049      2.23           300.75  -0.0250
RC       5      0.0089      1.21           438.62  -0.0347
BB       5      0.0040      2.25 **        283.22  -0.0074

Core      [R.sub.sh] = [R.sub.10] x [Q.sub.10.sup.
origin       ((MAT - 10/10)] x (a + b x rain)

         [R.sup.2]      P     SEE     AIC

BT       0.92          0.36   0.74
RC       0.83          0.52   1.10
BB       1.00         <0.01   0.01   -12.76
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Author:Zimmermann, M.; Davies, K.; de Zimmermann, V.T.V. Pena; Bird, M.I.
Publication:Soil Research
Article Type:Report
Date:May 1, 2015
Words:10058
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