Calibración de una sonda de ondas transversales

The diel variability in surface CO2 flux rates and subsurface CO2concentration and δ13CCO2 profiles has

important implications for the interpretation of Dry Valley soil CO2 flux measurements. The results

presented in this study are particularly relevant to studies investigating ecosystem activity and response to environmental change, and C turnover times in the Dry Valleys, as well as CO2 flux

studies in other deserts (e.g. Ma et al., 2013; Brummell et al., 2014).

Previous studies of Dry Valley soil CO2 fluxes have not considered surface and subsurface CO2

dynamics in such detail, and have based their interpretations of surface CO2 flux rates on the

following two assumptions: 1) that maximum surface CO2 flux rates occur at the warmest times of

the day and represent the maximum rate of soil CO2 production, and 2) that surface CO2 fluxes are

representative of CO2 production within the soil (i.e. the system is in steady-state). This study has

demonstrated that these two assumptions do not hold for all sites, hence new approaches to characterisation of Dry Valley soil CO2 fluxes are necessary.

6.3.1

Comparisons of Dry Valley ecosystem activity

The misinterpretation of surface CO2 flux rates as representing the rate of CO2 production within the

soil is problematic for studies comparing the timing and magnitude of ecosystem activity between sites. Firstly, the results presented in this study have demonstrated that Dry Valley CO2 dynamics are

characteristically non-steady. Therefore, the surface CO2 flux is not representative of soil CO2

production at the time of measurement. Characteristic response times (t) for diffusive systems are related to length scales (L) and diffusivity (D) by t = L2_{/D (Anderson and Anderson, 2010). For the}

effective diffusivity of the coarse-textured soils studied (0.03 cm2 _s-1_{), perturbations in CO2}

production at 15 cm depth would take ~2 h to affect surface fluxes. Therefore, periods of steady- state production that persist for up to 2 h are probably necessary to yield steady surface CO2 flux

rates. This appears to occur in soils where biological CO2 production is low (Site B), but CO2 dynamics

in soils where biological CO2 production is relatively high (Site A) are characterised by high spatial and

temporal variability in subsurface CO2 production, hence steady-state is rarely observed. Secondly,

site-specific differences in the timing of maximum CO2 production are likely to be regulated by local

soil temperature and moisture availability, as well as the functionality of the biological community present. This means that local site conditions must be carefully monitored to ensure comparability between datasets. Finally, this study shows that surface CO2 flux rates alone do not provide an

indication as to the relative proportions of biotic and abiotic contributions to the flux. Consequently, comparisons between ecosystem activity based on surface CO2 flux rates may be misleading.

6.3.2

Carbon turnover times

Estimates of C turnover times are contingent on reliable measures of heterotrophic soil respiration. The caveats discussed above suggest that existing estimates of C turnover times may be erroneous. At sites where there is a substantial abiotic component to the flux, turnover times calculated on the basis that soil CO2 fluxes represent biological CO2 production will be underestimates. Any abiotic

component to the flux means that C utilisation rates will be less than previously assumed, hence C pools will have a longer turnover time. Alternatively, at sites with high biological activity, turnover times could be underestimates if the surface CO2 flux rates on which they are based did not capture

flux rates at the time of maximum CO2 production. This has implications for the legacy and spatial

subsidy models of C supply. Current estimates of C turnover time, based on in situ CO2 flux rates,

range from 23 to 130 years (Burkins et al., 2001; Elberling et al., 2006), which is incompatible with the legacy model. However, it has been suggested that there may be at least 2 pools of organic C involved in Dry Valley C cycling: a small, labile organic C pool which is sustained via in situ autotrophic productivity, and a larger, more recalcitrant organic C pool derived from ancient glacial and lake sediments (Burkins et al., 2001; Barrett et al., 2005). Regardless of the organic C pools involved, if soil

CO2 flux rates used in calculations of C turnover times are overestimates, then current ecosystem

activity may in fact be able to be sustained by legacy C.

6.3.3

Field and laboratory manipulations

The results presented in this study can also be used to inform laboratory studies that utilise Dry Valley soils. As with temperate soils, laboratory studies of Dry Valley soil microbial activity also show a greater respiratory response to increased soil temperatures (Elberling et al., 2006; Hopkins et al., 2006). However, most of these studies also increased moisture content and provided additions of C and N (Hopkins et al., 2006; Hopkins et al., 2008; Sparrow et al., 2011), thereby providing favourable conditions for biological activity. No studies have attempted to simulate the diel temperature variability that characterises field conditions, instead maintaining a relatively high but stable temperature equivalent to the maximum summertime soil temperature (Hopkins et al., 2006; Hopkins et al., 2008; Sparrow et al., 2011). Without temperature variability, there is no mechanism for abiotic CO2 production, and thus on the basis of increased respiration rates under consistently

warm temperatures (and following substrate addition), these studies have concluded that soil CO2

production is of biological origin. However, without a genuine simulation of field conditions, results from subsidy experiments do not provide sufficient evidence to preclude abiotic CO2 production

under field conditions. Similarly, laboratory decomposition studies that show δ13_{C values of emitted}

CO2 are similar to the δ13C values of source organic materials (Hopkins et al., 2009) simply confirm

the inference of a biological response to increased temperature and substrate addition. Again, this would be expected in a laboratory environment, but it cannot be used to infer that biological processes dominate CO2 production in the field. As steady-state (with respect to temperature)

laboratory experiments cannot simulate the range of processes occurring in the field, such studies will generally eliminate abiotic processes. Consequently, conclusions from studies conducted at a constant temperature must be re-evaluated before being assumed to fully describe CO2 dynamics in

the field.

Field manipulations such as those incorporating substrate additions, and artificially increasing soil temperature and moisture content, will also benefit from insight gained through this study. In particular, the timing and frequency of soil CO2 flux measurements is critical, and may significantly

influence the interpretations made. Further insight could be readily gained from studies such as Barrett et al.’s (2008a) tracer study, which used a 13_{C-labelled sugar to track heterotrophic CO}

2

production from nematodes. If the measured surface CO2 fluxes (from which the nematode

contribution to surface fluxes was calculated) included an abiotic component, then the contribution