General patterns of responses
Soil surface CO2 flux (RS) provides an integrated result of biological CO2 production throughout the soil column, changes in soil CO2 diffusivity in the soil profile, and in some areas geological processes. Soil CO2 is produced primarily by heterotrophic activity (i.e., decomposer organisms) and by autotrophic activity (i.e., living roots and mycorrhizae), where most of the produced CO2 is released to the atmosphere (Raich and Schlesinger, 1992; Schlesinger and Andrews, 2000).
Increase in RS following rewetting of dry soils have been reported in multiple terrestrial ecosystems and various land-use types, including pulses observed in cropland (Kessavalou et al., 1998), grazing pasture (Xu and Baldocchi, 2004), forest (Kim et al., 2010b), grassland (Joos et al., 2010), savannas (Castaldi et al., 2010) and desert (Sponseller and Fisher, 2008). Incubation experiments have yielded similar patterns, showing RS increases in soils from cropland (Beare et al., 2009), grazing pasture (Wu et al., 2010b), forest (Fierer and Schimel, 2003), grassland (Xiang et al., 2008), peatland (Goldhammer and Blodau, 2008) and desert (Sponseller and Fisher, 2008) ecosystems. For example, in an upper Sonoran Desert ecosystem, RS increased up to 30-fold immediately following experimental rewetting, and returned to background levels within 48 h (Sponseller, 2007). In soil moisture manipulations in a Norway spruce plantation, drought and rewetting treatments increased the annual CO2 flux by 51% (Borken et al., 1999). Lee et al. (2004) estimated that the peak RS flux in a single intensive storm amounted to a loss of 0.18 t C ha-1 to the atmosphere, or 5 − 10% of the annual net ecosystem production in a mid-latitude forest. These studies have reported responses ranging from short-term (ca. 6 − 24 h) up to 30 d responses (Table 1, Fig. 4), and relative RS changes ranging from 40% to >10,000% increases (Table 1, Fig. 2). Together, these studies support that hypothesis that rewetting a variety of soil types can have substantial affects on the C balance of terrestrial ecosystems (Borken et al., 1999; Lee et al., 2004; Xu et al., 2004). However, we caution that most of these studies lack the high temporal sampling resolution necessary to capture the full dynamic of the pulse (Groffman et al., 2006; Muhr et al., 2009, Vargas et al 2011).
It is important to recognize that RS could be suppressed during or after rainfall as previous studies have reported: 1) large (10-fold) decreases during light rainfall in arable soils (Rochette et al., 1991); 2) sharp RS decreases in no-tillage agricultural fields (Ball et al., 1999); and 3) reduced CO2 grassland fluxes with artificially increased variability of rainfall in mesic grasslands (Knapp et al., 2002). Possible explanations for these reduced RS rates are: 1) increased water in the soil pore space reduce soil CO2 diffusivity rates (Rochette et al. 1991; Šimůnek and Suarez, 1993); 2) soil CO2 may dissolve into the infiltrating water (Johnson et al., 2008); and 3) the restriction of the soil macro-porosity by the rainfall reduces soil air-filled pore space enhances anaerobiosis and reduces aerobic respiration (Linn and Doran 1984; Ball et al. 1999; Davidson et al., 2000). In the following sections we focus on the positive impact of rewetting on RS.
Mechanisms and drivers
Two broad mechanisms responsible for increased RS following rewetting have been commonly hypothesised: 1) enhanced microbial metabolism by substance supply, and 2) changes in physical protection of organic matter. First, microbial metabolism can be enhanced by the availability of accumulated substrates during soil drying periods. A large proportion of microorganisms, fine roots and mycorrhizae die during drought (Clein and Schimel, 1994; Teepe et al., 2001); these dead cells tend to have low C:N ratios and could rapidly decompose during rewetting (Kieft et al., 1987; Van Gestel et al., 1993). Microorganisms accumulate high concentration of solutes (osmolytes) to retain water inside the cell during drought conditions (Harris, 1981), which rapidly decompose on rewetting (Fiere and Schimel, 2003; Schimel et al., 2007). Root exudates from reviving plants could thus significantly affect soil surface flux (Crow and Wieder, 2005; Curiel Yuste et al., 2007). Second, rewetting could disrupt soil aggregates, exposing physically protected organic matter and increase the accessibility of substrate that can be rapidly mineralized (Groffman and Tiedje, 1988; Appel, 1998). Other physical mechanisms that can influence gas flux include infiltration, reduced diffusivity and gas displacement in the soil. For example, the infiltration of rainwater may displace CO2 that accumulates in soil pore spaces during dry periods (Huxman et al., 2004).
The magnitude of RS increases following rewetting depends on: 1) the size of soil organic pool; 2) the quality of organic matter, determined by its age, origin, and extent to which these substrates are protected from microbial attack by adsorption to clay surfaces and inclusion in micro-aggregates; and 3) the properties of soil biota (Van Gestel et al., 1993). Soil moisture conditions before rewetting also influence the response (Orchard and Cook, 1983; Cable et al., 2008), as can the length and severity of drought periods (Unger et al., 2010), and rain pulse size (Sponseller, 2007; Chen et al., 2009). Based on our literature review, we identified the existence of a threshold in soil moisture at 12−20% gravimetric moisture content, below which a substantial increases in RS after rewetting typically is observed (Davidson et al., 1998; Xu and Qi, 2001; Rey et al., 2002; Yuste et al., 2003; Dilustro et al., 2005; Cable et al., 2008; Chou et al., 2008; Kim et al., 2010b; Misson et al., 2010).
The effects of rewetting may decline with successive drying and rewetting cycles, possibly as a result of a limited pool of labile substrates that have built up over time or during the dry season (Schimel and Mikan, 2005; Goldberg et al., 2008). Importantly, Fernández et al. (2006) suggested that substrate availability, rather than soil moisture, influenced the duration of the CO2 pulse in response to rain events, while Vargas et al (2010b) noted that RS pulses may be driven not only by labile substrate availability, but also plant photosynthesis rates following the rain event. In addition, management practice (mowing or tillage) (Steenwerth et al., 2010), vegetation type (Shi et al., 2011) and high soil temperatures (Jager and Bruins, 1975; Borken et al., 1999) could influence the magnitude of the response of soil CO2 flux following rewetting of dry soils.
Berard, A., Bouchet, T., Sevenier, G., Pablo, A.L., Gros, R., 2011. Resilience of soil microbial communities impacted by severe drought and high temperature in the context of Mediterranean heat waves. European Journal of Soil Biology 47, 333-342.
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In the context of Climate Change, the increasing of frequency and intensity of droughts and heat waves constitutes a serious threat for agroecosystems in the Mediterranean region. Soils and their functions may be impacted by these extreme events through changes in the biomass, composition and activities of edaphic microbial communities. We designed an experiment to investigate changes over time in the microbial biomass, composition (EL-FAME profiles) and functions (catabolic responses) after severe drought and high temperature disturbances. Impacts were assessed using indoor soil microcosms under controlled drought and high temperatures, mimicking various stress scenarios and durations in conditions of severe drought and heat wave. Drought and heat wave restructured the soil microbial communities over the course of the experiment. This may be a consequence of inhibition and/or killing of sensitive species and selection of tolerant species by the disturbances applied, but also of the proliferation of fast-growing species after environmental soil conditions had been restored. Heating dry soil at 50 °C had a stronger effect than only drying. Moreover, above a critical threshold of heat wave duration, soil microbial communities may have undergone a drastic biomass killing and restructuring associated with a shift in physiological traits. In this experimental context, resilience of microbial catabolic functions was not observed and in consequence ecosystem processes such as carbon mineralization and sequestration in soil may be affected.
Boot, C., 2011. Microbial response to drying and rewetting: osmotic and matric effects. Plant and Soil 348, 99-102
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Chatterjee, A., Jenerette, G.D., 2011. Changes in soil respiration Q10 during drying-rewetting along a semi-arid elevation gradient. Geoderma 163, 171-177
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Wetting induced increases in soil CO2 efflux (R) from dry soils have been repeatedly reported, however little is known about the sensitivity of the pulse to temperature. To address this knowledge gap changes in temperature sensitivity of soil CO2 efflux (Q10) during repeated drying–rewetting (DW) events were experimentally quantified for soils collected both under canopy and interspace microenvironments at three elevations on Santa Rosa Mountain in southern California. Five field-replicated surface soil samples for each location were incubated at 13, 19, 25 and 31 °C. At each temperature, three consecutive DW cycles were performed by adding water to 40% water holding capacity. Instantaneous R was measured immediately after wetting and repeatedly until the soils were dry (< 2% of added water). Soil R responses were averaged above and below 20% WHC and considered as wet and dry fluxes, respectively. Wet and dry soil R responses were separately modeled using the Arrhenius equation and activation energy (Ei) was determined using non-linear mixed-level modeling. Soil R at 25 °C (flux25) increased with elevation gradient with a decrease in required Ei values. Negative relationship between flux25 and Q10 supported the carbon-quality hypothesis, whereas, Q10 values > 2 supported a temperature sensitive metabolic pulse throughout repeated DW events for soils across the mountain. Including variation in Arrhenius temperature kinetics with precipitation patterns has the potential to improve predictions of the precipitation pulse induced C loss across large spatiotemporal scales.
Research highlights
► Precipitation induced soil respiration pulses have a strong temperature sensitivity. ► Microenvironment influences Q10 change over drying at higher elevations. ► Inclusion of pulse induced Q10 variation is critical for global flux modeling.
Freeze-thaw cycles might also alter the chemical composition of SOM - as shown by sugar analysis:
ReplyDeleteSchmitt A, Glaser B, Borken W and Matzner E (2008) Repeated freeze-thaw cycles changed organic matter quality in a temperate forest soil. J Plant Nutr Soil Sci 171:707-718
Marañón-Jiménez, S., Castro, J., Kowalski, A.S., Serrano-Ortiz, P., Reverter, B.R., Sánchez-Cañete, E.P., Zamora, R., 2011. Post-fire soil respiration in relation to burnt wood management in a Mediterranean mountain ecosystem. Forest Ecology and Management 261, 1436-1447
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After a wildfire, the management of burnt wood may determine microclimatic conditions and microbiological activity with the potential to affect soil respiration. To experimentally analyze the effect on soil respiration, we manipulated a recently burned pine forest in a Mediterranean mountain (Sierra Nevada National and Natural Park, SE Spain). Three representative treatments of post-fire burnt wood management were established at two elevations: (1) “salvage logging” (SL), where all trees were cut, trunks removed, and branches chipped; (2) “non-intervention” (NI), leaving all burnt trees standing; and (3) “cut plus lopping” (CL), a treatment where burnt trees were felled, with the main branches lopped off, but left in situ partially covering the ground surface. Seasonal measurements were carried out over the course of two years. In addition, we performed continuous diurnal campaigns and an irrigation experiment to ascertain the roles of soil temperature and moisture in determining CO2 fluxes across treatments. Soil CO2 fluxes were highest in CL (average of 3.34 ± 0.19 μmol m−2 s−1) and the lowest in SL (2.21 ± 0.11 μmol m−2 s−1). Across seasons, basal values were registered during summer (average of 1.46 ± 0.04 μmol m−2 s−1), but increased during the humid seasons (up to 10.07 ± 1.08 μmol m−2 s−1 in spring in CL). Seasonal and treatment patterns were consistent at the two elevations (1477 and 2317 m a.s.l.), although respiration was half as high at the higher altitude.
Respiration was mainly controlled by soil moisture. Watering during the summer drought boosted CO2 effluxes (up to 37 ± 6 μmol m−2 s−1 just after water addition), which then decreased to basal values as the soil dried. About 64% of CO2 emissions during the first 24 h could be attributed to the degasification of soil pores, with the rest likely related to biological processes. The patterns of CO2 effluxes under experimental watering were similar to the seasonal tendencies, with the highest pulse in CL. Temperature, however, had a weak effect on soil respiration, with Q10 values of ca. 1 across seasons and soil moisture conditions. These results represent a first step towards illustrating the effects of post-fire burnt wood management on soil respiration, and eventually carbon sequestration.
Research highlights
► Post-fire wood management altered soil respiration. ► Soil CO2 fluxes were highest where the burnt wood covered the ground. ► Basal values were registered during summer and increased during humid seasons. ► Soil respiration was mainly controlled by moisture, temperature had a weak effect. ► Watering after drought boosts CO2 effluxes, which decreased to basal values as the soil dried. ► Soil respiration was half at the higher elevation.
Parton, W., Morgan, J., Smith, D., Del Grosso, S., Prihodko, L., LeCain, D., Kelly, R., Lutz, S., 2011. Impact of Precipitation Dynamics on Net Ecosystem Productivity. Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02611.x.
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Net ecosystem carbon dioxide (CO2) productivity (NEP) was measured on shortgrass steppe (SGS) vegetation at the USDA Central Plains Experimental Range in northeastern Colorado from 2001 to 2003. Large year-to-year differences were observed in annual NEP, with > 95% of the net carbon uptake occurring during May and June. Low precipitation during the 2002 April to June time period greatly reduced annual net carbon uptake. Large precipitation events (> 10 mm day−1) promoted carbon uptake, while small precipitation events (< 10 mm day−1) enhanced heterotrophic respiration and resulted in a net loss of carbon from the system. Large precipitation event enhanced carbon uptake was attributed to increased soil water content (SWC), which promotes plant photosynthesis. The large precipitation events which occurred from July to October have lower increases in daytime net CO2 uptake (NEPd) due to the presence of low live plant biomass compared to earlier in the growing season. Live aboveground plant biomass (AGB), solar radiation, and SWC were the major variables that controlled NEPd, while AGB, SWC, and relative humidity control nighttime respiration losses (NEPn). Aboveground plant biomass is the most important variable for controlling both NEPd and NEPn dynamics. These results suggest that the major factor controlling growing season NEPn is the amount of carbon fixed via photosynthesis during the day. Heterotrophic soil respiration is greatly enhanced for one to two days following rainfall events with daily rainfall events > 5 mm having a similar increase in respiration (> 3.00 g m Cm−2 d−1). In addition, the size of the heterotrophic respiration pulse is independent of both the amount of time since the last rainfall event and the time of occurrence during the growing season.
Sainju, U.M., Caesar-TonThat, T., Caesar, A., 2011. Comparison of soil carbon dioxide flux measurements by static and portable chambers in various management practices. Soil and Tillage Research, doi:10.1016/j.still.2011.1010.1020
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Portable chamber provides simple, rapid, and inexpensive measurement of soil CO2 flux but its effectiveness and precision compared with the static chamber in various soil and management practices is little known. Soil CO2 flux measured by a portable chamber using infrared analyzer was compared with a static chamber using gas chromatograph in various management practices from May to October 2008 in loam soil (Luvisols) in eastern Montana and in sandy loam soil (Kastanozems) in western North Dakota, USA. Management practices include combinations of tillage, cropping sequence, and N fertilization in loam and irrigation, tillage, crop rotation, and N fertilization in sandy loam. It was hypothesized that the portable chamber would measure CO2 flux similar to that measured by the static chamber, regardless of soil types and management practices. In both soils, CO2 flux peaked during the summer following substantial precipitation and/or irrigation (>15 mm), regardless of treatments and measurement methods. The flux varied with measurement dates more in the portable than in the static chamber. In loam, CO2 flux was 14–87% greater in the portable than in the static chamber from July to mid-August but 15–68% greater in the static than in the portable chamber from late August to October in all management practices. In sandy loam, CO2 flux was 10–229% greater in the portable than in the static chamber at all measurement dates in all treatments. Average CO2 flux across treatments and measurement dates was 9% lower in loam but 84% greater in sandy loam in the portable than in the static chamber. The CO2 fluxes in the portable and static chambers were linearly to exponentially related (R2 = 0.68–0.70, P ≤ 0.01, n = 40–56). Although the trends of CO2 fluxes with treatments and measurement dates were similar in both methods, the flux varied with the methods in various soil types. Measurement of soil CO2 flux by the portable chamber agreed more closely with the static chamber within 0–10 kg C ha−1 d−1 in loam soil under dryland than in sandy loam soil under irrigated and non-irrigated cropping systems.
Fenner, N., Freeman, C., 2011. Drought-induced carbon loss in peatlands. Nature Geosci 4, 895-900
ReplyDeletePeatlands store vast amounts of organic carbon, amounting to approximately 455 Pg. Carbon builds up in these water-saturated environments owing to the presence of phenolic compounds—which inhibit microbial activity and therefore prevent the breakdown of organic matter. Anoxic conditions limit the activity of phenol oxidase, the enzyme responsible for the breakdown of phenolic compounds. Droughts introduce oxygen into these systems, and the frequency of these events is rising. Here, we combine in vitro manipulations, mesocosm experiments and field observations to examine the impact of drought on peatland carbon loss. We show that drought stimulates bacterial growth and phenol oxidase activity, resulting in a reduction in the concentration of phenolic compounds in peat. This further stimulates microbial growth, causing the breakdown of organic matter and the release of carbon dioxide in a biogeochemical cascade. We further show that re-wetting the peat accelerates carbon losses to the atmosphere and receiving waters, owing to drought-induced increases in nutrient and labile carbon levels, which raise pH and stimulate anaerobic decomposition. We suggest that severe drought, and subsequent re-wetting, could destabilize peatland carbon stocks; understanding this process could aid understanding of interactions between peatlands and other environmental trends, and lead to the development of strategies for increasing carbon stocks.
Xu, X., Luo, X., 2012. Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season. Geoderma 170, 118-126
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The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ, rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO2 pulses increased from 0.84 to 2.08 g CO2–C m− 2 d− 1 with the intensity of wetting up to ca. 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH4–C m− 2 d− 1 with the intensity of wetting. Wetting dry forest floor increased N2O fluxes from 6.2 to 25.9 μg N2O–N m− 2 d− 1, but there was no significant difference between all experimental wetted plots. The rainfall-induced N2O pulses with increasing wetting intensity were opposite to that of the CO2 pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO2 flux and CH4 uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q10 value of the temperature response of CO2 fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO2 pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.
Highlights
► Wetting intensities were manipulated by subsequently maintained soil moisture levels. ► In situ GHG fluxes from forest floor differently responded to wetting intensity. ► Wetting increased soil microbial biomass and metabolic quotient differently. ► Wetting could significantly affect the sizes of soil active C and N pools.
Estop-Aragonés, C., Blodau, C., 2012. Effects of experimental drying intensity and duration on respiration and methane production recovery in fen peat incubations. Soil Biology and Biochemistry, doi:10.1016/j.soilbio.2011.1012.1008
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Drying and rewetting to a variable extent influence the C gas exchange between peat soils and the atmosphere. We incubated a decomposed and compacted fen peat and investigated in two experiments 1) the vertical distribution of CO2 and CH4 production rates and their response to drying and 2) the effects of temperature, drying intensity and duration on CO2 production rates and on CH4 production recovery after rewetting. Surface peat down to 5 cm contributed up to 67% (CO2) and above 80% (CH4) of the depth-aggregated (50 cm) production. As CO2 production sharply decreased with depth water table fluctuations in deeper peat layers are thus not expected to cause a substantial increase in soil respiration in this site. Compared to anaerobic water saturated conditions drying increased peat CO2 production by a factor between 1.4 and 2.1. Regarding the effects of the studied factors, warmer conditions increased and prolonged drying duration decreased CO2 production whereas the soil moisture level had little influence. No significant interactions among factors were found. Short dry events under warmer conditions are likely to result in greatest peaks of CO2 production rates. Upon rewetting, CH4 production was monitored over time and the recovery was standardized to pre-drying levels to compare the treatment effects. Methane production increased non-linearly over time and all factors (temperature, drying intensity and duration) influenced the pattern of post-drying CH4 production. Peat undergoing more intense and longer drying events required a longer lag time before substantial CH4 production occurred and warmer conditions appeared to speed up the process.
Highlights
► In degraded fen >50% of peat respiration concentrated in uppermost 5 cm of peat. ► Depth dependency explains lack of CO2 flux response to water table change. ► Peat respiration influenced by temperature and drying duration but not intensity. ► Recovery of methanogenesis from drought accelerates with soil temperature. ► Recovery of methanogenesis from drought slows with drying duration and intensity.
Harms, T.K., Grimm, N.B., 2012. Responses of trace gases to hydrologic pulses in desert floodplains. J. Geophys. Res. 117, G01035
ReplyDeletePulsed hydrologic inputs interact with antecedent moisture conditions to shape biogeochemical dynamics in many ecosystems, but the outcomes of these interactions remain difficult to predict. Hydrologic pulses may influence biogeochemical activity through several mechanisms: by providing water as a resource, providing limiting nutrients or substrates that fuel particular biogeochemical pathways, or determining redox conditions. Antecedent moisture conditions may modify the relative importance of each of these potential mechanisms, by influencing accumulation of labile carbon and nutrients, the severity of water limitation to biological processes, and longer-term effects on abiotic conditions, including redox. We experimentally applied hydrologic pulses of different sizes (1-cm and 20-cm events) to soils of desert floodplains and assessed responses of trace gases (CO2, CH4, NO, and N2O) in dry and monsoon seasons to test these mechanisms. Size of the hydrologic pulse strongly interacted with antecedent soil-moisture conditions to determine emissions of some trace gases. Following dry antecedent conditions, water addition stimulated emissions of CO2, CH4, and NO, but not N2O, and larger experimental pulses resulted in larger fluxes. In the monsoon season, responses to water addition were muted and size of the hydrologic pulse had no effect, except for CH4 emission, which increased in response to the 20-cm event. Seasonal contrasts indicated that antecedent moisture conditions constrain the effects of hydrologic pulses on biogeochemical processes, whereas contrasts among responses of different trace gases demonstrated that mechanisms controlling emissions of particular gases are water limitation (CO2), in situ production of nitrogen substrates (NO), or redox conditions (CH4). Strong and predictable interactive effects of water inputs and antecedent conditions indicate that extended droughts may cause elevated emissions of gaseous C and NO following the return of precipitation, whereas larger floods or longer wet seasons are expected to dampen gaseous fluxes, which may contribute to conserving soil C and nutrients within floodplains.
Song, W., Chen, S., Wu, B., Zhu, Y., Zhou, Y., Li, Y., Cao, Y., Lu, Q., Lin, G., 2012. Vegetation cover and rain timing co-regulate the responses of soil CO2 efflux to rain increase in an arid desert ecosystem. Soil Biology and Biochemistry 49, 114-123
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Climate models often predict that more extreme precipitation events will occur in arid and semiarid regions, where C cycling is particularly sensitive to the amount and seasonal distribution of precipitation. Although the effects of precipitation change on soil carbon processes in desert have been studied intensively, how vegetation cover and rain timing co-regulate the responses of soil CO2 efflux to precipitation change is still not well understood. In this study, a field manipulative experiment was conducted with five simulated rain addition treatments (natural rains plus 0%, 25%, 50%, 75%, 100% of local annual mean precipitation) in a desert ecosystem in Northwest China. The rain addition treatments were applied with 16 field rain enrichment systems on the 10th day of each month from May to September, 2009. Soil water content, soil temperature and soil CO2 efflux rates were measured in both bare and vegetated soils before and after the rain addition during a 3-week period for each rain treatment. The response magnitude and duration of soil CO2 efflux to rain addition depended not only on the rain amount but also on the type of vegetation covers and the timing of rain addition treatments. Soil water content responded quickly to the rain addition regardless of rain amount and timing, but soil CO2 efflux increased to rain addition only in May–July but not in late growing season (September). In addition, soil CO2 efflux from the bare and vegetated soils showed similar increase to rain additions in May–July, but they demonstrated distinct responses to rain addition in September. The differences in the responses of soil CO2 efflux to rain addition between the bare and vegetated soils could be explained by the root activities stimulated by added rain water, while the difference in soil CO2 efflux response to rain addition among treatment times could be attributed to soil water condition prior to rain addition and/or soil temperature drop following rain addition. Thus, both vegetation cover and rain timing can co-regulate responses of soil CO2 efflux to future precipitation change in arid desert ecosystems, which should be considered when predicting future carbon balance of desert ecosystems in arid and semiarid regions.
Ma, S., Baldocchi, D.D., Hatala, J.A., Detto, M., Curiel Yuste, J., 2012. Are rain-induced ecosystem respiration pulses enhanced by legacies of antecedent photodegradation in semi-arid environments? Agricultural and Forest Meteorology 154-155, 203-213
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Ecosystemrespiration (Reco) is highly variable in semiarid ecosystems. After a period of drought, Reco can jump to a high value in response to rain events, and afterward it decays exponentially with time. To better understand the timing, size, and duration of rain-inducedrespirationpulses, we examined 57 rain events from over 23 site-years of eddy covariance measurements at an open annual grassland, a woodland understory, and a peatland pasture in California, USA. To explain these findings, we conducted a factorial litter-watering experiment on grass and soil plots exposure to varying amounts of sunlight and water. To interpret and compare the population of rain-inducedrespirationpulses, we fitted the data with a two pool, 4-parameter exponential decay model.
At the field scale, the total amounts of CO2 emission emanating from a single and sustained respirationpulse (mean ± standard deviation) were 44.4 ± 38.0, 24.2 ± 17.8, and 94.6 ± 45.8 gC m−2 at the open grassland, the woodland understory, and the peatland pasture, respectively. The large variations in precipitation of these respiration-triggering rain events were associated with 73–84% of variation in the total amount of CO2 emission.
Litter that experienced antecedentphotodegradation tended to enhance respirationpulses. Biotic and abiotic processes involved in the dynamic of respirationpulses were pulse-specific, and we summarized them into two pulse scenarios. This study illustrates an emerging connection of ecosystem processes, and these findings may help to improve models on the dynamics of ecosystem CO2 cycling.
Highlights
► The highlight of this study is to make a link between rain-inducedrespirationpulses and legacies of antecedentphotodegradation by analyzing a dataset over 23 site-years of eddy covariance measurements, applying a factorial litter-watering experiment, and modeling labile carbon pools involved in respirationpulses. ► Our data show that litter, combined with precipitation, enhanced the size of respirationpulses. ► This study is done timely when photodegradation and its sequent effects on carbon fluxes start attracting the attentions of the carbon flux community. ► No one has reported the potential effects of antecedentphotodegradation on ecosystemrespirationpulses yet to our knowledge.
Ohkubo, S., Iwata, Y., Hirota, T., 2012. Influence of snow-cover and soil-frost variations on continuously monitored CO2 flux from agricultural land. Agricultural and Forest Meteorology 165, 25-34
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Changes in cryospheric snow accumulation, snowmelt, and soil freezing and thawing might influence the ground-surface CO2 flux and cumulative winter CO2 flux from agricultural land. We continuously observed CO2 flux using automatically closing chambers at an untreated control plot and a plot with snow removal in northern Japan. The CO2 in soil pores at 10-cm depth increased by 6.5 ppmv day−1 as soil began to freeze, but it increased dramatically (to 49 ppmv day−1) after snowmelt water infiltrated the soil and froze. The soil-frost layer constrained gas diffusion into the air, and the barrier strengthened as the frozen snowmelt water decreased the air volume in soil pores. Leached gas CO2 from the freezing snowmelt water also increased gas CO2 concentration in soil. As the soil thawed, the CO2 concentration decreased drastically, at 790 ppmv day−1. However, these changes had little effect on CO2 flux. The soil CO2 concentration remained stable after snow cover reached 30 cm in the control plot. Low CO2 flux in both plots occurred during the winter. No clear relation was found between CO2 flux and snow depth or soil-frost depth because of the small CO2 source at this site. We also considered how the presence of the chamber influenced soil temperatures and water contents. During the snow-free season, the chamber mitigated diurnal changes in soil temperature. The daily average soil temperature differed from that in the natural state by −1.7 °C to 6.3 °C. This fluctuation of temperature corresponded to the fluctuation of CO2 flux, which ranged from 91% to 143% of the CO2 flux in the natural state based on the temperature-response equations. The chamber had little influence on the soil temperature during the snow-cover period, and did not influence soil water content throughout the study period. Cumulative winter CO2 emissions were 17.2 gC m−2 (over 143 days) in the control plot and 13.4 gC m−2 (over 151 days) in the treated plot (10.0 and 7.5% of annual accumulation, respectively).
Highlights
► We observed CO2 gas dynamics over agricultural bare soil in the snow-covered season. ► Ground surface CO2 flux was evaluated continuously with automatic chambers. ► The soil-frost layer increased soil CO2 concentration but did not affect the CO2 flux. ► During freeze-thaw events, the CO2 flux varied concomitantly with soil temperature. ► The cumulative CO2 flux during the winter period was 10.0% of the annual flux.
Harrison-Kirk, T., Beare, M.H., Meenken, E.D., Condron, L.M., 2012. Soil organic matter and texture affect responses to dry/wet cycles: Effects on carbon dioxide and nitrous oxide emissions. Soil Biology and Biochemistry, http://dx.doi.org/10.1016/j.soilbio.2012.1010.1008
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Soil organic matter (SOM) content and texture are important factors affecting carbon (C) and nitrogen (N) mineralisation under constant soil moisture but their effects on organic matter mineralisation and associated biogenic gas (carbon dioxide (CO2) and nitrous oxide (N2O)) production during dry/wet cycles is poorly understood. A laboratory incubation study was conducted to quantify CO2 and N2O production during sequential dry/wet cycles and under constant soil moisture conditions along a gradient of SOM contents in two soil types representing different texture classes (silt loam vs. clay loam). Three soil moisture treatments were established: wet (WW; field capacity), moderately dry (MD; 120% of soil moisture content (SMC) at wilting point (WP)) and very dry (VD; 80% of SMC at WP). To each of the two ‘dry’ treatments two different dry/wet treatments were applied where the soils were either maintained continuously dry (MD & VD) or subjected to three sequential 20-day long dry/wet cycles (MDW & VDW) during the treatment phase of the experiment. At field capacity soil moisture content, the rate of C mineralisation increased with increases in SOC content and the increase per unit of C was twice as high in silt loam (0.30 mg CO2-C g−1 SOC d−1) as in clay loam (0.13 mg CO2-C g−1 SOC d−1) soils. N2O-N emissions also increased with increasing in SOC content. However, in contrast to C mineralisation, the effect was four-fold greater for clay loam (1.38 μg N2O-N g−1 SOC d−1) than silt loam (0.32 μg N2O-N g−1 SOC d−1) soils. Following rewetting, the VDW and MDW soils produced a short-term C mineralisation flush that was, on average, 30% and 15% greater, respectively, than in WW soils. However, the flush of C mineralisation was not sufficient to compensate for the reduction in mineralisation during the drying phase of each cycle, resulting in a lower total C mineralisation from MDW and VDW soils, on average, compared with WW soils over the three sequential dry/wet cycles. The C mineralisation flush also remained a relatively constant proportion of the total C mineralised from both silt loam (23%) and clay loam soils (22%), irrespective of their SOC content. In contrast, the short-term flush of N2O that followed rewetting of dry soil accounted for 62% and 68% of the total N2O emissions from silt loam and clay loam soils, respectively. On average, the total N2O emissions from dry/wet treatments imposed on silt loam and clay loam soils were 33% and 270% greater, respectively, than from the WW treatments, though the effect varied greatly and depended on SOC content. Overall, N2O emissions were highest where we had a combination of fine texture, an adequate supply of available C (i.e. high SOM content), and a water-filled pore space (WFPS) > 0.60 cm cm−3 at field capacity. Prediction of C mineralisation over dry/wet cycles using mineralisation data from soils at constant moisture content is possible, but knowledge of the stress history for the soil would be required to improve accuracy. The prediction of N2O-N emissions during dry/wet cycles using emission data from soils at constant moisture was very inaccurate, due to the inherent spatial variability of N2O emissions.
Highlights
► C mineralisation increased with increasing SOC, silt loam twice clay loam. ► N2O emissions increased with increasing SOC, clay loam four times silt loam. ► CO2 and N2O response to dry/wet cycles depended on SOC content. ► CO2 flush following rewetting was 22–23% of the total C mineralised. ► N2O flush following rewetting was 62–68% of total N2O emissions.
Jin, V.L., Haney, R.L., Fay, P.A., Polley, H.W., 2013. Soil type and moisture regime control microbial C and N mineralization in grassland soils more than atmospheric CO2-induced changes in litter quality. Soil Biology and Biochemistry 58, 172-180.
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Global change-induced alterations in litter quality and soil moisture regime will likely impact grassland C and N dynamics, but how these changes interact with edaphic properties across the landscape is unclear. We measured the effects of litter quality, soil type, soil moisture level, and soil drying-rewetting frequency (D-RW) on microbial C and N mineralization of litter and soil organic matter (SOM) in a full-factorial, controlled incubation experiment. Four levels of litter quality (no litter; or litter from Bouteloua curtipendula grown under 280, 380, 500 μL L−1 CO2) were surface-applied to three contrasting soils common to Blackland Prairie landscapes: an upland Mollisol, a lowland Vertisol, and a fluvial Alfisol. Different soil moisture regimes were tested by incubating soils at four moisture levels (air-dry, 25%, 35%, or 50% water-holding capacity, WHC) and by drying-rewetting soils 0, 1, 2, 4 or 8 times over the 112-d incubation period. Litter additions stimulated microbial activity, increasing total CO2 production (i.e. C mineralized from litter + SOM decomposition) up to 17× more than no-litter controls (average 3×) and decreasing net N mineralization up to −3× less (average −0.5×) due to greater microbial N immobilization. Neither C nor N mineralization, however, was affected by litter quality. For all soils, litter decomposition increased with increasing WHC and D-RW frequency, but the average percent of total CO2 derived from litter was a negative function of SOM content. Similarly, net N mineralization also was positively correlated with soil WHC and affected most strongly by soil type (Alfisol < Mollisol < Vertisol). Net N mineralization responses to D-RW events was also soil-specific, with Alfisol soils showing no response and Mollisol and Vertisol soils decreasing after 4 D-RW events. Our results suggest that predicted changes in rainfall patterns and its interactions with soil type across the landscape will control short-term C and N mineralization responses in grasslands to a greater extent than atmospheric CO2-induced changes in litter C:N ratio for this common species of prairie grass.
Gallo, E., Lohse, K., Ferlin, C., Meixner, T., Brooks, P., Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways. Biogeochemistry, 10.1007/s10533-013-9927-0
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Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO2 and N2O immediately following wetting were among the highest ever published (up to 1,588 mg C m−2 h−1 and 3,121 μg N m−2 h−1). Mean post-wetting CO2 and N2O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m−2 h−1; 168 and 187 μg N m−2 h−1) than in the sand channels (45 mg C m−2 h−1 and 7 μg N m−2 h−1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.
Warren, C.R., 2014. Response of osmolytes in soil to drying and rewetting. Soil Biol. Biochem. 70, 22-32.
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• Water deficits were imposed slowly by withholding water from large mesocosms.
• A broad spectrum of small organic compounds was assessed using mass spectrometry.
• Osmolytes were increased 10-fold by water deficits.
• Osmolytes accounted for a substantial fraction of microbial C.
Abstract
The accumulation and subsequent release of microbial osmolytes in response to drying and rewetting are thought to be key players in C and N dynamics, yet studies on soils have failed to support this hypothesis. The aim of this experiment was to determine how low-molecular weight compounds, and osmolytes in particular, are affected by drying and rewetting. Water deficits were imposed slowly by withholding water for 21 weeks from large (200 L) mesocosms vegetated with a globally widespread grass Themeda triandra. A broad spectrum of small molecules in extracts was identified and quantified by capillary electrophoresis–mass spectrometry and gas chromatography–mass spectrometry. Compared with controls, drought-stressed mesocosms contained >10-fold larger amounts of known microbial osmolytes: ectoine, hydroxyectoine, betaine, proline-betaine, trigonelline, proline, trehalose, arabitol. The pool of osmolytes accounted for 3.6% of CHCl3 labile TOC in control mesocosms and 17% of CHCl3 labile TOC in drought-stressed mesocosms. There was no evidence that rewatering led to a large pulse of osmolytes in free solution. Instead osmolytes decreased to control concentrations within 1–3 h of rewatering – probably indicating rapid uptake by microbes and plants. Results of this study suggest that osmolytes can account for a substantial fraction of microbial C, and are at least one of the ways that soil microbes cope with water deficits.
Semenov, V.M., Kogut, B.M., Lukin, S.M., 2014. Effect of repeated drying-wetting-freezing-thawing cycles on the active soil organic carbon pool. Eurasian Soil Science 47, 276-286.
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Samples of soddy-podzolic soil (long-term overgrown fallow and continuous bare fallow), gray forest soil (forest, farming agrocenosis), and a typical chernozem (virgin steppe, forest area, farming agrocenosis, continuous bare fallow) have been incubated under stable conditions; other samples of these soils have been subjected to six drying-wetting-incubation-freezing-thawing-incubation cycles during 136 days. The wetting of dried soils and the thawing of frozen soils result in an abrupt but short increase in the emission rate of C-CO2 by 2.7–12.4 and 1.6–2.7 times, respectively, compared to the stable incubation conditions. As the soil is depleted in potentially mineralizable organic matter, the rate of the C-CO2 emission pulses initiated by disturbing impacts decreases. The cumulative extra production of C-CO2 by soils of natural lands for six cycles makes up 21–40% of that in the treatments with stable incubation conditions; the corresponding value for cultivated soils, including continuous clean fallow, is in the range of 45–82%. The content of potentially mineralizable organic matter in the soils subjected to recurrent drying-wetting-freezingthawing cycles decreased compared to the soils without disturbing impacts by 1.6–4.4 times, and the mineralization constants decreased by 1.9–3.6 times. It has been emphasized that the cumulative effect of drying-wetting-freezing-thawing cycles is manifested not only in the decrease in the total Corg from the soil but also in the reduction of the mineralization potential of the soil organic matter.
Yu, Z., Wang, G., Marschner, P., 2014. Drying and rewetting-Effect of frequency of cycles and length of moist period on soil respiration and microbial biomass. European Journal of Soil Biology 62, 132-137.
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Rewetting of dry soil induces a flush of respiration, but less is known about the effect of the number of drying and rewetting (DRW) cycles on the distribution of respiration and how the length of the moist period influences respiration and microbial biomass in residue-amended soil. Two experiments were conducted. In the first experiment, the effect of number of DRW cycles on soil respiration and microbial biomass was assessed. We exposed a sandy loam amended with finely ground pea residues (C/N 26) to up to three DRW cycles, each cycle consisting of one week drying and one week moist incubation. Soils which were maintained moist throughout served as controls. The flush in respiration was greatest in the first DRW where it was 3 times higher than in the following cycles, but did not differ between the second and third DRW. Cumulative respiration at the end of the experiment was highest in the constantly moist soil and lowest with three DRW cycles (20% lower than in the constantly moist soil). Microbial biomass C did not differ between the constantly moist treatment and the DRW treatments on days 14 and 28, but at the end of the experiment (d 42) the MBC concentration was 40% higher in the treatments with 2 and 3 DRW cycles than in the constantly moist treatment. The second experiment was conducted to test the hypothesis that the amount of C respired at the end of the preceding cycle may influence respiration in the second cycle. In this experiment, the soil was maintained moist or subjected to two DRW cycles where the length of the moist period in the first cycle lasted between one and five days followed by 7 days dry. The second moist period was three days in all DRW treatments. Cumulative respiration at the end of the experiment was lowest in the soil with one moist day in the first cycle (1 mg CO2–C g−1) and highest in the constantly moist control (1.6 mg CO2–C g−1). Among the DRW treatments, cumulative respiration over the entire incubation period (two DRW cycles) was about 30% higher in treatments with four to five moist days in the first DRW than in the treatments with one, two or three moist days which did not differ in cumulative respiration. Cumulative respiration in the second cycle was about 30% higher in treatments with 1–2 moist days in the first cycle compared to treatments with 3–5 moist days. Both cumulative respiration in the second DRW cycle and in the second moist period were negatively correlated with cumulative respiration at the end of the first period. Microbial biomass C (MBC) differed little between the moisture treatments except for a 50% higher MBC concentration at the end of the first moist period in the treatment with 1 moist day. It can be concluded that cumulative respiration is reduced by DRW and that the negative effect of DRW on cumulative respiration is exacerbated if the moist period after rewetting is short. The finding that cumulative respiration in the second DRW cycle is negatively correlated with cumulative respiration in the first cycle indicates that substrate availability at the start of the second cycle plays an important role in respiration in the second cycle.
Shi, A., Marschner, P., 2014. Drying and rewetting frequency influences cumulative respiration and its distribution over time in two soils with contrasting management. Soil Biol. Biochem. 72, 172-179.
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Understanding the factors determining cumulative respiration upon rewetting of dry soil is critical for predicting C efflux from soils. The response of respiration to drying and rewetting may be influenced by land management due to its effect on the soil organic C pool and differ between soil size fractions. An incubation experiment was conducted with soils collected from two plots with a long history of different management (wheat-fallow and permanent pasture). The soils were sieved to 4–10 mm and <2 mm to obtain two size factions. There were five moisture treatments with the same length (48 days). The constantly moist control (CM) was maintained at 50% of WHC throughout. In the drying and rewetting (DRW) treatments, the number of dry and moist days was equal but the number of DRW events ranged from one to four (1–4DRW). Respiration was measured daily, microbial biomass C (MBC) was determined six days after rewetting in each DRW cycle and on day 48 (end of the experiment). The proportion of soil in the 4–10 mm size fraction decreased over time with a greater decrease in pasture than in wheat soil and in the DRW treatments compared to the constantly moist treatment (CM). Cumulative respiration at the end of the experiment was greater in the <2 mm than in the 4–10 mm fraction in both soils and was highest in CM and 1DRW. In wheat soil, cumulative respiration decreased from 1DRW to 3DRW, whereas it decreased only between 2 and 3DRW in pasture soil. In treatments with two to four DRW, the proportion of total cumulative respiration was lowest in the last cycle. In 2DRW, cumulative respiration was smaller in the second than in the first moist period whereas the reverse was true for 3DRW and 4DRW. Cumulative respiration in the second moist period was greater in 3DRW than in 2DRW (8 and 12 prior moist days) whereas cumulative respiration in the third moist period was greater in 4DRW than in 3DRW (12 and 16 prior moist days). At the end of the experiment, the MBC concentration in the 4–10 mm fraction was unaffected by moisture treatment, whereas in the <2 mm fraction, it was greatest in CM and lowest in 4DRW. We conclude that the response of respiration to drying and rewetting is more strongly influenced by management than size fraction. In a given soil, the cumulative respiration upon rewetting is influenced not only by the number of DRW cycles but also the number of moist days prior to rewetting.
Ma, J., Zheng, X.-J., Li, Y., The response of CO2 flux to rain pulses at a saline desert. Hydrological Processes 26, 4029-4037.
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As the substantial component of the ecosystem respiration, soil CO2 flux is strongly influenced by infrequent and unpredictable precipitation in arid region. In the current study, we investigated the response of soil CO2 flux to rain pulses at a saline desert in western China. Soil CO2 flux was measured continuously during the whole growing season of 2009 at six sites. We found that there were remarkable changes in amplitude or diurnal patterns of soil CO2 flux induced by rainfall events: from bimodal before rain to a single peak after that. Further analysis indicated that there is a significant linear relationship (P < 0.001) between soil CO2 flux and soil temperature (Tsoil). However, a hysteresis between the waveform of diurnal course of CO2 flux and Tsoil was observed: with soil CO2 flux always peaked earlier than Tsoil. Furthermore, a double exponential decay function was fitted to the soil CO2 flux after rainfall, and total carbon (C) releases were estimated by numerical integration for rainfall events. The relative enhancement and total C release, in association with the rain pulses, was linearly related to the amount of precipitation. According to the size and frequency of rainfall events, the total amount of C release induced by rain pulses was computed as much as 7.88 g C·m–2 in 2009, equivalent to 10.25% of gross primary production. These results indicated that rain pulses played a significant role in the carbon budget of this saline desert ecosystem, and the size of them was a good indicator of rain-induced flux enhancement.
Leon, E., Vargas, R., Bullock, S., Lopez, E., Panosso, A.R., La Scala Jr, N., Hot spots, hot moments, and spatio-temporal controls on soil CO2 efflux in a water-limited ecosystem. Soil Biol. Biochem. DOI: 10.1016/j.soilbio.2014.05.029
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Soil CO2 efflux is the primary source of CO2 emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO2 efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO2 efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO2 efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO2 efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO2 efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO2 efflux, and during the wet season the emergence of a hot spot of soil CO2 efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO2 efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO2 efflux in water-limited ecosystems.