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Rain pulse response of soil CO 2 exchange by biological soil crusts and grasslands of the semiarid Colorado Plateau, United States

2011; American Geophysical Union; Volume: 116; Issue: G3 Linguagem: Inglês

10.1029/2011jg001643

2156-2202

D. R. Bowling, Edmund E. Grote, Jayne Belnap,

Lichen and fungal ecology

Journal of Geophysical Research: BiogeosciencesVolume 116, Issue G3 Free Access Rain pulse response of soil CO2 exchange by biological soil crusts and grasslands of the semiarid Colorado Plateau, United States D. R. Bowling, D. R. Bowling [email protected] Department of Biology, University of Utah, Salt Lake City, Utah, USASearch for more papers by this authorE. E. Grote, E. E. Grote Southwest Biological Science Center, U.S. Geological Survey, Moab, Utah, USASearch for more papers by this authorJ. Belnap, J. Belnap Southwest Biological Science Center, U.S. Geological Survey, Moab, Utah, USASearch for more papers by this author D. R. Bowling, D. R. Bowling [email protected] Department of Biology, University of Utah, Salt Lake City, Utah, USASearch for more papers by this authorE. E. Grote, E. E. Grote Southwest Biological Science Center, U.S. Geological Survey, Moab, Utah, USASearch for more papers by this authorJ. Belnap, J. Belnap Southwest Biological Science Center, U.S. Geological Survey, Moab, Utah, USASearch for more papers by this author First published: 08 September 2011 https://doi.org/10.1029/2011JG001643Citations: 55AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract [1] Biological activity in arid grasslands is strongly dependent on moisture. We examined gas exchange of biological soil crusts (biocrusts), the underlying soil biotic community, and the belowground respiratory activity of C3 and C4 grasses over 2 years in southeast Utah, USA. We used soil surface CO2 flux and the amount and carbon isotope composition (δ13C) of soil CO2 as indicators of belowground and soil surface activity. Soil respiration was always below 2 μmol m−2 s−1 and highly responsive to soil moisture. When moisture was available, warm spring and summer temperature was associated with higher fluxes. Moisture pulses led to enhanced soil respiration lasting for a week or more. Biological response to rain was not simply dependent on the amount of rain, but also depended on antecedent conditions (prior moisture pulses). The short-term temperature sensitivity of respiration was very dynamic, showing enhancement within 1–2 days of rain, and diminishing each day afterward. Carbon uptake occurred by cyanobacterially dominated biocrusts following moisture pulses in fall and winter, with a maximal net carbon uptake of 0.5 μmol m−2 s−1, although typically the biocrusts were a net carbon source. No difference was detected in the seasonal activity of C3 and C4 grasses, contrasting with studies from other arid regions (where warm- versus cool-season activity is important), and highlighting the unique biophysical environment of this cold desert. Contrary to other studies, the δ13C of belowground respiration in the rooting zone of each photosynthetic type did not reflect the δ13C of C3 and C4 physiology. Key Points Temperature sensitivity of respiration by BSC is highly dependent on moisture C3 and C4 grasses of Colorado Plateau do not separate into cool/warm season Isotopes of soil CO2 can be difficult to interpret in low-flux biomes 1. Introduction [2] Ecosystem processes in dryland regions are principally dependent on moisture, which is often the most limiting factor for activity of desert organisms [Noy-Meir, 1973, 1974]. There is a growing appreciation that the seasonality of precipitation in these areas can be as important as the amount of precipitation in influencing ecosystem processes, including C dynamics. Much of the southwest United States and northwest Mexico receives summer rain during the North American monsoon [Adams and Comrie, 1997], and the influence is particularly strong in the warm Chihuahuan and Sonoran deserts [Loik et al., 2004]. The cold deserts of the Intermountain region are not strongly influenced by the monsoon; the Great Basin receives little summer rain, and the Colorado Plateau an intermediate amount [Higgins et al., 1997]. Based on these regional differences in seasonality of precipitation, there are likely fundamental differences in the responses of desert organisms to rain and their influences on C cycling processes. However, knowledge of soil carbon (C) cycling processes in cold deserts of North America lags behind that for the better studied warm deserts. [3] Precipitation in arid regions is dominated by small events [Huxman et al., 2004; Lauenroth and Bradford, 2009; Reynolds et al., 2004]. In the Canyonlands region of the Colorado Plateau of southeast Utah (location of the present study), 71% of rain pulses are smaller than 5 mm, and 35% are smaller than 1 mm (Figure 1). Contrasting with the fairly regular summer monsoon precipitation of the warm deserts, moisture events in the cold deserts are fairly evenly distributed throughout the year, although there is a little less precipitation in June, and a little more in September and October [Schwinning et al., 2008]. In winter, cold desert soils are near or sometimes below freezing, and precipitation leads to recharge of soils (Figure 2), providing stored water that plants can access during the growing season [Bowling et al., 2010; Obrist et al., 2004; Prater and DeLucia, 2006; Wohlfahrt et al., 2008]. Summer rain falls on hot dry soils with little soil recharge, leading to a functional distinction between winter and summer precipitation that influences ecological processes throughout the cold deserts [Gebauer and Ehleringer, 2000; Kulmatiski et al., 2006; Prevéy et al., 2010; Schwinning et al., 2003, 2005b]. Figure 1Open in figure viewerPowerPoint Frequency distribution of precipitation events at Corral Pocket during 1999–2008. Pulses are defined as single or cumulative events separated by 24 or more rain-free hours. A total of 363 pulses occurred during this period, with 1747 h where rain was recorded. Figure 2Open in figure viewerPowerPoint (a) Seasonal pattern of maximum and minimum daily soil temperature (10 cm depth) at Corral Pocket during 2000–2006. Daily means and standard deviations of hourly data are shown. (b) Seasonal pattern of soil moisture (10 cm depth) at Corral Pocket during the same period, with all years overlapped. Each year is a different color; study years were 2003 (thick blue) and 2005 (thick green dashed). Large variation during winter periods indicates ice in the soil. [4] Biological soil crusts (biocrusts) are communities of organisms including cyanobacteria, mosses, lichens, and algae, covering the top few millimeters of the soil surface [Belnap, 2006; Belnap et al., 2003]. Co-occurring with plants, they can cover up to 70% of the surface in drylands around the globe. These drylands compose 40% of the Earth's land surface. Biological soil crusts serve important functional roles, including stabilizing soils against erosion, influencing infiltration of water and germination of plant seeds, and C and nitrogen fixation. Due to their occurrence at the surface of the soil, they dry quickly and are hence very responsive to moisture pulses and temperature. Since soil surface conditions can change frequently, biocrusts are difficult to study in their natural environment; the present study is among very few conducted of biocrust gas exchange in the field. [5] The pulsed nature of precipitation leads to highly variable soil moisture, particularly near the surface (Figure 2b). This temporally dynamic moisture environment exerts a first-order control on most soil and plant processes. Desert organisms respond differently to rain pulses of varying size. Even the smallest events will influence biocrusts, intermediate pulses might wet the subsurface biotic community to a few cm depth, and typically only larger events are used by plants for carbon gain or growth of roots or shoots [Austin et al., 2004; Belnap et al., 2005]. Hence, desert organisms may respond to moisture pulses in fundamentally different ways. [6] It is unclear how the seasonality of precipitation of the cold deserts (primarily winter moisture with diminished summer monsoon influence) affects carbon cycle processes in these regions. C4-dominated grasslands which receive reliable summer rains associated with the North American monsoon gain most of their carbon in the summer [Emmerich and Verdugo, 2008; Mielnick et al., 2005; Scott et al., 2006, 2009], but the cold desert grasslands of the Great Basin and Colorado Plateau exhibit C uptake primarily in spring [Bowling et al., 2010; Ivans et al., 2006; Obrist et al., 2003]. The warm-season/cool-season distinction does not appear to apply to ecosystem-scale carbon exchange in these colder regions. Seasonal differences in physiological activity of grasses are commonly observed in the warm deserts, with C3 grasses generally more active in spring, fall and winter and C4 grasses generally more active in the summer [Cable, 1975; Everett et al., 1980; Fay et al., 2003; Kemp, 1983; Kemp and Williams, 1980]. [7] The lack of understanding of soil carbon exchange processes in the cold deserts motivated this study. The objective was to examine the response of grassland soils of the Colorado Plateau to natural moisture pulses, with a primary focus on soil CO2 exchange. We investigated the physiological and biogeochemical activity of the soil community (including biocrusts, soil biota, plant roots and rhizosphere communities), using measurements of in situ soil CO2 and CO2 isotopes, and soil surface CO2 exchange. The dynamics of activity of the soil community were assessed in response to moisture pulses over the cold and warm seasons. Soil surface fluxes were examined over a wide range of ambient conditions to elucidate the primary environmental controls on biocrusts and soil community activity. We hypothesized that moisture and temperature, in that order, were the primary factors influencing soil community activity. Following the traditional distinction of cool-season and warm-season grasses, we further hypothesized that the soil communities surrounding both types would be most active in spring, but that C4 grasses would respond to summer moisture more than the C3 grasses, influencing the dynamics of activity of their soil communities. 2. Methods Study Location [8] This research was conducted at two semiarid grassland sites, located ∼10 km apart, in southeast Utah, USA. Mean annual temperature is 12.0°C, and mean annual precipitation is 216 mm (1965–2008, Western Regional Climate Center, http://www.wrcc.dri.edu/). At these sites, about 33% of precipitation occurs during summer (July–September) and 67% during the rest of the year (Table 1). The first site, Corral Pocket (38.09°N, 109.39°W, 1520 m elevation), is a mixed C3–C4 grassland, dominated by the perennial bunchgrasses Hilaria jamesii (C4) and Stipa hymenoides (C3) and the C3 shrub Coleogyne ramosissima, with other C3 and C4 grasses and annuals making up a small percentage of total plant cover. Annual vegetation and biological soil crust cover at Corral Pocket vary widely [Belnap et al., 2009] in response to moisture and the 6–8 weeks of livestock grazing that occurs in late fall and winter. Biological soil crusts at this site were cyanobacterially dominated, with very low biomass (LOD class 1 [Belnap et al., 2008]). During the year of this study (2003), vegetation cover was very low, with only 14% of the ground surface covered by plants [Bowling et al., 2010]. Soils were sandy loams. Table 1. Precipitation in Canyonlands National Park During the Study Years Compared to the Long-Term (1965–2008) Meana Year Unit October–December Preceding Year January–March April–June July–September Total 2003 mm 52.6 67.3 16.8 47.2 183.9 2005 mm 65.3 57.7 39.9 109.2 271.8 2003 percent of long-term (91) 167 (41) (67) (87) 2005 percent of long-term 113 144 (98) 156 129 Long-term mm 58.0 40.2 40.9 70.1 211.3 a Fall precipitation (October–December) is shown for the preceding year. Deficits relative to the long-term mean are indicated in parentheses when expressed as percent of long-term mean. Data are from Western Regional Climate Center, http://www.wrcc.dri.edu/, for the site "Canyonlands, The Needle" which is ∼2 km from Squaw Flat and ∼10 km from Corral Pocket. [9] The second site, Squaw Flat (SF, 38.14°N, 109.79°W, 1540 m), was located in Canyonlands National Park, and research was conducted there in 2005. This grassland site is dominated by H. jamesii (C4), which was invaded by the annual grass Bromus tectorum (C3) sometime before 1940 (the "historically invaded" site of Belnap and Phillips [2001]). However, the current work was conducted in an area within the grassland that was strongly dominated by Hilaria with very little Bromus cover. Squaw Flat, located within the national park, has not been grazed since 1974, and crusts are still recovering from this disturbance (LOD class 2–4 [Belnap et al., 2008]), with higher cyanobacterial biomass than Corral Pocket and some moss. During 2005, vegetation cover was 15% perennials and 8% annuals, and the remainder of the sandy loam soil was covered with biocrusts. Further details for these sites, including extensive information about the soils, can be found elsewhere [Bowling et al., 2010; Goldstein et al., 2005]. During 2003, the Canyonlands region was in the fourth consecutive year of severe drought [Bowling et al., 2010]. Precipitation was substantially below normal in 2003, particularly during April–September (Table 1). In contrast, precipitation in 2005 was above normal for most of the year. Meteorological Measurements [10] A variety of environmental parameters were monitored at the sites, including precipitation, soil temperature and moisture at several depths, and photosynthetic photon flux density (PPFD). These were measured using standard methods and sensors as described elsewhere [Bowling et al., 2010]. Environmental data were collected hourly. Additional data were obtained for Corral Pocket from the CLIM-MET monitoring network (http://esp.cr.usgs.gov/info/sw/clim-met/index.html). [11] The time period for a precipitation "pulse" was defined as one or more cumulative hours containing measurable precipitation, with intervals of 24 h minimum between events. Thus, a rainstorm of several hours, followed by a few dry hours, followed by more rain, would be considered a single event. Total precipitation falling within the time period of a single event is referred to as "pulse size." CO2 Within the Soil: Continuous Measurements—Corral Pocket and Squaw Flat [12] In this paper we distinguish soil CO2 (CO2 mole fraction in the soil air space, μmol CO2 mol air−1) from soil CO2 flux (either soil respiration or net CO2 uptake, the flux density through the plane at the soil surface, μmol CO2 m−2 s−1). Soil CO2 was measured in situ at 5 and 15 cm depth using solid-state optical sensors (GMT222, Vaisala, Inc., Woburn, MA). Sensors were enclosed in capped polyvinyl chloride tubes, and inserted vertically in the soil into a hole drilled with an auger bit. The temperature of the sensor tip was monitored with a thermocouple, and corrections for sensor temperature were made following Tang et al. [2003]. Laboratory tests over a wide temperature range (data not shown) showed that these corrections led to an accuracy of about 10% of reading (e.g., 1000 ± 100 μmol mol−1). [13] Due to calibration drift during warm-up, sensors were left on continuously, with the unavoidable artifact that they physically heated the soil as much as 5°C above ambient soil temperature at the same depth. It was not possible to calibrate the sensors frequently; as a result, they drifted over the season, and at some times the readings were lower than ambient CO2 in the air. For these reasons, soil CO2 data from the Vaisala sensors were interpreted in a relative sense (for example, relative changes following a moisture pulse). [14] To evaluate relative differences among C3 and C4 belowground plant activity and the interspace between plants, CO2 sensors were installed within the rooting zones of individual (1) Stipa hymenoides, (2) Hilaria jamesii plants, and (3) in the interspace between plants (2 depths per location type, 6 total sensors) at Corral Pocket. At Squaw Flat, failure and drift of 4 sensors led to the use of a single reliable sensor each at 5 and 15 cm, located in the interspace within 10 cm from Hilaria plants. CO2 Within the Soil: Flasks—Corral Pocket [15] At Corral Pocket, permanent 100 m transects were established within the rooting zones of individual Stipa or Hilaria plants or interspace locations (n = 10 samples collected per location type, located roughly in a straight line every 10 m). While the roots of individual bunchgrasses probably overlapped somewhat with their neighbors, only 14% of the ground surface was covered with vegetation, with significant interspace between the bunchgrasses. At each location, 6.4 mm OD stainless steel tubing was inserted into the soil to a depth of 15 cm, and a preevacuated (<4 Pa) 100 mL glass flask (34–5671, Kontes Glass, Vineland, NJ) was attached via a filter (15 μm, Nupro SS-4FW-15, Swagelok, Solon, OH) to the tubing. The stopcock was opened, and the flask was slowly filled to ambient pressure through the filter over several minutes, to minimize influence of sampling on the diffusive profile of soil gases. A worst-case calculation, assuming totally dry soils and using measured soil porosity, indicates that a spherical volume of 3.7 cm radius would be disturbed to achieve a 100 mL sample, centered at the 15 cm depth. [16] Tubing was left in place for the entire 2003 season and soil gas samples collected several times during June–October to evaluate seasonal differences in interspace and C3 and C4 belowground plant activity. The CO2 and the carbon isotope ratio (δ13C) of CO2 in the flasks was measured on an isotope ratio mass spectrometer (IRMS, Delta Plus XL, Thermo-Finnigan, Bremen, Germany) as described by Schauer et al. [2005]. Spatial variability of soil CO2 along each transect was considerably greater than the analytical accuracy of ∼1 μmol mol−1. The CO2 data are presented relative to the World Meteorological Organization scale, and δ13C relative to the Vienna PDB scale (±0.1‰). Calculation of δ13C of Belowground Respiration: Corral Pocket [17] The δ13C of belowground respiration was compared roughly every 3 weeks for Stipa, Hilaria, and interspace locations at Corral Pocket. The δ13C of belowground respiration was calculated from mixing relationships with CO2 and δ13C of CO2 in the soil gas (measured) and in the air (assumed) as described below. The CO2 and δ13C of CO2 in the air was estimated from mean values for summer 2003 at Niwot Ridge, Colorado [Bowling et al., 2005]. The exact values for the air are not needed, as we compared the Stipa, Hilaria, and interspace locations, and these are derived from mixing lines (thus, arbitrary values could be used). [18] Two separate methods were used to calculate the δ13C of belowground respiration. The first is based on linear combination of two gases (the Keeling method), and the other is based on the physical principles of soil gas transport (the Davidson method). Diffusional isotopic fractionation is treated differently by the two models, but both assume steady state conditions in the soil (an important limitation discussed later). The Keeling method utilized a 2-ended mixing model [Keeling, 1958]. Ordinary least squares regressions were performed of δ13C versus 1/CO2 from all the soil gas samples in a treatment on a particular sampling date, and the intercept of the regression line (minus 4.4‰) was taken to represent the δ13C of belowground respiration, with the standard error of that intercept used as a measure of uncertainty. The soil gas is subjected to a diffusive fractionation of 4.4‰ [Cerling et al., 1991]; the diffusive isotopic effect on soil gas mixing relationships has been described elsewhere [Bowling et al., 2009]. Subtraction of 4.4‰ facilitates comparison of the soil-respired gas with δ13C of C3 and C4 biomass, and with the second method. [19] For the Davidson method, equation (1) was used to estimate δ13C of belowground respiration, which Davidson [1995] referred to as δJ This equation was derived from Fick's first law for 12CO2 and 13CO2 separately. Here C and δ refer to CO2 and δ13C of CO2, respectively, and the subscripts s and a refer to soil and air, respectively. In our case, equation (1) was applied for each soil sampling location individually, for each treatment and each sampling date, and the mean and standard deviation of the resulting δJ population are presented. [20] In the text that follows, we use δ13CBR to refer to the δ13C of belowground respiration calculated via either method, and indicate in the figures which method was used. On some of the sampling dates, part of the soil flask collection occurred late in the afternoon or early evening, and the remainder was finished the following morning. On these dates the early and late samples were analyzed separately to calculate δ13CBR. The δ13C of Plant Biomass: Corral Pocket [21] Grass samples were collected at Corral Pocket in May 2003, as described by Bowling et al. [2010]. Fully expanded green leaves and stems were dried to constant mass, finely ground, and the δ13C of their bulk tissue was measured via IRMS (DeltaS, Thermo-Finnigan, Bremen, Germany). Soil Surface CO2 Flux: Handheld Chambers—Corral Pocket [22] At Corral Pocket in 2003, permanent collars to measure soil respiration were established at each of 36 interspace locations, in a 6 × 6 collar configuration, with the corners of each plot located at 20 m spacing along a 100 × 100 m grid. Soil collars were made of polyvinyl chloride, 10.2 cm OD × 8 cm tall, inserted to 4 cm in the soil, and left in place for the season. Soil respiration was measured roughly every 3 weeks during 2003, usually over a 2–3 h period in the afternoon. Respiration measurements were made using a portable photosynthesis system with a soil chamber (LI-6400 and LI-6400-09, LI-COR, Lincoln, NE). A minimum of 20 randomly selected collars were measured on each sampling date. Collars in which annual plants were infrequently found growing were not measured on that date, and the aboveground vegetation was removed in those collars. Soil Surface CO2 Flux: Automated Chambers—Squaw Flat [23] Two transparent automated flux chambers were used to measure net soil gas exchange (which included respiration and/or carbon uptake by biocrusts) at Squaw Flat in 2005. The design of Riggs et al. [2009] was used, which included a polyvinyl chloride collar and a pneumatically operated lid with a frame in the soil. The original design was modified to include hourly calibration with zero and span of known CO2 mole fraction, measurement of in-line water vapor using a modified relative humidity sensor (HMP-35C, Campbell Scientific, Inc., Logan UT) and measurement of gas temperature using a thermocouple. Flux calculations included corrections for dilution and band broadening due to water vapor. The collars were 38 cm tall × 38 cm inner diameter and contained a soil surface area of 0.11 m2. Each collar was inserted into the soil in the interspace between plants by driving to a depth of 35 cm, leaving 3 cm of the chamber protruding above the soil surface, resulting in an intact soil column within the collar. The frame and lid were then installed by excavating around the soil collar, placing the lid on the intact soil column/collar, and the frame buried. This caused disturbance to the soil within 10 cm horizontally outside of the chamber, but the column within the chamber was subjected to minimal disturbance. Biological soil crusts were present in the chambers but not vascular plants. [24] Measurements for each of the chambers were made once per hour with a closed-chamber period of 4 min. Clear polycarbonate lids allowed sunlight to enter the chambers. Laboratory tests showed a reduction in PPFD through the lid of 10–20%. The transparent chambers had a small effect on temperature in the chamber in the field (usually an increase). Of 12,894 hourly measurements available in the 2 chambers, 83.8% exhibited 0–1°C change in chamber air temperature during the measurement, and 92.9% showed 0–2°C change. Soil crust temperature was less affected, with 93.7% of hours 0–1°C, and 98.5% 0–2°C. Data gaps were caused by pump failures, power failures, calibration gas loss due to animal damage to tubing, tumbleweeds obstructing the lid, and other problems. [25] Near-surface soil moisture was monitored within one of the chambers just below the soil surface (∼1 cm depth) using a horizontally oriented water content reflectometry probe (CS615, Campbell Scientific, Inc., Logan, UT). Crust temperature was measured in each chamber using thermocouples buried a few mm below the surface (the junction just deep enough to avoid exposure to direct solar radiation). 3. Results Belowground Soil CO2 and Surface CO2 Flux: Corral Pocket and Squaw Flat [26] Most precipitation events at Corral Pocket in 2003 were small, but several events exceeded 5 mm (Figure 3). While these larger events had varying influences on soil moisture (Figure 3b), fairly substantial changes in belowground CO2 occurred followed each event. Continuously measured CO2 agreed favorably with flask measurements, with both showing high spatial variability in soil CO2. Belowground CO2 initially peaked around day 135 (15 May) and gradually decreased as the soil dried, with four independent periods of sustained increase afterward (Figure 3c). Diel oscillation in CO2 was observed at both depths, and was correlated with soil temperature at each depth (data not shown). The diel peaks of CO2 were out of phase with each other by a few hours at the two depths. An 8.6 mm rain event on day 135 led to a sustained increase in CO2 that lasted for ∼20 days at 15 cm depth (Figure 3c). In contrast, the CO2 increase following a similar pulse on day 170 (19 June) lasted only a few days. A 2 mm pulse on day 215 (3 August) led to a very brief increase in CO2, which occurred when soils were at their hottest (Figure 2) and very dry. Soil respiration fluxes were typically less than 0.5 μmol m−2 s−1 except on a few sampling dates (Figure 3d). A notable exception was on day 171 (20 June) after 9.1 mm rain, when fluxes were higher than on other sampling days (1.7 μmol m−2 s−1). The temporal dynamics of belowground CO2 following rain (Figure 3c) indicate that periods of higher soil respiration were probably missed by the infrequent manual flux measurement schedule (Figure 3d). Figure 3Open in figure viewerPowerPoint Precipitation, soil moisture, belowground CO2, and soil surface CO2 flux at Corral Pocket in 2003. (a) Total precipitation pulse size, (b) soil moisture (10 cm depth), (c) soil CO2 at 5 and 15 cm depth (mean of 3 sensors per depth), and the mean (±1 SD) soil CO2 in 20–30 flasks (15 cm depth), and (d) mean (±1 SD) surface CO2 flux measured in 20 or more collars located in the interspace between plants. Positive fluxes indicate carbon loss. [27] Precipitation during 2005 was fairly evenly distributed throughout the year in this area (Figure 4a; note that precipitation data are from Corral Pocket, ∼10 km from Squaw Flat, and some events may not have occurred at both locations). There were six rain pulses greater than 10 mm, which are large considering the distribution in Figure 1. Soil moisture did not show a pronounced seasonal trend at either depth (compare Figures 2b and 4b), but this was likely due to variability in individual sensor response or placement, since other moisture sensors at Squaw Flat did show seasonal patterns similar to those in Figure 2 (data not shown). The soil surrounding the moisture sensor at 1 cm depth dried quickly and a seasonal pattern was not observed at this depth. Figure 4Open in figure viewerPowerPoint Precipitation (measured at Corral Pocket), and soil moisture, belowground CO2, and soil surface CO2 flux at Squaw Flat in 2005. (a–c) Conceptually the same as in Figure 3, with depths indicated (no flasks were collected). (d) Surface CO2 flux measured hourly in two transparent automated chambers, and (e) daily (24 h) means of the data in Figure 4d (both chambers). Positive fluxes indicate carbon loss, and negative fluxes indicate carbon uptake by biological soil crusts. [28] Belowground CO2 at Squaw Flat was near 400 μmol mol−1 in winter and again in late fall (Figure 4c). There was a general increase in belowground CO2 as the soil warmed during the first third of the year (up to day 150, 30 May). Moisture pulses during this time did not result in increased belowground CO2, although the surface flux did increase following moisture near day 50 (19 February, Figure 4d). Soil respiration during this time gradually increased from near zero in midwinter to a daily mean of ∼0.5 μmol m−2 s−1 (Figure 4e). Rain on day 154 (3 June) wet the soil and led to substantial increases in belowground CO2 and soil respiration (Figure 4). Increased belowground CO2 persisted for ∼20 days, similar to what occurred at Corral Pocket during the same time of year (Figure 3c). Monsoon rains after day 200 (19 July) led to increases in belowground CO2 and surface CO2 flux but increases were less sustained than those earlier in the year. Soil respiration decreased systematically during soil drying on days 140–154 (late May to early June), 230–250 (late August to early September), and 273–