Conversion of Oak Woodlands to Vineyards - PubAg - USDA

Published online July 6, 2006

Effects of Land Use on Soil Respiration: Conversion of Oak Woodlands to Vineyards Eli A. Carlisle,* Kerri L. Steenwerth, and David R. Smart conversion of natural systems to perennial agricultural systems, and the added spatial heterogeneity it imposes at the landscape scale, is reflected in recent attempts to quantify terrestrial C fluxes over larger continental scales (Chen et al., 2000; Pacala et al., 2001) which are poorly defined. Soil CO2 efflux, or “soil respiration,” is one of the more important components of ecosystem C budgets. Soil respiration consists of organic matter oxidation, root respiration, and rhizosphere respiration (i.e., microbial consumption of root exudates and contents of sloughed cells) (Hanson et al., 2001). Soil respired CO2 represents the ultimate oxidative fate of soil C, and the C lost from terrestrial ecosystems occurs mainly through soil respiration (Amundson, 2001). These C losses are generally many times greater than losses from leaching and erosion (Hao et al., 2001; Lal, 2003; Pardini et al., 2003), although erosion can be a major contributor in some systems (Jacinthe et al., 2004). Agriculture related emissions are thought to contribute to approximately 26% of the total global anthropogenic CO2 flux (Zhang et al., 2002). Soil and/or ecosystem respiration has been characterized for a limited number of Mediterranean oak biomes (see Table 1). These investigations have focused on environmental controls on soil respiration and natural ecosystems where soils are undisturbed. For example, Pinõl et al. (1995) and Joffre et al. (2003) examined soil respiration for Quercus ilex montane forests in Spain, and Reichstein et al. (2002) tracked ecosystem respiration of mixed holm oak forests in Italy. Reichstein et al. (2002) and Joffre et al. (2003) both observed that soil moisture content was a better estimator of soil respiration rate than soil temperature, while Pinõl et al. (1995) found that soil temperature rather than soil moisture was the better predictor at their sites. These studies begin to document the influence of Mediterranean climates on soil respiration, and provide information about the magnitude of fluxes in natural systems. Nonetheless, both agricultural and urban encroachment continue to change these landscapes and information is lacking on how conversion to cropping systems influences the magnitude of the CO2 flux as well as source– sink relations. We are unaware of any studies that have examined the influence of land use conversion on C cycling in Mediterranean ecosystems dominated by perennial woody plants. In this investigation we hypothesized that conversion of oak woodlands to vineyards increases the proportion of C respired from more recalcitrant C sources in the vineyard soils relative to the oak woodland soils. At the same time, we proposed that the magnitude of soil

Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.

ABSTRACT We examined constraints on soil CO2 respiration in natural oak woodlands, and adjacent vineyards that were converted approximately 30 yr ago from oak woodlands, in the Oakville Region of Napa Valley, California. All sites were located on the same soil type, a Bale (variant) gravelly loam (fine-loamy, mixed, superactive, thermic Cumulic Ultic Haploxeroll) and dominated by C3 vegetation. Seasonal soil CO2 efflux was greatest at the oak woodland sites, although during the summer drought the rates of soil CO2 efflux measured from oak sites were generally similar to those measured from the vineyards. Soil profile CO2 concentrations at the oak woodland sites were lower below 15 cm despite higher CO2 efflux rates. Soil gas diffusion coefficients for oak sites were larger than for vineyard sites, and this indicated that the apparent discrepancy in soil profile carbon dioxide concentration ([CO2]) may be caused by a diffusion limitation. Soil profile [CO2] and d13C values showed substantial temporal changes over the course of a year. Vineyard soil CO2 was more depleted in 13 CO2 below 25 cm in the soil profile during the active growing season as indicated by more negative d13C ratios. This result indicated that different C sources were being oxidized in vineyard soils. Annual C losses were less from vineyard soils (7.02 6 0.58 Mg C ha21 yr21) as compared to oak soils (15.67 6 1.44 Mg C ha21 yr21), and both were comparable to losses reported in previous investigations.

I

N THE CENTRAL PACIFIC COAST region

of North America, there has been extensive conversion of oak woodlands and oak woodland–grasslands to agricultural ecosystems consisting of woody perennials like grapevine. Estimates show nearly one million vineyard acres in California, and about half of it is located in the coastal regions (Heaton and Merenlender, 2000) and Sierra Nevada foothills. Understanding the influence of land use conversion on the carbon (C) cycle will improve our knowledge of the origin and magnitude of carbon dioxide (CO2) production and consumption processes, as CO2 contributes more to radiative forcing than any other greenhouse gas (McCarthy et al., 2001). Deforestation of woodlands and replacement with cultivated agricultural systems and pastures has resulted in substantial release of soil C (Guo and Gifford, 2002; Lal, 2002; Norby et al., 2002; Olsson and Ardo, 2002; Dahlgren et al., 2003)—an estimated 30% decrease in soil organic carbon (SOC) occurs in temperate soils within 20 yr after first cultivation (West and Post, 2002). Our lack of knowledge of C cycle processes following Department of Viticulture & Enology, University of California, Davis, One Shields Avenue, Davis, CA 95616-8749. K.L. Steenwerth, present address: USDA/ARS, Crops Pathology and Genetics Unit, University of California, Davis, One Shields Avenue, Davis, CA 956168749. Received 29 Apr. 2005. *Corresponding author (ecarlisle@ ucdavis.edu). Published in J. Environ. Qual. 35:1396–1404 (2006). Special Submissions doi:10.2134/jeq2005.0155 ª ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: [CO2], carbon dioxide concentration; SOC, soil organic carbon.

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Table 1. Annual soil respired CO2–C losses reported for Mediterranean forest and perennial agriculture ecosystems. Ecosystem

Site

Soil respired CO2–C


21

Eucalyptus savanna Quercus ilex woodlands Quercus ilex woodlands Pinus halepensis forest Thompson seedless vineyard

Australia

Mg C ha yr 14.3

Reference

21

Chen et al. (2003)

Spain

1.8 –3.4

Pinõl et al. (1995)

France

3.8 –5.8

Raich and Schlesinger (1992) J. Gru¨nzweig, personal communication Williams (1996)

Israel California

4.5 3.6 – 6.2

respiration declines under cultivation of a perennial crop because long-term cultivation decreases nonrecalcitrant C in the plow layer. To accomplish these objectives, we monitored net soil CO2 efflux, CO2 concentration, and d13C of CO2 with depth for three oak woodland sites and three adjacent vineyard sites that were converted from oak woodlands approximately 30 yr ago. Since long-term cultural practices at the vineyard site (cultivation and tractor passes for management activities) have changed soil physical characteristics, we also examined soil physical properties that might constrain soil CO2 efflux. MATERIALS AND METHODS Study Sites The study consisted of three oak woodland and three vineyard sites with known land use histories in the Oakville Region of Napa Valley, California. The vineyard sites were formerly part of the adjacent oak woodlands before their conversion to vineyards. The oak woodlands were mainly composed of Quercus lobata Neé and Q. lobata hybrids, and had functional canopy diameters of .15 m. All oak sites had .80% canopy cover. The understories in the oak woodlands were a mix of annual C3 grasses and forbs. The vineyard sites were planted to Vitis vinifera L. cv. Merlot grafted onto two locally common rootstocks, 101-14 Mgt. (V. riparia 3 V. rupestris cv. 101-14 Millardet de Gramanet), and 1103 P (V. berlandieri 3 V. rupestris cv. 1103 Paulsen). The vineyards were converted directly from oak woodlands 30 to 32 yr ago (Far Niente Winery, historical records). All sites in this study were located on the same soil type, a Bale (variant) gravelly loam. The Oakville region averages 825.5 mm of annual precipitation and has a mean annual temperature of 14.38C (CIMIS, California Department of Water Resources). All sites were located between 40 and 55 m altitude above sea level. The vineyards had 2.42-m-wide rows, and 2.19-m vine spacing (1868 vines ha21). The rows are tilled twice each year (during April of 2003) and the vineyard berm is sprayed with glyphosate [N-(phosphonomethyl)glycine] annually. Sulfur was applied to the vines twice during the year. The vineyard berms, a 61-cm-wide strip immediately underneath the vines, were not physically disturbed during any vineyard management activities, and this is where the soil efflux collars were installed. No irrigation was applied during the course of the experiment to the vineyards in which the soil efflux collars were installed.

Soil Characteristics Approximately 3 kg of mineral soils (0- to 20-cm depth) were collected at each of three locations in each site, bulked, sieved (2-mm mesh), and air-dried. Total C and N contents

were measured by combustion using a C and N analyzer on a mass spectrometer (Hydra 20/20; PDZ Europa, Crewe, UK). Soil pH was measured according to the saturated paste method. Particle size analysis was determined by soil suspension, and soil bulk density was determined by the core method (8-cm diameter by 6-cm depth).

Soil Efflux Measurements To develop an idea of annual soil C lost through soil respiration, we measured soil CO2 efflux in situ using a nondispersive infrared gas analyzer (Model LI-6400/6400-09; LI-COR, Lincoln, NE). Twelve permanent 10.16-cm-diameter polyvinyl chloride (PVC) collars were inserted 2 cm into the mineral soil 1 mo before the first measurement. All vegetation within the collars was removed. A pair of collars was installed at each of the three oak woodland and vineyard sites (n 5 6 for each land-use type). Vineyard soil respiration collars were placed within the vine rows and underneath the canopies. The collars at the oak woodland sites were placed underneath canopies where there was greater than 80% oak canopy cover. All efflux measurements were corrected for vapor pressure, soil temperature, and chamber temperature. Percent gravimetric soil moisture content at 0 to 20 cm was determined following each efflux measurement. The soil respiration measurements were made between approximately 1200 and 1400 h every 2 to 3 wk from October 2002 to December 2003. Measurements of diurnal CO2 efflux indicated a 20% decline at dusk and early morning during the active growing season, with rates reaching a plateau between approximately 1100 and 1500 h (data not shown). In December 2002 the area received 510 mm of rainfall and the sites were inaccessible during the scheduled measurement interval. Once we obtained annual CO2–C efflux measurements, we calculated annual soil C losses by integrating the efflux rate for each site over time and incorporating the observed nighttime decrease. This allowed us to estimate a value for annual soil C loss.

Soil Profile Carbon Dioxide Measurements Soil gas was sampled bimonthly from an array of permanently established stainless steel gas sampling tubes constructed as described by Pinõl et al. (1995). Arrays of six tubes were installed at depths of 15, 25, 45, 65, 85, and 105 cm in the three oak woodland and vineyard sites. All arrays were installed 1 mo before the first sampling date. Each array was established within 2 m of two soil efflux collars. Before sampling from each soil gas tube, 10 mL of air were removed and discarded. Samples of soil gas (13 mL) were then withdrawn and injected into a 12-mL vacutainer (Labco Limited, High Wycombe, UK). Samples of ambient CO2 were obtained 10 cm above the soil surface at each array on each sampling date. The samples were analyzed for 13C abundance at the University of California, Davis, stable isotope facility using a mass spectrometer (Geo 20/20; PDZ Europa). The 13C abundance in soil CO2 was expressed as a ratio according to the following formula:

d13 C 5 (Rsample /Rstandard 2 1) 3 1000 per mil 13

12

[1]

where Rsample and Rstandard were the respective C to C ratios for the sample and a universal standard (PeeDee Belemnite [PDB]; Farquhar et al., 1989). A gas chromatograph with a capillary column focused CO2 before entering the mass spectrometer, and was used to obtain measures of sample [CO2] with an in-line thermal conductivity detector.


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Diffusion Coefficients

Soil Moisture and Temperature

Intact cores of soil (8-cm diameter 3 6-cm depth) were removed from areas adjacent to the gas sampling arrays. The intact cores were placed in gas diffusion chambers (Rolston, 1986) with a known concentration of Freon-12 gas in the reservoir chamber. Freon concentrations in the reservoir were measured every 15 min on a Model 310 gas chromatograph (GC) (SRI Instruments, Torrance, CA) with a flame ionization detector (FID) to determine the rate of diffusion through the soil core (Rolston, 1986). Modeled values for the diffusion coefficients were determined using the Buckingham–Burdine– Campbell (BBC) gas diffusivity model (Moldrup et al., 1999):

Oak woodland soils had higher soil moisture than vineyard soils throughout most of the year (P , 0.05; Fig. 1a). The peak moisture level for all sites occurred in January of 2003 after the area received 510 mm of rainfall. The last rainfall event of the 2002–2003 rainy season (October 2002–May 2003) occurred on 15 May, and there was no further precipitation until the second week of November of 2003. Despite the differences in moisture content, measured soil temperatures were not significantly different between oak woodland and vineyard sites over the course of the year (Fig. 1b).

Dp /D0 5 B2 (e/B)213/b

[2]

where Dp is the soil gas diffusion coefficient, D0 is the gas diffusion coefficient in air (m2 air s21), B is the total soil porosity, e is the air-filled porosity, and b is the Campbell poresize distribution parameter. The b parameter was estimated from the clay fraction (CF) of the respective soils according to the equation described by Rolston (1986):

b 5 13:6CF 1 3:5

Soil Carbon Dioxide Efflux Soil respiration increased in oak woodland and vineyard sites immediately after the first precipitation

[3]

Statistical Analyses Soil CO2 efflux was analyzed according to a repeated measures ANOVA. Profile CO2 and d13C measurements were analyzed using a mixed model ANOVA. The relationships between soil CO2 efflux and soil moisture content and temperature were analyzed using multiple regressions. All statistical analyses were performed using SAS Version 8.3 (SAS Institute, 1990), and levels of significance were based on the probability of committing a Type I error of P , 0.05.

RESULTS Mineral soils at 0 to 20 cm in the vineyards and the oak woodlands had similar textures, but differed in total C and N contents, mineral N, and bulk density (Table 2). Total C contents in the oak woodland soils had nearly twice the C content as the vineyard soils. Total N content followed a similar pattern, with the oak woodland soils having 67% more N than the vineyard. Vineyard soils had considerably lower extractable NH41 contents (P , 0.05), while differences in extractable NO32 contents were nonsignificant. Table 2. Means and the standard errors of the means (n 5 6) for total soil C, total soil N, KCl-extractable NH41–N and NO32–N, texture (% sand, silt, clay), bulk density (rb), and measured diffusion coefficients (Dg). Property Total C, % Total N, % 1 21 NH4 –N, mg kg 2 21 NO3 –N, mg kg Sand, % Silt, % Clay, % 23 rb, g cm 0 – 6 cm 6 –12 cm 40 – 46 cm Dg, cm 2 s21 0 – 6 cm 6 –12 cm 40 – 46 cm

Vineyard

Oak

2.48 6 0.03 0.21 6 0.01 3.02 6 0.25 2.75 6 0.26 49.33 6 1.03 33.67 6 1.23 17.00 6 0.01

4.63 6 0.13 0.35 6 0.01 12.23 6 1.36 2.20 6 0.14 49.67 6 1.37 37.67 6 1.21 12.67 6 0.52

1.25 6 0.06 1.33 6 0.10 1.45 6 0.05

1.02 6 0.05 1.16 6 0.08 1.20 6 0.02

0.0033 6 0.0008 0.0018 6 0.0011 0.0029 6 0.0009

0.0057 6 0.0031 0.0036 6 0.0016 0.0091 6 0.0011

Fig. 1. Bimonthly measurements of (a) soil gravimetric water content, (b) soil temperature, and (c) soil CO2 efflux. Points represent means (n 5 6), and error bars indicate standard error. Soil moisture and temperature measurements were taken to a 20-cm depth.



event following the summer dry season (November of both 2002 and 2003; Fig. 1a). After the abrupt increase in soil CO2 efflux following first autumn rain, subsequent efflux rates during the wet season declined to levels that were still higher than observed during the previous summer. Efflux rates gradually increased during the late winter and early spring until a peak was reached in the middle of May (Fig. 1c). At this time, soils were still moist, and soil temperatures were high (19.78C for the oak woodland sites, and 21.98C for the vineyard sites; Fig. 1a and 1b). These warm moist conditions had the greatest effect on CO2 production at the oak woodland sites with peak values at 9.89 6 0.80 mmol m22 s21 versus 3.16 6 0.36 mmol m22 s21 for the vineyard. Soil efflux rates were more strongly correlated with moisture content (multiple regression adjusted R2 5 0.3052; P , 0.0043) than by soil temperature (adjusted R2 5 0.0028; P 5 0.0580) at the oak woodland sites. Vineyard soil efflux rates were not well explained by either soil moisture content (P 5 0.2756) or temperature (P 5 0.4149).

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With few exceptions, oak woodland soils tended to have greater respiration rates than the vineyard soils at any given time. Thus, the oak woodland soils had substantially greater annual efflux of CO2 than the vineyard soils (P 5 0.0055; Fig. 1c). The integrated average annual loss of C from the oak woodland soils through soil respiration was 15.76 6 1.44 Mg C ha21 (mean 6 SE, n 5 3), while it was only 7.02 6 0.58 Mg C ha21 from the vineyard soils.

Soil Profile Soil [CO2] was higher throughout much of the vineyard soil profile than the oak woodland profile (P 0.05; Fig. 2). The d13C ratios varied significantly with depth (P , 0.01; Fig. 3), with CO2 becoming increasingly depleted in 13C at depths below 25 cm. The annual average d13C at depths below 30 cm was 221.83 6 0.13‰ (mean 6 SE, n 5 12) in vineyard soils, and 220.20 6 0.16‰ in oak woodland soils.

Fig. 2. Seasonal means and standard errors of the means of soil profile CO2 concentrations in oak woodland and vineyard soils. During the winter and spring some sites were so waterlogged as to prevent sample gas extraction. As a result winter and spring points are composed of n 5 3–6 data points while summer and fall points are composed of n 5 12 data points.


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Fig. 3. Seasonal means and standard errors of the means of soil profile CO2 d13C values in oak woodland and vineyard soils. During the winter and spring some sites were so waterlogged as to prevent sample gas extraction. As a result, winter and spring points are composed of n 5 3–6 data points while summer and fall points are composed of n 5 12 data points.

Soil [CO2] and CO2 d13C showed seasonal trends that could be broken down temporally into “dry” (summer and fall) and “wet” (winter and spring) seasons. Soil [CO2] was higher throughout the vineyard soil profile than in the oak woodland soil during the dry seasons (Fig. 2; P , 0.05). During the dry season, oak woodland soil CO2 was consistently more enriched in 13C throughout the profile (Fig. 3; P , 0.01). Soil CO2 d13C values were at their most enriched during the summer when profile [CO2] values were at their lowest (Fig. 3). During the wet season, the oak woodland soil profiles had higher [CO2] and more depleted d13C at the 15- and 25-cm depths (Fig. 2 and 3). Soil profile [CO2] was greatest during the wet season, and d13C patterns were similar among oak woodland and vineyard soils during this period (Fig. 2 and 3). In the wet season, vineyard soils produced CO2 that was significantly

(P , 0.05) more enriched in 13C at the 15- and 25-cm depths, while the oak woodland soils produced more 13 C enriched CO2 in the lower depths (Fig. 2 and 3).

Soil Physical Properties Oak woodland soils tended to have higher soil CO2 diffusion coefficients than the vineyard soils (Table 2). Modeled values showed the same trend (Fig. 4), although the absolute rates calculated according to the model were higher than the measured values at all three depths. Modeled diffusion coefficients for the oak woodland soils were greater at all moisture levels than were the vineyard diffusion coefficients (Fig. 4). It should be noted that air-filled porosity drives the soil gas diffusion model, not moisture content (Rolston, 1986). Even though oak woodland soils had greater gravimet-



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Fig. 4. Modeled diffusion coefficients for vineyard and oak woodland soils for two depths (0–6 and 6–12 cm) as calculated for different soil water contents.

ric moisture contents, the oak woodland soils, with their greater total porosity, had higher air-filled porosities than the vineyard soils.

DISCUSSION In these seasonally warm and dry Mediterranean climates, soil respiration in oak woodlands was driven more by soil moisture content than by temperature. Carbon dioxide efflux increased abruptly in the oak woodlands following the first autumn precipitation event (Fig. 1c), and remained elevated throughout the period when gravimetric soil moisture content was above approximately 15% (Fig. 1a), even though soil temperatures decreased through the fall and winter (Fig. 1b). Several studies have examined the mechanisms behind this initial flush of CO2 (Orchard and Cook, 1983; Van Gestel et al., 1993; Lundquist et al., 1999; Franzleubbers et al., 2000), and it is likely that similar factors played a role in this system as well. These factors include water infiltration resulting in physical displacement of the soil atmosphere, releasing CO2 accumulated in soil air space (Van Gestel et al., 1993; Smart and Pen˜uelas, 2005). Water addition also increases concentrations of dissolved organic carbon (DOC) by providing a solvent for previously lysed cell contents, and thus increasing microbial mineralization activity (Orchard and Cook, 1983; Van Gestel et al., 1993; Lundquist et al., 1999). When oaks leaf out during spring (March 2003), subsequent increases in photosynthetic activity may also contribute to the seasonal variation we observed in soil respiration. Rates of soil CO2 efflux were significantly lower from vineyard soils, but none of the seasonal changes observed in the oak woodlands were evident in vineyard soils even after the grapevines leafed out at the be-

ginning of April 2003 (see Fig. 1). The comparatively slow rates of vineyard soil CO2 efflux may have been a consequence of having lower total soil C (Table 2). In a parallel laboratory incubation of these soils under controlled conditions, vineyard soils produced less CO2 than oak woodland soils under equal soil moisture content and temperature conditions (Carlisle, unpublished data). In the soil profile, neither soil moisture content at 20 cm nor soil temperature explained significant variation in CO2 efflux. These data suggest that another factor, such as availability of labile C, may have limited soil respiration in the vineyards more than water or temperature. The presence of high [CO2] in vineyard soil profiles in comparison with oak woodlands suggested that soil physical properties caused increased diffusive resistance to CO2 movement (Gaertig et al., 2002). Higher bulk densities and smaller soil CO2 diffusion coefficients (Dg) in vineyard soils at all depths measured (Table 2) supported this hypothesis. Hashimoto and Suzuki (2002) found that diffusion coefficients decreased with depth in forest soils as a consequence of decreasing porosity at lower depths. Our soils also showed diminished porosity with depth (Table 2), but the change was greater for the cultivated vineyard soils. Long-term annual cultivation of soils increases bulk density (Onwualu and Anazodo, 1989; Pagliai et al., 2004; Table 2), diminishes porosity, and thus alters the effective soil gas diffusivities. In addition to lower CO2 production, lower diffusion coefficients may account for the lower rates of CO2 efflux and higher CO2 concentrations in vineyard soil profiles. Seasonality (i.e., wet or dry season) influenced soil profile [CO2] in the oak woodland and vineyard soils. During the summer, the [CO2] was at its lowest in both soils, most likely as a result of decreased microbial activity and increased movement of CO2 through the


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soil profile in comparison to the wet season. Greater soil microbial activity and root respiration from annual grasses and forbs under the oaks may have contributed to the higher surface CO2 concentrations in the oak woodlands during the wet season (see Fig. 2). Respiration by roots of the perennial oaks may have been higher in the oak woodland soils, as a large percentage of the total root mass exists within 50 cm of the surface (Millikin and Bledsoe, 1999). The contribution by oak and grapevine roots probably was negligible until spring since the oaks and grapevines did not start to leaf out until the third week of March and the first week of April, respectively. The high [CO2] in soil profiles during the “wet” season in both soils may be attributed to high soil moisture content, which caused lower diffusion rates of CO2 through the profile. In studies of fir and beech systems, it has also been noted that [CO2] increases as soil moisture content does (Certini et al., 2003). There was an obvious seasonal shift in d13C of CO2 to a more positive value in both the oak woodland and vineyard soils beneath approximately 15 cm (Fig. 3), and the shift was much larger in the oak woodland soils. The shift to a more enriched d13C value corresponded to a decrease in soil moisture content (Fig. 1a), increased soil temperature (Fig. 1a), and a decline in soil [CO2] (Fig. 2). The isotopic signature of soil respired C is generally representative of the C source oxidized (Amundson et al., 1998), and SOC becomes increasingly enriched in 13C as depth increases before stabilizing (Ehleringer et al., 2000; Accoe et al., 2002; Fessenden and Ehleringer, 2003). Older SOC is more enriched in 13C than fresh SOC and litter. In addition, the soil atmosphere CO2 should theoretically be enriched by roughly 4.4‰ relative to the respired source at each depth due to differences between the diffusivities of 12CO2 and 13CO2 (Amundson et al., 1998). Thus, we hypothesized that soils with a steady input of 13C depleted plant material such as the oak woodland sites should be relatively less enriched in 13C, at least at the surface, than soils from the vineyards. Conversely, surface soils in vineyards should be more enriched in 13C, since aboveground C inputs have been restricted, and root biomass, which is slightly enriched in 13C relative to aboveground tissues (Ehleringer et al., 2000), provides a relatively greater proportion of C to the soil. A laboratory incubation experiment provided supporting data for the above. Surface soils (20 cm) from the same sites incubated under controlled laboratory conditions indicated that oak woodland soils were about 1.0 6 0.2‰ (mean 6 SE, n 5 6) more depleted in 13C, as compared with the vineyard soils (Carlisle, unpublished data). Our field results, on the other hand, did not support this contention. Under field conditions, where root respiration is a major component of soil CO2 efflux and soil moisture and temperature conditions are not controlled, the d13C of soil CO2 showed trends similar to the laboratory data during winter and spring at the depths of 15 and 25 cm (Fig. 3). However, during the dry summer and fall seasons, the vineyard soils had more depleted d13C values for CO2 at the uppermost two depths (Fig. 3). This shift in d13C in the top 25 cm was attributed to increased microbial and annual herbaceous

root activity during the wet season in the oak woodland sites that was not present in the vineyard sites. Differences between the results in the field versus those in the incubation experiment also suggest that abiotic factors influenced the d13C of the soil CO2. The greater d13C values throughout the profile in soils during the summer as compared to the wet season may have been moisture related. Other studies in annual crops, such as maize and winter wheat (Schu¨ßler et al., 2000) and coniferous forests (Fessenden and Ehleringer, 2003), have also observed an enrichment the d13C of soil CO2 during the dry summer season. The highly enriched d13C values in the oak woodland soils during the summer may have been a result of easier exchange of atmospheric CO2 (d13C of 2 8‰) into the soil profile as diffusion constants increased with lower soil moisture, at least for the 15-cm depth. The more positive CO2 d13C values of the remainder of the oak woodland soil profile relative to the vineyard soil profile may have been produced by deeper (.1 m), older, and more highly degraded SOC (Melillo et al., 1989; Ehleringer et al., 2000). As the soil gradually dried down and efflux rates approached their lowest values (Fig. 1c), CO2 produced from the deeper SOC may have moved more easily through the soil profile. Over the course of the entire year, the oak woodland soil CO2 was significantly more enriched in 13C throughout the year in the lower profile depths (45–105 cm). This suggests that different relative contributions of organic matter oxidation and root and rhizosphere respiration in the oak woodland versus vineyard systems may play a role in the d13C of profile CO2. Grapevine roots, including the rootstocks 1103P and 101-14 Mgt, have deep and uniform depth distributions, as compared with oak woodlands (Smart et al., 2005). The generally more negative values of d13CO2 in the vineyard profile either reflect that a greater proportion of 13C-depleted recalcitrant C pools are undergoing oxidation (Wedin et al., 1995), or that a greater proportion of soil respiration is derived from root and rhizosphere respiration, which, while more enriched in 13C relative to aboveground biomass, is still less enriched than older microbial degraded SOM (Ehleringer et al., 2000). These data do not necessarily fully support our hypothesis that vineyard soil respiration would be derived from more recalcitrant sources of C than oak woodland soils. Rather, it suggests that root respiration at depth is a more important component of respiratory C losses in the vineyard and that older more recalcitrant C sources have been greatly depleted in the vineyard soil in comparison to the oak woodland soils. Our values for annual soil respired C losses for the vineyard and oak woodland systems are high, but within the range reported for other ecosystems (Table 1). Vineyard soil respiration measurements were taken in the vineyard berms only where annual forbes and grasses are eliminated. The vineyard inter-rows supported some annual grasses. Our estimated annual soil respiration of these vineyards might be slightly greater if this were taken into account. Our measures represent respiration rates within the oak canopy, and our sites had nearly


continuous canopy cover (.80%). In woodlands on different soil types, with greater oak densities, or oak savannas with lesser oak canopy cover, annual soil respiration C losses will surely differ from our estimates.


CONCLUSIONS The conversion of natural ecosystems to agriculture results in loss of soil organic C. Such loss is a key driver of global change processes (Tilman et al., 2001), and has a strong influence on soil carbon flow. Nonetheless, patterns and processes involved in carbon retention in Mediterranean environments like California have received little attention. This investigation has shown that the study oak woodland sites lose significantly more soil CO2 than adjacent vineyards. Net C losses from the vineyard soils can be estimated from organic C contents (Table 2) at roughly 33 Mg per hectare since the vineyards were converted from oak woodlands 30 to 32 yr ago (Carlisle, unpublished data). Furthermore, soil physical properties that influence soil gas diffusion were altered. Cultural practices such as tillage and vineyard preparation had large impacts on SOC pools and SOC distribution through the soil profile. Soil [CO2] and CO2 d13C values from this investigation have shown that the respiration sources in the soil profile change with season and depth, and that soil moisture content has a large influence on soil respiration d13C values. Our estimates point to the clear need to develop a more acute understanding of the contribution of belowground production in perennial cropping systems, as well as in the perennial systems from which they were converted. ACKNOWLEDGMENTS Our work was supported by the Kearney Foundation of Soil Science through Grant #2001-18 awarded to D.R. Smart, and a Ph.D. fellowship awarded to Eli A. Carlisle. We thank Dianne Louie for technical assistance and consultation, Daniel Bosch and Mitchell Klug of the Robert Mondavi Corporation for working with us to obtain field sites for the project, and three anonymous reviewers for their helpful suggestions.

REFERENCES Accoe, K., P. Boeckx, O. Van Cleemput, G. Hofman, Y. Zhang, R. Li, and C. Guanxiong. 2002. Evolution of the d13C signature related to total carbon contents and carbon decomposition rate constants in a soil profile under grassland. Rapid Commun. Mass Spectrom. 16: 2184–2189. Amundson, R. 2001. The carbon budget in soils. Annu. Rev. Earth Planet. Sci. 29:535–562. Amundson, R., L. Stern, T. Basiden, and Y. Wang. 1998. The isotopic composition of soil and soil-respired CO2. Geoderma 82:83–114. Certini, G., G. Cort, A. Agnelli, and G. Sanesi. 2003. Carbon dioxide efflux and concentrations in two soils under temperate forests. Biol. Fertil. Soils 37:39–46. Chen, W., J. Chen, and J. Cihlar. 2000. An integrated terrestrial ecosystem carbon-budget model based on changes in disturbance, climate, and atmospheric chemistry. Ecol. Modell. 135:55–79. Chen, X., L.B. Hutley, and D. Eamus. 2003. Carbon balance of a tropical savanna of northern Australia. Ecosyst. Ecol. 137:405–416. Dahlgren, R.A., W.R. Horwath, K.W. Tate, and T.J. Camping. 2003. Blue oak enhance soil quality in California oak woodlands. Calif. Agric. 57:42–47. Ehleringer, J.R., N. Buchmann, and L.B. Flanagan. 2000. Carbon iso-

1403

tope ratios in belowground carbon cycle processes. Ecol. Appl. 10: 412–422. Farquhar, G.D., J.R. Ehleringer, and K.T. Hubick. 1989. Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant Physiol. Plant Mol. Biol. 40:503–538. Fessenden, J.E., and J.R. Ehleringer. 2003. Temporal variation in d13C of ecosystem respiration in the Pacific Northwest: Links to moisture stress. Oecologia 136:129–136. Franzleubbers, A.J., R.L. Haney, C.W. Honeycutt, H.H. Schornberg, and F.M. Hons. 2000. Flush of carbon dioxide following rewetting of dried soil relates to active organic pools. Soil Sci. Soc. Am. J. 64: 613–623. Gaertig, T., H. Schak-Kirchner, E.E. Hildenbrand, and K.V. Wilpert. 2002. The impact of soil aeration on oak decline in southwestern Germany. For. Ecol. Manage. 159:15–25. Guo, L.B., and R.M. Gifford. 2002. Soil carbon stocks and land use change: A meta analysis. Global Change Biol. 8:345–360. Hanson, P., N. Edwards, C.T. Garten, and J.A. Andrews. 2001. Separating root and soil microbial contributions to soil respiration: A review of methods and observations. Biogeochemistry 48:115–146. Hao, Y., R. Lal, R.C. Izaurralde, J.C. Ritchie, L.B. Owens, and D.L. Hothem. 2001. Historic assessment of agricultural impacts on soil and soil organic carbon erosion in an Ohio watershed. Soil Sci. 166:116–126. Hashimoto, S., and M. Suzuki. 2002. Vertical distributions of carbon dioxide diffusion coefficients and production rates in forest soils. Soil Sci. Soc. Am. J. 66:1151–1158. Heaton, E., and A.M. Merenlender. 2000. Modeling vineyard expansion, potential habitat fragmentation. Calif. Agric. 54:12–20. Jacinthe, P.A., R. Lal, L.B. Owens, and D.L. Hothem. 2004. Transport of labile carbon in runoff as affected by land use and rainfall characteristics. Soil Tillage Res. 77:111–123. Joffre, R., J. Ourcival, S. Rambal, and A. Rocheteau. 2003. The key role of topsoil moisture on CO2 efflux from a Mediterranean Quercus ilex forest. Ann. For. Sci. 60:519–526. Lal, R. 2002. Soil carbon sequestration in China through agricultural intensification, and restoration of degraded and desertified ecosystems. Land Degrad. Dev. 13:469–478. Lal, R. 2003. Offsetting global CO2 emissions by restoration of degraded soils and intensification of world agriculture and forestry. Land Degrad. Dev. 14:309–322. Lundquist, E.J., K.M. Scow, L.E. Jackson, S.L. Uesugi, and C.R. Johnson. 1999. Rapid response of soil microbial communities from conventional, low input, and organic farming systems for a wet/dry cycle. Soil Biol. Biochem. 31:1661–1675. McCarthy, J.J., O.F. Canziani, N.A. Leary, D.J. Dokken, and K.S. White (ed.) 2001. Climate Change 2001: The scientific basis. Intergovernmental Panel on Climate Change Third Assessment Report. Cambridge Univ. Press, Cambridge. Melillo, J.M., J.D. Aber, A.E. Linkins, A. Ricca, B. Fry, and K. Nadelhoffer. 1989. Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter. Plant Soil 115: 189–198. Millikin, C.S., and C.S. Bledsoe. 1999. Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California. Plant Soil 214:27–38. Moldrup, P., J. Olesen, T. Yamguchi, K. Schjonning, and D.E. Rolston. 1999. Modeling diffusion and reaction in soils: IX. The Buckingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil. Soil Sci. Soc. Am. J. 164:542–551. Norby, R.J., P.J. Hanson, E.G. O’Neill, T.J. Tschaplinski, J.F. Weltzin, R.A. Hansen, W. Cheng, S.D. Wullschleger, C.A. Gunderson, N.T. Edwards, and D.W. Johnson. 2002. Net primary productivity of a CO2–enriched deciduous forest and the implications for carbon storage. Ecol. Appl. 12:1261–1266. Olsson, L., and J. Ardo. 2002. Soil carbon sequestration in degraded semiarid agro-ecosystems—Perils and potentials. Ambio 31:471–477. Onwualu, A.P., and U.G.N. Anazodo. 1989. Soil compaction effects on maize production under various tillage methods in a derived savannah zone of Nigeria. Soil Tillage Res. 14:99–114. Orchard, V.A., and F.J. Cook. 1983. Relationship between soil respiration and soil moisture. Soil Biol. Biochem. 15:447–453. Pacala, S.W., G.C. Hurtt, D. Baker, P. Peylin, R.A. Houghton, R.A. Birdsey, L. Heath, E.T. Sundquist, R.R. Stallard, P. Ciais, P.


1404


Moorcroft, J.P. Caspersen, E. Shevliakova, B. Moore, G. Kohlmaier, E. Holland, M. Gloor, M.E. Harmon, S.M. Fan, J.L. Sarmiento, C.L. Goodale, D. Schimel, and C.B. Field. 2001. Consistent land- and atmosphere-based U.S. carbon sink estimates. Science (Washington, DC) 292:2316–2320. Pagliai, M., N. Vignozzi, and S. Pellegrini. 2004. Soil structure and the effect of management practices. Soil Tillage Res. 79:131–143. Pardini, G., M. Gispert, and G. Dunjo. 2003. Runoff erosion and nutrient depletion in five Mediterranean soils of NE Spain under different land use. Sci. Total Environ. 309:213–224. Pinõl, J., J.M. Alcan˜iz, and F. Roda´. 1995. Carbon dioxide efflux and pCO2 in soils of three Quercus ilex montane forests. Biogeochemistry 32:53–69. Raich, J.W., and W.H. Schlesinger. 1992. The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus Ser. B 44:81–99. Reichstein, M., J.D. Tenhunen, O. Roupsard, J.M. Ourcival, S. Rambal, S. Dore, and R. Valentini. 2002. Ecosystem respiration in two Mediterranean evergreen holm oak forests: Drought effects and decomposition dynamics. Funct. Ecol. 16:27–39. Rolston, D. 1986. Gas diffusivity. p. 1089–1102. In A. Klute (ed.) Methods of soil analysis. Part 1. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI. SAS Institute. 1990. SAS user’s guide: Statistics. SAS Inst., Cary, NC. Schu¨ßler, W., R. Neubert, I. Levin, N. Fischer, and C. Sonntag. 2000. Determination of microbial versus root-produced CO2 in an agricultural ecosystem by means of d13CO2 measurements in soil air. Tellus Ser. B 52:909–918.

Smart, D.R., and J. Pen˜uelas. 2005. Short-term CO2 emissions from planted soil subject to elevated CO2 and simulated precipitation. Appl. Soil Ecol. 28:247–257. Smart, D.R., E. Schwass, A. Lakso, and L. Morano. 2005. Grapevine rooting patterns: A comprehensive analysis and review. p. 153–174. In Soil Environment and Vine Mineral Nutrition Proc., San Diego. 28 June 2004. Am. Soc. of Enology and Viticulture, Davis, CA. Tilman, D., J. Fargione, B. Wolff, C. D’Antonia, A. Dobson, R. Howarth, D. Schindler, W.H. Schlesinger, D. Simberloff, and D. Swackhamer. 2001. Forecasting agriculturally driven global environmental change. Science (Washington, DC) 292:281–284. Van Gestel, M., R. Merckx, and K. Vlassak. 1993. Soil drying and rewetting and the turnover of 14C-labelled plant residues: First order decay rates of biomass and non-biomass 14C. Soil Biol. Biochem. 25:125–134. Wedin, D.A., L.L. Tieszen, B. Dewey, and J. Pastor. 1995. Carbon isotope dynamics during grass decomposition and soil organic matter formation. Ecology 76:1383–1392. West, T.O., and W.M. Post. 2002. Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Sci. Soc. Am. J. 66:1930–1946. Williams, L.E. 1996. Grape. p. 851–881 In E. Zamski and A.A. Schaffer (ed.) Photoassimilate distribution in plants and crops: Source-sink relationships. Marcel Dekker, New York. Zhang, Y., L. Changsheng, X. Zhou, and B. Moore, III. 2002. A simulation model linking crop growth and soil biogeochemistry for sustainable agriculture. Ecol. Modell. 151:75–108.