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Soil Science Society of America Journal 66:2002-2008 (2002)
© 2002 Soil Science Society of America

DIVISION S-7—FOREST & RANGE SOILS

Enzyme Activities and Carbon Dioxide Flux in a Sonoran Desert Urban Ecosystem

Douglas M. Green*,a and Michelle Oleksyszynb

a Environmental Resources Program, Arizona State University East, Mesa, AZ 85212
b Dep. of Plant Biology, Arizona State Univ. Main, Tempe, AZ 85282

* Corresponding author (dm.green{at}asu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Urban expansion into wildlands significantly changes soil processes such as nutrient cycling and organic matter processing. Knowledge of these changes is important so that the impact of urbanization on ecosystems may be assessed. We measured the activities of invertase, cellulase, and CO2 flux in mesiscape, xeriscape, and in remnant desert patches in a rapidly urbanizing south central Arizona Sonoran desert ecosystem. In this system, mesiscapes are irrigated watered lawns, xeriscapes include low water-use vegetation, and desert remnants include undeveloped areas within the urban matrix. Invertase activity ranged from 2.4 to15 mg glucose equivalents (GE) g-1 24 h-1. Invertase activities in mesiscapes during January exceeded desert remnant sites by a factor of six and xeriscape sites by a factor of two. Cellulase activity ranged from 48 to 406 µg GE g-1 24 h-1. Cellulase activity in mesiscapes during January significantly exceeded desert remnant and xeriscape sites by a factor of two. Mesiscape soils were up to 18.4°C cooler than xeriscape soils and had the lowest average temperatures (20.7°C). The average temperature of desert remnant soils was 27.4°C. Over the study period, CO2 flux rates ranged from 0.212 to 1.760 g m-2 h-1. Maximum rates of CO2 flux rates occurred in the spring and summer, and flux rates were lowest during the winter months. Winter peaks of enzyme activity are attributed to the onset of dormancy in C-4 grasses in the fall and establishment of winter lawns by homeowners.

Abbreviations: CAP-LTER, Central Arizona Long Term Ecological Research • GE, glucose equivalents


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
URBANIZATION has numerous impacts on soils. The urbanization process increases spatial heterogeneity of the soilscape by creating recently altered or modified soils in close proximity to older, naturally occurring soil bodies (Craul, 1992; Schleuß et al., 1998). Urban soils often contain considerable amounts of anthropogenic materials such as construction wastes, bricks, and wood (Sukopp et al., 1979; Schleuß et al., 1998). Gilbert (1991) and Craul (1992) summarized differences in other physical properties of urban and nonurban soils, including reduced soil structure, compaction, surface crusting, restricted aeration and drainage, and modified temperature regime.

Although information on the biology of urban soils is limited, published studies indicate that urban soils have reduced numbers and diversity of organisms compared with natural or seminatural soils (Gilbert, 1991; Harris, 1991). Earthworm (Lumbricus terrestris) biomass declines along a gradient of increasing urbanization in Belgium (Pil and Josens, 1995). Microbial activity of soils in northwest Germany declines with increasing urbanization (Beyer et al., 1995). When compared with natural or seminatural soils, urban soils typically have reduced organic matter input and reduced quantities of humic acid and humin, contributing to a reduction in water stable aggregates (Craul, 1992; Beyer et al., 1995).

We suggest that urbanization of soils may also have impacts on soil enzyme activities. Soil enzymes occupy a pivotal role in catalyzing reactions associated with organic matter decomposition and nutrient cycling. Invertase and cellulase are important enzymes in organic matter decomposition. Cellulase catalyzes hydrolysis of cellulose to D-glucose. Cellulose is the most abundant polysaccharide of plant cell walls and represents a significant input to soils (Richards, 1987). Invertase hydrolyzes sucrose to fructose and glucose. Invertase occurs in plant tissues and soil organisms (Skujins, 1976). Enzyme activity in soils reflects not only enzymes in soil solution and living tissue, but also enzymes bound to soil colloids and humic substances (Skujins, 1976; Nannipieri et al., 1990).

Enzymes have been suggested by some researchers as potential indicators or monitoring tools to assess soil quality (Dick, 1994; Bandick and Dick, 1999) and bioremediation activities (Margesin et al., 2000). Enzyme activity is influenced by soil conditions such as organic matter content (Pancholy and Rice, 1973; Lalande et al, 1998; Kandeler et al., 1999a), moisture (Bergstorm et al, 1998; Ross and Speir, 1984), and temperature (Tscherko et al., 2001). Enzyme activity is also impacted by management including cultivation (Bandick and Dick, 1999; Kandeler et al., 1999b), fertilization (Ross et al., 1995; Ajwa et al., 1999), and burning (Ajwa et al., 1999). To date, little research has examined enzyme activities in urban and nonurban sites.

The purpose of this study was to enzyme activity and CO2 flux in urban and nonurban sites that occupy the same soil type. The overall objective of this study was to quantify the activities of invertase and cellulase and CO2 flux in residential mesiscapes, xeriscapes, and desert remnant sites. Specifically, how do enzyme activities and CO2 flux differ between low water-use xeriscapes, often constructed to mimic desert systems and desert remnant sites in an urbanized region of the lower Sonoran desert ecosystem.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Our study was conducted from June 1998 to May 1999 in the Central Arizona Long Term Ecological Research (CAP-LTER) site, located in south central Arizona, USA (33.34' N lat., 111.99' W long.). The CAP-LTER encompasses the city of Phoenix and Maricopa County, a rapidly expanding urban center. Between 1990 and 2000, the population of Maricopa County grew from 2.1 to 3.1 million people (U.S. Census Bureau, 2001). Study sites were located in the southern Phoenix metropolitan area in the Ahwatukee Foothills and South Mountain Park where the elevation is about 380 m. The climate of the study area is hot and dry, with summer temperatures averaging 30.8°C and winters have an average temperature of 11.3°C. Average annual precipitation is 19.4 cm with approximately 50% occurring as summer thunderstorms and the remainder associated with winter frontal systems originating from the Pacific Ocean (Schmidli, 1996). During the study period, temperature and precipitation were very close to the 30-yr average.

Our study areas were chosen at random from a group of homeowners who volunteered to participate in the study. Study areas were chosen to include homes with well-watered lawns and other landscape vegetation (mesiscapes) and those with low water-use vegetation (xeriscapes). We use landscape in this paper to indicate the immediate area around the home that is maintained by the homeowner. Mesiscape homes were characterized by the presence of a lawn area dominated by Bermuda grass (Cynodon dactylon). Xeriscape homes were characterized by the presence of water conserving shrubs and trees such as Texas ranger (Leucophyllum frutescens), Mexican bird of paradise (Caesalpina mexicana), Sonoran paloverde (Ceridium praecox), and Chilean mesquite (Prosopsis chilensis). Xeriscape homes generally had very limited lawn areas and a gravel covered soil surface. A local mountain preserve located approximately 2 km from the study area was used as a source of control sites. Control sites were located on level areas of similar soils at an elevation of 400 m. Vegetation within the preserve consisted of species typical of lower elevation Sonoran desert: creosote bush (Larrea tridentata), triangle-leaf bursage (Ambrosia deltoidea), and saguaro (Carnegiea gigantea). The study consisted of three mesiscape, three xeriscape, and three desert remnant sites.

The majority of the study areas are classified as Rock Land (USDA, 1974) Association. The Rock Land Association contains 70% rock land, 15% rough broken land, 10% Cavelt soils (Loamy mixed, superactive, hyperthermic, shallow Typic Petrocalcids) and 5% Pinamt soils (Loamy-skeletal, mixed, superactive, hyperthermic Typic Calciargids). The parent materials of this association are granite, schist, andesite, and conglomerate. Soils on our study sites were sandy loam texture ranging from 6.3 to 12.7% clay and from 33.6 to 43.6% coarse fragments (>2 mm diam.) (Table 1). Desert remnant soils had the largest clay content (12.7%) and coarse-fragment content (43.6%). Mesiscape sites had the smallest clay content (6.3%) and the smallest coarse-fragment content (33.6%). Mesiscape sites had the largest organic C contents (1.23%), and desert remnant had the smallest (0.54%) (Table 2). Soil pH ranged from 7.6 to 8.4. All soils have a hyperthermic temperature regime which has a mean annual temperature >22°C.


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  		Table 1. Mean values (SE) of percentage of sand, clay, textural class, bulk density, and percentage of coarse fragments of mesiscape, xeriscape, and desert remnant sites in south central Arizona.

 

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  		Table 2. Mean values (SE) of pH, percentage of organic C, total C, total N, and C/N ratios of mesiscape, xeriscape, and desert remnant sites in south central Arizona.

 
At each study site we established a permanent sampling plot approximately 10 by 10 m. Initially, 10 soil cores were collected to a depth of 30 cm from each site with a core sampler lined with a 2.5-cm diam. butyrate sampling tube. These cores were combined to characterize the study sites. Soil texture was determined using the hydrometer method of Bouyoucos (1962). The percentage of coarse fragments (>2 mm) was determined from the weight of material retained after sieving the initial 10 cores through a 2-mm sieve. Soil pH was determined using an electrode and a 1:1 soil/water mixture (Thomas, 1996). We quantified organic C using the Walkely-Black method (Nelson and Sommers, 1996). Total C and total N were determined with a Perkin Elmer 2400 autoanalyzer (Bremmer, 1996; Nelson and Sommers, 1996). Five 5-cm diam. by 10-cm deep cores were collected to determine bulk density at each site (Blake and Hartge, 1986).

Each month, five cores each of a 2.5-cm diam. by 30 cm were collected from the permanent sampling plot at each site. Sample tubes were capped and placed on ice for transportation to the laboratory. All soils were stored at 4°C until analysis. Soil moisture and enzyme activity were determined from these samples. Soil moisture was determined gravimetrically by drying a sample to a constant weight at 105°C. Invertase (EC 3.2.1.26 ß-D-fructoglucanoside fructohydrolase) and cellulase (EC 3.2.14 ß-glucanohydrolase) activities were determined with the method of Schinner et al. (1995). Invertase activity was determined after incubating samples for 3 h at 50°C at pH 5.5. Cellulase activity was determined after incubating samples for 24 h at 50°C at pH 5.5. All soils were analyzed field moist. All enzyme determinations were made in triplicate. All values are reported on an oven dry basis.

Soil CO2 flux and temperature was measured with a P.P. Systems model EGM-2 Infrared Gas Analyzer (P.P. Systems, Haverhill, MA) at the same time cores for enzyme analysis were collected. At each site, ten 10.2-cm diam. by 3.7-cm tall PVC pipes were installed flush with the soil surface and used as collars for the measurement of soil respiration. Soil CO2 flux was not measured in October or December because of equipment failures. At the same time CO2 flux was sampled, soil temperature was measured at a 10-cm depth with a type T thermocouple and a digital thermometer.

We used a one-way ANOVA to test for differences within each month of invertase and cellulase activities and CO2 flux between mesiscapes, xeriscapes, and desert remnant sites. Differences between sites were tested with Fisher's protected least significant difference procedure (Petersen, 1985). We used a significance level of P = 0.05 for ANOVAs and mean separations. The functional dependence of soil invertase, cellulase, and CO2 flux on soil temperature and moisture were tested with correlation and multiple regression procedures (Zar, 1984). Regressions with P < 0.05 were considered significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Soil Temperature and Moisture
Soil temperatures varied from 7.5°C at the mesiscape sites to 41.3°C at the desert xeriscape sites (Fig. 1) . At each sampling period, mesiscape sites were consistently cooler than either the desert remnant or the xeriscape sites. Desert remnant sites were the warmest at all sampling occurrences. Minimum and maximum temperature readings occurred in February and August respectively. Soil moisture ranged from 15.1% in mesiscape sites to 0.5% in desert remnant sites. Average soil moisture was highest in mesiscapes, and lowest in desert remnant sites.



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  		Fig. 1. Average soil temperatures (°C) at 10-cm depth in mesiscape, xeriscape and desert remnant sites in south central Arizona. Numbers are means ± SE. *, ** and *** indicate significance between treatments at P < 0.05, 0.01, and 0.001 level, respectively.

 
Enzyme Activities
Invertase activities ranged from 15 mg GE g-1 24 h-1 in mesiscape sites in January to 2.4 mg GE g-1 24 h-1 in desert remnant sites in August. Mesiscape sites showed a distinct seasonal peak in winter months and a seasonal low in the summer months (Fig. 2) . No seasonal pattern was discernable for desert remnant or xeriscape sites. Differences between land-use types were most pronounced in the winter months, when invertase activity of the mesiscape sites was almost twice as much as the desert remnant sites and over three times greater than the xeriscape sites. No significant differences among land-use types were found in the spring and early summer months. Differences between desert remnant and xeriscape sites were significant only in December and January. In each of these of these months, the invertase activities were greater in the desert remnant sites when compared with the xeriscape sites.



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  		Fig. 2. Seasonal change in invertase activity in mesiscape, xeriscape and desert remnant sites in south central Arizona. Numbers are means ± SE. *, ** and *** indicate significance between treatments within months at P < 0.05, 0.01, and 0.001 level, respectively.

 
The highest and lowest cellulase activity, ranging from 406 ± 41.7 µg GE g-1 24 h-1 in January to 49 ± 7.85 µg GE g-1 24 h-1 in March, occurred in mesiscape sites (Fig. 3) . No seasonal pattern of enzyme activity and site was discernable. Cellulase activity was significantly higher in the mesiscape sites than desert remnant or xeriscape sites in the months of May, June, August, and September. Although cellulase activity in the mesiscape site was higher in July this difference was not significant. Cellulase activity mesiscapesin the winter (January and February) was significantly greater than that of desert remnant and xeriscape sites. Differences between desert remnant and xeriscape sites were not significant with the exception of March, when activity in the xeriscape sites were higher than either the mesiscape sites or the desert remnant sites.



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  		Fig. 3. Seasonal change in cellulase activity in mesiscape, xeriscape, and desert remnant sites in south central Arizona. Numbers are means ± SE. *, ** and *** indicate significance between treatments within months at P < 0.05, 0.01, and 0.001 level, respectively.

 
Correlations and regressions of soil temperature, moisture, and flux explained up to 76% of the observed variation in enzyme activities (Table 3). Soil temperature alone was the single most important predictor of enzyme activity for cellulase and invertase. Correlations of invertase activity with soil temperature in mesiscapes and desert remnant sites had an r of 0.72 and 0.76, respectively, and were highly significant. Correlations and regressions of invertase activity in xeriscape sites were not significant. The correlation between soil temperature and cellulase activity was significant for mesiscape and xeriscape sites. Soil temperature and moisture explained the largest percentage of cellulase activity in desert remnant soils with an R2 of 0.41 and a P value of 0.002.


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  		Table 3. Best predictors, coefficients of determination for correlation (r) and multiple regression (R2) and significance levels (P) for regressions of invertase and cellulase activities and CO2 flux in mesiscape, xeriscape, and desert remnant sites in south central Arizona.

 
Carbon Dioxide Flux
Soil CO2 flux ranged from 1.76 ± 0.541 g m-2 h-1 in mesiscape sites in April to 0.212 g m-2 h-1 in desert remnant sites in March (Fig. 4) . Mesiscape sites had larger CO2 flux in the late spring and summer and smallest flux rates in the winter months. Xeriscape and desert remnant sites did not show a seasonal pattern. For all sampling months except March, CO2 flux was significantly larger in mesiscapes than desert remnants or xeriscapes. Xeriscape sites had significantly greater CO2 flux rates than desert remnant sites in July and September. Although CO2 flux rates in xeriscapes were generally larger than desert remnants, the differences were not significant.



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  		Fig. 4. Seasonal change in CO2 flux in mesiscape, xeriscape, and desert remnant sites in south central Arizona. Numbers are means ± SE. *, ** and *** indicate significance between treatments within months at P < 0.05, 0.01, and 0.001 level, respectively.

 
The best predictor for CO2 flux in mesiscape and xeriscape sites was soil temperature (Table 3). In desert remnant sites, the only significant predictor of CO2 flux was soil moisture.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In this study of mesiscapes, xeriscapes, and desert remnant sites, we found large differences in enzyme activities and CO2 flux between sites. Rates of enzyme activity and soil CO2 flux were largest in the mesiscape sites. Differences between xeriscape and desert remnant were generally nonsignificant.

Although soil temperature was the best predictor of enzyme activity, there were significant differences between sites. Invertase activities in mesiscape sites were generally higher and showed a strong seasonal variation with peaks in the winter months. These peaks may be due to homeowner activities. Many homeowners, including all those where our plots were located, maintain a winter and summer lawn. Summer lawns are dominated by Bermuda grass. During mid-October summer lawns are forced into dormancy by reduced irrigation and close mowing. The lawn surface is then overseeded with perennial, annual, or hybrid ryegrass (Lolium spp.) and fertilized to stimulate winter growth. The fertilizer applied is most commonly steer (bos taurus) manure. Maintenance of a large, active rhizosphere and larger C inputs by that rhizosphere may have been a factor contributing to the seasonally high invertase activity in mesiscape sites. Roots and rhizosphere organisms are thought to be major sources of invertase and other enzyme accumulation in soils (Skujins, 1976; Bopaiah and Shetty, 1991). Higher activities of invertase in uncultivated pasture compared with cultivated sites were considered by Bandick and Dick (1999) to be a result of a more extensive rhizosphere system in the uncultivated sites. Other studies have found larger activities of enzymes with decreasing frequency of cultivation (Klein and Koths, 1980; Dick, 1984; Angers et al., 1993). Release of invertase from the plants as they enter dormancy may have also contributed to the winter peak in invertase activity. This process has been suggested to account for winter peaks of other enzyme activities in Juniper-galletta (Juniperus monosperma – Hilaria jamesii) woodlands (Krämer and Green, 2000), Canadian mixed prairie and fescue (Festuca spp.) grassland soils (Dormaar et al., 1984), and in Russian Mollisols (Khaziyev, 1977).

Soil moisture was not a significant predictor of enzyme activity in our study with the exception of cellulase in desert remnant sites. Soil moisture may have been a poor predictor of enzyme activity in mesiscapes because of irrigation of these sites by homeowners. Irrigation may have kept soil moisture from becoming a limiting factor in these sites. Variability of soil moisture in our mesiscaped sites was low. Xeriscape sites in our study were covered with a layer of gravel ranging from about 2.5 to 10 cm thick. Interception of rainfall by this gravel layer may have kept the soil moisture in the xeriscape soils below that needed to stimulate invertase or cellulase activity. In desert remnant sites, there may have been enough water to stimulate invertase activity when the soils were sampled. Our sampling was not tied to precipitation events, and soils were relatively dry when they were sampled because of high evaporation rates coupled with small rainfall amounts. Soil moisture was not correlated with invertase activity in considerably more mesic New Zealand pasture soils (Ross et al., 1995). In fertile pasture soils in Australia, however, invertase activity was linked to soil moisture (Ross and Speir, 1984). Soil moisture was a significant predictor of cellulase activity only in desert remnant sites. Soil moisture had a limited influence on phosphatase activities in a semiarid woodland of Arizona with a cinder mulch averaging 2.4 cm deep (Krämer and Green, 2000).

Seasonal patterns of CO2 flux rates in mesiscapes followed those of soil temperatures, with seasonal lows in winter months and peaks in the summer months. Soil temperature is an important driver of CO2 flux rates (Mielnick and Dugas, 2000; Fang and Moncrieff, 2001). Warm soil temperatures and available moisture may have contributed to high flux rates in the spring and summer months. Soil temperatures in desert remnant sites followed the same general pattern, but were 5 to 10°C warmer over the year than the mesiscape sites. Soil CO2 flux in these sites however did not show a strong seasonal trend probably because of moisture limitation. The functional dependence of flux rates on soil moisture is well known (Mielnick and Dugas, 2000) and indicated in the desert remnant sites of this study by the significance of soil moisture on flux rates.

Land use has significant impacts on soil processes and functions. Even within the same land-use type, this study found significant differences in enzyme activities and CO2 flux depending upon site management. Mesiscape sites had higher enzyme activities and CO2 flux than xeriscape sites. Additional research will be necessary to assess the importance of these differences to C models for urban areas such as Phoenix.


   ACKNOWLEDGMENTS
 
We wish to thank all homeowners for permission to work on their properties, and those individuals who helped with field research and laboratory analysis. This research was supported by Central Arizona-Phoenix Long-term Ecological Research project (National Science Foundation grant DEB-9714833).

Received for publication February 5, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Green, D. M.
Right arrow Articles by Oleksyszyn, M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Green, D. M.
Right arrow Articles by Oleksyszyn, M.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Green, D. M.
Right arrow Articles by Oleksyszyn, M.
Related Collections
Right arrow Other Soil Types
Right arrow Urban Soils
Right arrow Soil Biochemistry
Right arrow Soil Biology

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