Plant and Soil 227: 127–137, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years Mark A. Williams, Charles W. Rice∗ & Clenton E. Owensby
Department of Agronomy, Kansas State University, Manhattan, KS 66506-5501, USA. Corresponding author∗ Received 16 March 1999. Accepted in revised form 3 July 2000
Key words: elevated CO2 , microbial activity, microbial biomass, soil water, soil C and N
Abstract Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO2 . Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO2 for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO2 compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO2 than ambient CO2 in the 5–15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO2 than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO2 treatments increased microbial activity relative to the ambient CO2 conditions. The increase in microbial activity under elevated CO2 in the 5–15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO2 than ambient CO2 . Our results suggest that decomposition is enhanced under elevated CO2 compared with ambient CO2 , but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO2 increases.
Introduction The CO2 concentration in the atmosphere of the Northern Hemisphere averages approximately 360 µmol mol−1 and is increasing at a rate of 1.7 µmol mol−1 per year (Lal et al., 1995). The direct effects of elevated atmospheric CO2 are typically increases in above-ground and below-ground productivity of C3 and sometimes C4 plants (Curtis et al., 1989; Dippery et al., 1995; Owensby et al., 1999). Plant physiological processes like C metabolism, water-useefficiency (WUE) and nutrient-use-efficiency (NUE) are enhanced with elevated CO2 (Knapp et al.,1993; ∗ FAX No: 785 532-6094. E-mail:
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Larigauderie et al., 1988; Owensby et al., 1993). In C4 -dominated tallgrass prairie, increases in WUE via reduction in plant transpiration in elevated CO2 may alleviate water stress while maintaining optimal C fixation (Owensby et al., 1997). Owensby et al. (1997) concluded that plant communities on ungrazed tallgrass prairie, and most C4 -dominated plant communities with periodic moisture stress, will have greater above- and below-ground plant production in elevated CO2 . The concomitant increase in C inputs from litter, root growth, and perhaps root exudation, and the more efficient use of resources such as N and water, impact the quantity and quality of energy and nutrient inputs into the soil (Cotrufo and Ineson, 1995; Gorissen et al., 1995, Kemp et al., 1994). The con-
128 sequences of these potential changes in plant tissue chemistry, increased C inputs and greater WUE will have pronounced effects on mineralization and decomposition, and the long-term response of the ecosystem to elevated CO2 (Rice et al., 1994). The balance between C inputs into soil from plants and exports from microbial respiration might create a new equilibrium and thus determine the potential for soil to serve as a C sink. Increases in C inputs may enhance energy availability for microbes, and so increase microbial activity and nutrient cycling (Zak et al., 1993). In contrast, increased C:N ratios of litter from plants grown in elevated CO2 may slow the rate of nutrient cycling (Hunt et al., 1991) and increase N immobilization (Diaz et al., 1993). Many questions have been raised about the effects of increased litter inputs on N cycling and N availability in soil (Jackson and Reynolds, 1996; Larigauderie et al., 1988; Rice et al., 1994). Increases in NUE tend to increase the C:N ratio of plant materials grown in elevated CO2 (Bazzaz, 1990; Curtis et al., 1989; Newton, 1991; Norby et al., 1986), but Kemp et al. (1994) measured no changes in litter C:N ratios in tallgrass prairie plants grown under elevated CO2 . Microbial biomass is considered a sensitive indicator of changes in quality and quantity of organic matter inputs (Follet and Schimel, 1989; Van Veen et al., 1989). Changes in microbial biomass C and N, microbial activity and inorganic N can indicate the effects of elevated CO2 on N dynamics. Particularly important is the effect of elevated CO2 on N cycling and availability because shortages in N could eventually attenuate the increased plant biomass production seen in short-term elevated CO2 studies. Because N is already limiting in most native ecosystems (Binkley and Hart, 1989; Owensby and Anderson, 1967), it could act to decrease total plant, microbial and ecosystem productivity. The end result may be reduced C assimilation and a net release or no change in soil C. Since the majority of grassland C is in relatively stable soil organic matter (Rice and Garcia, 1994) and has long turnover times (Stevenson and Cole, 1999), the greater C inputs by C4 perennial grasses could foster soil C storage. Greater soil water content increasing decomposition, and the potential for limiting nutrients such as N restricting plant growth could alter this hypothesis, and so a relatively long-term (8-yr) field study was initiated. In an earlier study of tallgrass prairie exposed to elevated CO2 , Rice et al. (1994) found increases in microbial C and N and greater microbial activity with elevated CO2 during a drought
year, but no changes in total soil C and N after 3 years of CO2 enrichment. To understand whether these properties would increase under longer exposure to elevated CO2 , we continued to monitor microbial biomass C and N, microbial activity, total soil C and N, inorganic N and seasonal soil water content in this tallgrass prairie exposed to elevated CO2 . We hypothesized that greater C inputs under elevated CO2 would increase microbial biomass C, with a concomitant increase in microbial N due to increased N demand; greater soil water content would increase microbial activity under elevated CO2 relative to ambient CO2 ; and greater C content and C:N ratios would occur in elevated CO2 soil compared to ambient conditions due to large increases in plant biomass production (Owensby et al., 1999).
Materials and methods Study site The experimental site was located in pristine (annually burned) tallgrass prairie north of Manhattan, KS (390 12◦ N, 960 35◦ W, 324 m above M.S.L.). Vegetation on the site was a mixture of C3 and C4 species, dominated by big bluestem (Andropogon gerardii Vitman) and indiangrass (Sorghastrum nutans (L.) Nash). Subdominants included Kentucky bluegrass (Poa pratensis L.), sideoats grama (Bouteloua curtipendula (Michx.) Torr.), and tall dropseed (Sporobolus asper var. asper (Michx.) Kunth). Members of the sedge family made up 5–10% of the composition. Average peak biomass of 425 g m−2 (dry wt.) occurs in early August, of which 35 g m−2 is from forbs (Owensby and Anderson, 1967). Soils in the area are transitional from Ustolls to Udolls (Tully series: fine, mixed, mesic, montmorillonitic, Pachic Argiustolls). The 30-year average annual precipitation is 840 mm, with 504 mm occurring from April through August. Treatments Circular open-top-chambers (4.5 m dia.) were established in early May 1989. Treatments were ambient CO2 - no chamber (NC), ambient CO2 - chamber (AC), and 2 X ambient CO2 - chamber (EC). Each treatment was replicated three times using a randomized complete block design. Enrichment of CO2 was supplied constantly from early April to late October. Soil samples were taken monthly from May through November at 0–5 and 5–15 cm depths starting in 1991.
129 The overall experimental design is explained in detail by Owensby et al. (1993). A no-chamber treatment was included in the experiment, and data from this treatment are included, but results and discussion will focus on the chamber treatments. Laboratory methods Percent soil organic C and N was determined by dry combustion of 15 g samples (dried and ground with mortar and pestle) in a Europa 20/20 IRMS (Crewe, Cheshire, UK). Soil pH was 6.6–6.8, and CaCO3 was absent, so no acid pre-treatment to remove inorganic C was necessary. Microbial biomass C and N were determined by the fumigation-incubation method (Jenkinson and Powlson, 1976). Soil used for estimation of microbial biomass C and N was stored (4 ◦ C) for no more than 2 weeks prior to analysis. To remove large roots and homogenize the soil, samples were sieved to pass a 4mm mesh. Soil (25 g) was added to duplicate125 mL Erlenmeyer flasks or for the last year of the study, 5 g of soil were added to six 160 mL serum bottles. When soil water content was less than 0.28 g g−1 (approx -150 kPa) water was added to achieve that soil water content. All samples were pre-incubated at 25 ◦ C for 5 days and then one-half of the replicates were fumigated with ethanol-free chloroform for 18–24 h. Fumigated and un-fumigated samples were sealed in 940-mL mason jars containing enough water to maintain a saturated atmosphere and then incubated for 10 days at 25 ◦ C. At the end of the incubation period, the headspace CO2 –C concentration was measured using a Shimadzu GC-8A gas chromatograph (Shimadzu Scientific Instruments Inc., Columbia, MO.) equipped with a 2 m Porapak Q column. After CO2 was measured, 100 mL of 1 M KCL were added to flasks containing 25 g soil and 20 mL were added to serum bottles with 5 g samples. Samples were shaken for 1 h on an orbital shaker at 300 RPM. The suspension was transferred to a 250 mL centrifuge bottle and centrifuged at 16 000 × g for 10 min. The supernatant was filtered through a nylon mesh (10 µm) and analyzed for NH4 + –N and NO3 − –N colorimetrically on an Alpkem Autoanalyzer (Alpkem Corp., Clackamas, OR). Ammonium-N was determined by the salicylatehypochlorite method (Crooke and Simpson, 1971) and NO3 − –N + NO2 − –N by the Griess-Ilosvay technique (Keeney and Nelson, 1982). Microbial biomass C and N were calculated using the equations of Voroney and Paul (1984).
Microbial activity was measured on freshly sampled soil (< 2 days), sieved as described previously. Field moist soil (20 g) was added to 160 mL serum bottles that were sealed with rubber stoppers and aluminum seals. Samples were incubated at 25 ◦ C for 48 h. Carbon dioxide concentration was measured four times during the incubation period to obtain a linear regression and CO2 was calculated as µg CO2 –C g−1 soil h−1 . Soil water content was measured by weight loss at 104 ◦ C for 48 h. Inorganic N was assessed by using the KCl extraction procedure described previously. Bulk density was measured in duplicate from each experimental unit in August 1997 using a hand-held probe (6 cm dia.) equipped with a sliding weight to retrieve samples from the 0 to 30 cm depths. The length of each soil core was measured in the field to the nearest 0.5 cm, placed in labeled brown paper sacks, transported to the laboratory and dried at 104 ◦ C for 62 h. Bulk densities were not significantly different between treatments, averaging 0.93 and 1.15 g soil cm−3 at the 0–5 and 5–15 cm depths, respectively. Data analysis Data from each year and depth were analyzed separately and conjointly by Proc Mixed (SAS Institute, 1996). Tests for normality were assessed, and all data presented met normality criteria. Many of the data shown are means derived from treatment by month effects; none tested significantly as crossover interactions. The model class statements were replication, treatment, month, year and depth. Least Significant Difference mean separation tests were used to determine where significant differences occurred. All results were considered significantly different at p < 0.10 unless noted otherwise.
Results General precipitation and temperature conditions that occurred during the study are shown in Table 1. Though some years may be relatively wet or dry on an average basis, soils in all years experience some degree of drying depending on rainfall periodicity, temperature, and evapo-transpiration. Average yearly soil water content for each sampling season are presented in Figure 1. The growing-season precipitation for each year, expressed as a percentage of the 30-year average growing-season precipitation, is also shown in Table 1.
130 Table 1. Monthly precipitation values in centimeters and yearly average of the growing-season daily maximum soil temperature at 10 cm depth Year
May
June
July
Aug.
Sept.
1991 1992 1993 1994 1995 1996
13 (92)a 4 (30) 28 (198) 8 (58) 37 (265) 20 (143)
5 (37) 9 (66) 28 (194) 14 (100) 11 (74) 10 (67)
5 (47) 34 (345) 34 (345) 10 (106) 9 (95) 13 (138)
6 (59) 5 (56) 17 (177) 8 (85) 8 (82) 8 (80)
4 (49) 14 (158) 9 (98) 1 (12) 13 (151) 8 (93)
Avg.
18.3b
12.8
17.5
8.7
8.2
Oct.
6 (79) 7 (91) 2 (27) 4 (58) 2 (29) 8 (108) 4.8
May 1– Nov 1 39 (56) 73 (104) 117 (167) 46 (66) 80 (115) 66 (94) 70.2
T◦ (C)C
32.8 28.5 28.4 28.8 25.5 25.2 28.2
a Numbers in parenthesis are percentages of the 30 year average. b Values in the last row are column averages. C This is the average of daily (growing season) maximum soil temperatures (10 cm depth).
Figure 1. Average yearly soil water content in the surface 15 cm as measured on a monthly basis for each sampling season (7 month sampling period). The percentages given were calculated as the amount of growing-season precipitation for each year divided by the 30-yr average growing-season precipitation.
Soil water content was significantly greater in EC over the entire study period at both depths relative to AC (p = 0.0012), averaging 0.27 and 0.25 (0–5 cm), and 0.32 and 0.28 g H2 O g−1 soil (5–15 cm) in EC and AC, respectively. Soil water content was significantly greater in EC relative to ambient conditions for all years separately (p < 0.1), except in 1996 at the 5–15 cm depth. Soil water content at that depth was always significantly greater in EC during the months of May, June and July (p < 0.1, Figure 2). The greatest differences in soil water content between EC and AC occurred during the driest years of 1991 and 1994. The wettest year, 1993, showed the third largest difference in soil water content between EC and AC. Although 1996 was a normal year for precipitation, an
Figure 2. Average monthly soil water content from 1991 to 1996 at 5–15 cm depth. Significant differences between the EC and AC treatments are indicated by ∗ (p < 0.1). Approximately 85% of plant growth had occurred by August 1st.
unusually cool summer reduced evapo-transpiration and decreased average (growing-season) daily maximum soil temperatures at 10 cm (Table 1). Results of greater soil water content in EC relative to AC measured on a mass basis are consistent with the results of Owensby et al. (1999) measured on a volumetric basis. Microbial activity was significantly greater in EC relative to AC at the 0–5 cm and 5–15 cm depth in the combined analysis of all years (p = 0.042, Tables 2 and 3) and for most years considered separately. Exceptions were the wet year of 1993 and the normal precipitation year of 1996 at the 0–5 cm depth and the two normal rainfall years of 1992 and 1996 at the 5– 15 cm depth. In all years, however, there was a general
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Figure 3. Average monthly microbial activity for 1991–1996 in (a) 0–5 cm and (b) 5–15 cm depths. Significant differences between the EC and AC treatments are indicated by ∗ , ∗∗ (p < 0.1, p
