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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Nitrogen Competition in a Tallgrass Prairie Ecosystem Exposed to Elevated Carbon Dioxide

Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

Corresponding author (cwrice{at}ksu.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Because N is a limiting nutrient in tallgrass prairie and most ecosystems, changes in N availability or N cycling could control the long-term response of ecosystems to elevated atmospheric CO₂. If more C is sequestered into the soil, then greater microbial demand for N could decrease plant-available soil N. Alterations in N dynamics such as plant uptake, N fixation, nutrient cycling, microbial utilization, and partitioning of N into plant and soil fractions ultimately could affect the capability of ecosystems to sequester C. Our objective was to determine if competition for N between plants and microorganisms changes after 8 yr of elevated CO₂ relative to ambient conditions. Treatments (three replications, randomized complete block design) were ambient CO₂–no chamber (NC), ambient CO₂–chamber (AC), and 2 x ambient CO₂–chamber (EC). Several short laboratory incubations assessed whether turnover rates of N in soil would be altered under elevated CO₂. Gross transformations of N were not altered significantly under elevated CO₂ compared with ambient conditions. To examine plant–microbial competition and altered allocation patterns of N under elevated CO₂, ¹⁵NH₄–N was added to 25-cm-diam. polyvinyl chloride (PVC) cores (15-cm depth) in the field, which were destructively sampled after 5 mo. Microbial biomass contained 75% of the total ¹⁵N that occurred in the soil organic matter (SOM) and, thus, appeared to be a significant regulator of plant-available N. The SOM under elevated CO₂ contained significantly more (>27%) ¹⁵N compared with ambient CO₂ conditions. Though a chamber effect was apparent, greater ¹⁵N in the SOM pool and greater percentage ¹⁵N SOM/percentage ¹⁵N plant suggest greater microbial demand for N under elevated CO₂.

Abbreviations: AC, ambient CO₂ with chamber • EC, elevated CO₂ with chamber • I, immobilization • M, mineralization • M_c NH⁺₄ consumption • i_p, NH⁺₄ production • N_p, NO^-₃ production • N_c, NO^-₃ consumption • NC, ambient CO₂ with no chamber • PVC, polyvinyl chloride • SOM, soil organic matter

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
ICE CORE DATA provide evidence that atmospheric CO₂ is at its highest concentration since at least 160000 yr ago (Lorius et al., 1989). Concentrations rose from 270 µmol mol^-1 in the late 1800s to 365 µmol mol^-1 in 1997 (Lorius et al., 1989) and may double preindustrial levels by the middle of the 21st century. Global terrestrial ecosystems and soil could be sinks for atmospheric CO₂. An understanding of the long-term effects of elevated atmospheric CO₂ on belowground processes and plant–soil interactions is crucial to predict future changes in ecosystem function and C storage.

Mineralization of organic matter by heterotrophic microorganisms is the primary mechanism for producing plant-available inorganic N. Conversion of organic N into NH⁺₄ by heterotrophs is the primary means of inorganic N formation in soils. The balance between organic N mineralized and inorganic N immobilized into microbial biomass is net N mineralization. Although knowledge of net N mineralization rates is important for assessing potentially plant-available inorganic N, measurements of gross transformation rates are needed to estimate nutrient turnover and assess the amount of N cycling through the plant-available inorganic pool. If nutrient turnover or N cycling is altered under elevated CO₂, understanding how these rates are modified will be important.

If microorganisms are C limited at any time during the year, then increased belowground productivity may elevate microbial metabolic processes and nutrient turnover. A strong microbial response to glucose addition in experiments by Bremer and van Kessel (1990) and Garcia and Rice (1994) showed that microorganisms can be limited by C in a cultivated grassland (1.3 mg C kg^-1, 0.17 mg N kg^-1) and native prairie (4.0 mg C kg^-1, 0.36 mg N kg^-1), respectively. Zak et al. (1993) postulated that greater N availability and organic matter turnover occurred in soil when poplar trees (Populus spp.) were grown in a low organic matter (0.2 mg C kg^-1 and 0.07 mg N kg^-1) sandy soil under elevated CO₂. Sites where N is limiting may be more constrained by N if C inputs into the soil are increased. Competition for N between plants and microorganisms could be enhanced because greater plant productivity and greater amounts of root C typically occur under elevated atmospheric CO₂ (Curtis et al., 1990; Owensby et al., 1999). Hence, the capacity for systems to have dynamic and multiple limiting factors (Owensby et al., 1999) underscores the need for experimentation and testing in natural ecosystems.

A study (Jackson et al., 1989) on a California annual grassland during the spring growth period provided evidence that microbes were better competitors for inorganic N than plants, especially for NH⁺₄–N. Kaye and Hart (1997) suggested that many ecologists believe that plants and heterotrophic microorganisms do not compete for soil resources because heterotrophic microorganisms are C limited, whereas temperate plants are known to be N limited. Still, many studies indicate that microorganisms mediate plant N uptake by controlling available N (Kissel and Smith, 1978; Norton and Firestone, 1996; Kaye and Hart, 1997; Dell, 1998), since greater C inputs into soil tend to increase N demand by microorganisms.

Long-term studies where ¹⁵N is added to soil at the beginning of the growing season can help assess whether soils, plants, and microorganisms alter the stores of N under elevated CO₂. If microorganisms utilize greater C inputs or higher soil water contents occur due to reduced water uptake by plants under elevated CO₂ (Owensby et al., 1999) and aids decomposition of organic matter (Zak et al., 1993), then the potential for greater mineralization and production of inorganic N could benefit plants and their productivity. If greater C inputs to soil constrain plant growth via greater microbial immobilization of N, then the potential for ecosystems to sequester C under elevated CO₂ will be limited (Diaz et al., 1993). Furthermore, because the N cycle is closely tied to the C cycle through the turnover of organic matter (van de Geijn and van Veen, 1993), understanding changes in internal N cycling caused by elevated CO₂ is important.

Some scientists have argued that N will become more limiting under elevated CO₂ and feed back to limit the increase in biomass production (Diaz et al., 1993). Though N use efficiency of plants may increase in systems under elevated CO₂ (Owensby et al., 1993), this may be offset by a decrease in available N. Microbial biomass N and microbial activity that regulates N transformations in soil were greater in tallgrass prairie under elevated CO₂ than under ambient conditions (Williams et al., 2000). Increases in microbial biomass N and microbial activity have led us to hypothesize that greater microbial N demand and greater plant growth may amplify plant–microbe competition under elevated CO₂. The addition of (¹⁵NH₄)₂SO₄ to soil in the early growing season (before plant growth) was used to test for changes in N allocation between plants and soil microbes. We expected greater sequestration of ¹⁵N into the microbial and SOM pools relative to plant biomass under elevated CO₂ than under ambient conditions. Larger pools of available C from greater root C inputs (Owensby et al., 1999) and greater microbial activity should increase microbial N transformation rates, especially immobilization.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Study Site
The experimental site was located in pristine (annually burned) tallgrass prairie north of Manhattan, KS (39.12°N, 96.35°W, 324 m above mean sea level). Vegetation on the site is a mixture of C₃ and C₄ species, dominated by big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash]. Members of the sedge family make up 5 to 10% of the composition. Principal forbs include ironweed [Vernonia baldwinii var. interior (Small) Schub.], western ragweed (Ambrosia psilostachya DC.), Louisiana sagewort (Artemesia ludoviciana Nutt.), and manyflower scurfpea [Psoralea tenuiflora var. floribunda (Nutt.) Rydb.]. Average aboveground peak biomass of 425 g m^-2 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-yr average annual precipitation is 840 mm, with 504 mm occurring from April through August.

Treatments
Circular open-top chambers (4.5 m in diam.) 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 continuously from early April to late October. The overall experimental design is explained in detail by Owensby et al. (1993).

Laboratory Methods
To estimate the gross N transformation rates, we used the method described by Davidson et al. (1991). Soils collected from the experimental plots in May and August, 1996, and April and May, 1997, were passed through a 4-mm sieve to remove the obvious larger roots and stored at 4°C until analysis (<10-d storage). Then soil subsamples (20 g) from each replicate treatment and depth were placed into four 125-mL Erlenmeyer flasks, sealed with parafilm, and allowed to incubate for 24 h (25°C). Two flasks from each replicate treatment and depth received 2 mg (2.0 atom % ¹⁵N) (NH₄)₂SO₄–N g^-1 soil, whereas the other two flasks received 2 mg (2.0 atom % ¹⁵N) KNO₃–N g^-1 soil. The ¹⁵N solution was added by slowly spraying 1 mL evenly over the soil using a 1-mL syringe. One set of each N addition was used as a control to assess the amount of N not recoverable from the soil. Five minutes after the N addition, the controls where shaken with 100 mL of 1 M KCl for 1 h at 300 rpm on an orbital shaker. The samples then were centrifuged (5°C) for 10 min at 15000 g. The other set of samples was sealed tightly with parafilm and incubated at 25°C in a humidified environment for 2 d. The samples then were shaken and extracted as described for the control samples.

Potassium chloride extracts were analyzed colorometrically for NH⁺₄ and NO^-₃ contents on an Alpkem Autoanalyzer (Alpkem Corp., Clackamas, OR). Ammonium N was determined by the salicylate-hypochlorite method (Crooke and Simpson, 1971) and NO^-₃ + NO^-₂–N by the Griess-IIosvay technique (Keeney and Nelson, 1982). Recovery of ¹⁵N in the KCl extracts was determined by the diffusion procedure as described by Brooks et al. (1989). Recovered N was then analyzed using a Europa 20/20 Mass Spectrometer (Europa Scientific, Crewe, Cheshire, UK). Mineralization was determined by adding ¹⁵NH⁺₄ and measuring the decline of the ¹⁵NH⁺₄ pool due to microbial mineralization of ¹⁴N into the soil. Nitrification was determined similarly by measuring the dilution of the added ¹⁵NO^-₃ from nitrified soil ¹⁴NH⁺₄. Gross rates of microbial consumption were calculated from the disappearance of the ¹⁵N label in the NH⁺₄ or NO^-₃ pools. Gross N transformation rates were calculated using the equations of Kirkham and Bartholomew (1954). Two different sets of equations were used depending on whether immobilization (I) was significantly greater (P < 0.1) than mineralization (M), or I and M were equal (not significantly different) on a sampling date. In cases were I = M, then only one set of numbers is reported.

Field Study
In late March, 1996, (1 mo before initiation of plant growth), PVC cores (25 cm diam. by 25 cm long) were inserted into the ground to a 15-cm depth using a metal hammer and a block of wood. About 44% of roots in tallgrass prairie soil occur in the top 10 cm of soil, and 70% in the top 20 cm (Rice et al., 1998).

In mid April, a solution of 105 mg N L^-1 (98% ¹⁵N) in the form of (NH₄)₂SO₄ was injected into each of the cores. Twenty-one 18-gauge, 15-cm-long spinal needles were inserted in an evenly spaced pattern into the soil. Syringes (10 mL) containing 6 mL of solution were used to inject 1 mL for every 2.5 cm of soil depth. Thus, each core received 13.2 mg of ¹⁵N (3 mg ¹⁵N g^-1 soil). Concentrations of inorganic N measured at the study site from 1991 to 1996 averaged 8.2 mg N g^-1 soil during May. At the time of injection, soil water content was 0.39 g H₂O g^-1 soil. The solution would have increased the soil water content to 0.41 g H₂O g^-1 soil. Though water-holding capacity of this soil has not been tested, surface soils had measured soil water contents of 0.54 and 0.45 g g^-1 soil during the study period at the 0- to 5- and 5- to 15-cm depths, respectively. We expect that leaching was not a problem during application, whereas the relatively high soil water contents facilitated diffusion of the ¹⁵NH⁺₄ throughout the soil in the cores.

Cores were removed from plots in early October and cut with a hacksaw to separate the 0- to 5- and 5- to 15-cm depths. Approximately 1000 g of soil below the core also was sampled to estimate the potential for leaching of the ¹⁵N, but <3% of the added N was found in the soil below the core.

Grass shoots were cut at the soil surface; fresh live roots and rhizomes were separated from the soil by hand, and all plant parts were dried (60°C, 3 d), weighed, and ground separately in a Wiley mill to pass an 800-mm screen. A visual inspection of the aboveground leaf material showed that 5 to 10% was green during removal of the cores. Soil was passed through a 4-mm sieve and well mixed. Soil water contents were measured by assessing the amount of soil water lost (15-g soil sample) during a 48-h period in a drying oven at 104°C. Approximately one-half of the soil at the 0- to 5- and 5- to 15-cm depths was placed separately in a 1% Na-hexametaphosphate solution for separation of smaller and finer roots by dispersion. The remaining soil was stored in plastic bags and stored at 4°C. The root solution was mixed for 4 to 8 h on a reciprocal shaker. Most of the fine roots were floating at the surface, but further extraction of the root solution through a series of sieves produced substantially more root material. Many fine roots in the root solution had to be discarded because of the intricate association of the delicate roots with organic matter, making separation impossible.

Triplicate 5-g soil samples were added to 163-mL serum bottles and were analyzed for microbial biomass C and N using the fumigation–incubation procedure of Jenkinson and Powlson (1976). Details of the procedure were outlined in an earlier paper (Rice et al., 1994). Microbial biomass C and N were calculated as described by Voroney and Paul (1984). Microbial ¹⁵N was extracted and estimated using the persulfate digestion (K₂S₂O₈) outlined by Cabrera and Beare (1993). Soil samples (25 g) were placed in 125-mL Erlenmeyer flasks. Treatment and control samples were treated similar to the fumigation–incubation procedure, except that after fumigation (48 h) and evacuation of the samples, both samples were mixed on an orbital shaker at 300 rpm with 100 mL of 0.5 M K₂SO₄. The suspension then was transferred to a 250-mL centrifuge bottle and centrifuged at 15000 g for 10 min. The supernatant then was filtered through a 10-mm nylon mesh. A 15-mL subsample of the supernatant was placed into glass test tubes, and mixed with 15 mL of low N (0.0005% N, K₂S₂O₈) oxidizing reagent (50 g L^-1 K₂S₂O₈, 30 g L^-1 H₃BO₃, 100 mL L^-1 3.75 M NaOH). Tubes were capped with screw caps containing Teflon liners and then autoclaved for 30 min at 120°C. After autoclaving, the samples were reweighed to assess the amount of moisture loss, which could be used to correct the NO^-₃ concentrations in solution. Nitrate and ¹⁵N content were then analyzed as stated above.

Organic N was measured by placing 50-g soil samples in 200 mL of 1 M KCl and mixing for 1 h on an orbital shaker at 300 rpm. The mixture was centrifuged for 10 min at 15000 g, and the KCl solution was decanted. This procedure was repeated two more times to remove as much inorganic N as possible. The soil was then washed two times with 200 mL distilled, deionized water. The soil was freeze-dried, crushed with a mortar and pestle into a fine powder, mixed, and 15-mg subsamples analyzed for total C and N and ¹⁵N content.

The amount of ¹⁵N in the nonmicrobial organic matter pool was estimated by subtracting microbial biomass ¹⁵N (fumigation–extraction) from total organic ¹⁵N.

Data were analyzed separately and conjointly by Proc Mixed (SAS Institute, 1996). Tests for normality were assessed, and all data presented met normality criteria. The model class statements were replication, treatment, 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

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Gross Transformations of Nitrogen
Neither rates of NH⁺₄ production (M_p) and consumption (M_c) nor gross nitrification (N_p) and NO^-₃ consumption (N_c) were significantly different among treatments (Tables 1–4). Consumption rates (M_c and N_c) for both depths tended to be larger than production rates (M_p and N_p), but this was especially the case for the surface 5 cm. Note that NH⁺₄ consumption includes both nitrification and NH⁺₄ immobilization (Davidson et al., 1991). Significantly greater (P < 0.0001) immobilization rates were measured in the top 5 cm of soil than in the 5- to 15-cm depth for M_p, M_c, N_p, and N_c. Rates of NH⁺₄ production and consumption were up to 10 times larger than those of NO^-₃ production and consumption at each depth, except during April when rates were quite high and approximately equal for M_p, M_c, N_p, and N_c. Although greater NH⁺₄ production and consumption occurred during the early part of the growing season (0–5 cm) compared with the August sampling date, a concomitant increase in rate variability also occurred.

View this table: [in this window] [in a new window]   Table 2. Gross rates of NH+4 production (Mp) and consumption (Mc) at the 5- to 15-cm depth

Pools of Nitrogen and Nitrogen-15 in Plants and Soil
Recovery of the added 13.2 mg of ¹⁵N from the soil cores was significantly (P = 0.078) greater in NC (84%) relative to AC (67%) and EC (71%). Because of the similar recovery rates between the two chamber treatments and the larger recovery from the NC treatment, values are presented as the percentage of recovered ¹⁵N (Table 5). The amount of soil recovered from each core was not significantly different (P = 0.67).

Approximately 0.18% of the N in the microbial biomass consisted of the added ¹⁵N, 1.2% of plant N was from added ¹⁵N, and 0.04% of the nonmicrobial SOM N was derived from added ¹⁵N. Microbial N (0–15 cm) contained 200 g N m^-2 compared with 10 g N m^-2 in plant roots, 10 to 20 g N m^-2 in rhizomes, and 5 g N m^-2 in shoots.

Total percentage of ¹⁵N recovered in the plant was not significantly (P = 0.62) different among treatments (Table 6). Percentage of recovered ¹⁵N in the soil organic pool was significantly greater in EC and NC than AC at the 5- to 15-cm depth (P = 0.088) but not at the 0- to 5-cm depth (P = 0.20, Table 5). The percentage of recovered ¹⁵N (5–15 cm depth) in the nonmicrobial SOM was significantly greater in EC and NC than in AC (P = 0.025; Table 6). The ratio of the percentage recovered ¹⁵N in the SOM relative to the plant was significantly (P = 0.033, Table 6) greater in EC and NC than in AC. Mass of surface litter in the cores tended to be greater in EC, but was not significantly (P = 0.26) different among treatments, averaging 188, 119, and 126 g m^-2 for EC, AC, and NC, respectively.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Gross Transformations of Nitrogen
Turnover rates of N followed the seasonal dynamics of greater soil inorganic N in spring, while lower turnover rates occurred with lower soil inorganic N in the mid growing season (Williams et al., 2000). High nitrification in the months of April and May probably was supported by the relatively large reservoir of NH⁺₄ that is found in tallgrass prairie soil during the early growing season (Garcia and Rice, 1994). Because nitrification is approximately equal to NH⁺₄ consumption and since NH⁺₄ consumption is the sum of both nitrification and heterotrophic NH⁺₄ immobilization, nitrification is a much greater sink for NH⁺₄ than heterotrophic immobilization during April. These results are consistent with data reported by Dell (1998) collected during April 1996.

The early growing-season months of April and May had much greater rates of soil NH⁺₄ production and consumption compared with those collected in August, indicating that nutrient turnover may have been limited by low soil water content (approximately -1500 Kpa) during August compared with April and May (-10 to -50 Kpa). The increase in variability as rates of NH⁺₄ production and consumption increased is difficult to interpret, but may be related to resource heterogeneity in the soil.

Plant–Microbe Competition (Chamber Effects)
Using the estimates of Rice et al. (1998), 40% of total root biomass in tallgrass prairie occurs below the depth (15 cm) sampled by our soil cores. Assuming these roots are equally active compared with roots in the surface 15 cm, an estimate of no more than (and probably much less than) 15% of the added N could have been transported below the cores by root growth. Furthermore, 3% of the added N was found in the soil below the cores, which could account for most or all of the unrecovered N in NC (16%), but only one-half of the N not recovered in the chamber treatments. Since it seems unlikely that root biomass in both EC and AC would have doubled the amount in NC at depths below 15 cm, greater denitrification might be responsible for the lower recovery rates in the chamber treatments compared with NC. Bremer et al. (1996) measured a 21 to 24% reduction in plant transpiration due to the chamber effect (AC compared with NC), which would increase soil water content and potentially promote denitrification.

Greater ¹⁵N in the soil organic fraction suggests that microbial demand for N is greater in NC relative to AC. No large changes in shoot or root production occurred in AC relative to NC (Owensby et al., 1999), indicating that available C and hence microbial N demand should be similar. On the other hand, potentially lower denitrification and decreased losses of added ¹⁵N in NC relative to AC would increase soil ¹⁵N. The greater SOM/plant ¹⁵N in NC relative to AC is more difficult to explain since C inputs and soil C contents were not significantly different between AC and NC.

Plant–Microbe Competition (Carbon Dioxide Effects)
Greater ¹⁵N in the soil organic fraction suggests that microbial demand for N increased under EC relative to AC. Greater amounts of available C and water in the elevated CO₂ soil may have increased microbial demand and capture of N, potentially increasing N as a limiting factor for plants. Though statistically correlated with greater ¹⁵N in the SOM of EC relative to AC, the greater SOM/plant ¹⁵N ratios in EC relative to AC provide a direct measure of altered plant–microbe competition due to elevated CO₂.

Dell (1998) found that an average of 77% of the ¹⁵N was accounted for in SOM, whereas the plant accrued 24%, illustrating the high N demand by the soil microorganisms. The same short-term distribution of added ¹⁵N with high immobilization into the SOM probably occurred in our cores during the first 6 d. Under elevated CO₂ greater amounts of root inputs occurred compared with ambient CO₂ (Owensby et al., 1999), thus increasing microbial demand for N. Similarly, Schimel et al. (1989) showed that microbial uptake of NH⁺₄–N was 4.2 to 9.2 times faster than plant uptake in an annual California grassland. The partitioning of ¹⁵N among soil and plant fractions (Table 6) in our 5-mo study was similar to that in a 6-mo study by Dell (1998), who found 60% of the recovered ¹⁵N in the SOM pool and 40% (for a plant/SOM ratio of 1.50) in the plant pool of the burned prairie after one growing season. Comparing the results of the 6-d and the 6-mo in situ core studies of Dell (1998) with our 5-mo study suggests that at least 22% of the ¹⁵N in the SOM pool at the beginning of the experiment was mineralized and transferred to the plant by the end of the growing season. Albeit this is a simplified unidirectional view of nutrient cycling, this estimate of microbial turnover of N could be a consequence of root C turnover as estimated by Seastedt and Hayes (1988).

In tallgrass prairie, many of the plant species, including the dominant big bluestem and indiangrass, persist and reproduce vegetatively by rhizomes (McKendrick et al., 1975). That study found that new roots appeared on tillers of both species only when green leaves were present (mid May to early June). They concluded that initial root growth depended on current photosynthate rather than stored food reserves, and that early N demand was met by using plant N reserves (rhizomes). When we injected the ¹⁵N solution into the soil cores, only small sprigs (5 to 10 cm tall) of green leaf material were apparent. Therefore, low elongation, growth, and activity of roots probably reduced plant demand for N relative to that in the peak growing season. Consequently, ¹⁵N taken up by the plant was first immobilized and then released to the plant via microbial mineralization.

McKendrick et al. (1975) and Hayes (1985) estimated that 18 to 45% of the N used by plants is stored in rhizomes and translocated to roots and shoots on an annual basis. The likely reason for more total recovered ¹⁵N (Table 5) in rhizomes of AC than EC is due to the significantly greater rhizome biomass in AC compared with EC (data not shown). This result contradicts the long-term results of greater shoot and root production measured by Owensby et al. (1999) and could be due to the spatial heterogeneity of rhizomes in soil. Since rhizomes are storage organs, they probably contribute little to direct plant uptake of N (due to low surface/volume ratio).

No differences in microbial biomass N or ¹⁵N were detected in this study, but microbial biomass N was significantly greater in the long term (1991–1996) under elevated CO₂ (Williams et al., 2000). Though no differences in gross transformation rates were found, a greater percentage of ¹⁵N in the nonmicrobial SOM pool of EC relative to AC may also suggest faster turnover of the highly ¹⁵N-enriched microbial biomass into the nonmicrobial SOM. In contrast, greater rates of root or labile organic matter (with relatively low ¹⁵N values) turnover would tend to dilute the SOM ¹⁵N pool. If this were true, greater ¹⁵N in EC would indicate slower turnover than in AC. A better understanding of the transfers and stores of mass between different nutrient pools in soil would help solve this problem.

Lower recovery of ¹⁵N in plant roots (and rhizomes), greater ¹⁵N in the SOM, and greater SOM/plant ¹⁵N ratios in EC than in AC suggest that microbes reduced plant supply for ¹⁵N. Our results demonstrated that during the time period of a growing season the microbial biomass controls plant N uptake in N-limited tallgrass prairie. This effect is exacerbated with the greater soil C inputs under elevated CO₂ compared with ambient conditions. Research on alterations of internal N cycling and storage within perennial plants in conjunction with a more thorough understanding of SOM dynamics will be crucial for understanding the combined microbial–plant response to rising atmospheric CO₂ in N-limited systems.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
This research was supported by the U.S. Department of Energy, Carbon Dioxide Research Division. Contribution no. 00-246-J of the Kansas Agric. Exp. Stn.

Received for publication February 14, 2000.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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