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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Carbon and Nitrogen Pools in a Tallgrass Prairie Soil under Elevated Carbon Dioxide

^a Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
^b 3121 Miller Plant Sciences Bldg., University of Georgia, Athens, GA 30606
^c Kansas State University, 2004 Throckmorton Plant Sciences Center, Dep. of Agronomy, Manhattan, KS 66506-5501

^* Corresponding author (cwrice{at}ksu.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Soil is a potential C sink and could offset rising atmospheric CO₂. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Our objective was to measure pools of soil C and N to assess the potential for C accrual and changes to N stocks as influenced by elevated atmospheric CO₂. Treatments (three replications, randomized complete block design) were ambient CO₂—no chamber (NC), ambient CO₂—chamber (AC), and two times ambient CO₂—chamber (EC). Long-term (290 d) incubations (35°C) were conducted to assess changes in the slow soil fractions of potentially mineralizable C (PMC) and potentially mineralizable N (PMN). Potentially mineralizable C was enhanced (P < 0.1) by 19 and 24% in EC relative to AC and NC soil at the 0- to 5- and 5- to 15-cm depths, respectively. Potentially mineralizable N was significantly greater by 14% at the 0- to 5-cm depth in EC relative to AC, but decreased by 12% in EC relative to NC (P < 0.1). Measurements of PMC indicate that increases in total soil C under elevated CO₂ in a previous study were a consequence of accrual into the slow pool. Relatively large amounts of new C deposited as a result of elevated CO₂ (C_new) remained in the soil after the 290-d incubation. In contrast to accumulation of C into the slow fraction, C_new was integrated into a passive fraction of soil organic matter (SOM). Accumulation of N was also detected in the whole soil, which cannot be explained by current estimates of ecosystem N flux.

Abbreviations: AC, ambient CO₂ with chamber • C_new, new carbon • EC, elevated CO₂ with chamber • MRT, mean residence time • NC, ambient CO₂ no chamber • PMC, potentially mineralizable C • PMN, potentially mineralizable N • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
NET PLANT PRODUCTIVITY and microbial decomposition are the two primary factors controlling ecosystem and soil C storage. The production of biologically resistant compounds and the protection capacity of the soil will further define the potential for soils to store C. If C sequestration can occur in soil under elevated CO₂, it is important to understand if changes in the rate of inputs can affect the amount and stability of this new C (C_new). On average, elevated CO₂ induced a 60% increase in root growth during an 8-yr study in the tallgrass prairie (Owensby et al., 1999).

Many studies show no effect of elevated CO₂ on soil or total ecosystem C (Oechel et al., 1994; Luo et al., 1996; Hungate et al., 1997; Van Kessel et al., 2000). Other studies report evidence of soil C sequestration (Leavitt et al., 1994; Wood et al., 1994; Williams et al., 2000), or provide evidence that more C is sequestered into other ecosystem components, such as plant biomass (Tissue et al., 1997). Since most of the studies were typically 4 yr or less in duration, the potential for long-term sequestration would be difficult to detect. Furthermore, different ecosystems are limited or controlled by a variety of factors, so that understanding why certain ecosystems respond and others do not will become important for understanding the potential for C sequestration under elevated CO₂.

Most of the C deposited into soil will be decomposed and respired as CO₂, but some of it will eventually become chemically and physically recalcitrant (Paul and Clark, 1996; Stevenson and Cole, 1999). In the three-pool model of SOM, active and microbial C and N are used synonymously as a fraction that is inherently decomposable and more vulnerable to decomposition when environmental conditions, such as temperature and water availability, are favorable. The passive soil fraction is defined as having mean residence times (MRTs) of centuries to millennia; the slow pool is intermediate between the active and passive pools with an MRT measured in decades (Paul, 1984). How the size of different organic matter pools could be altered will provide information on the capacity of soils to store C as a consequence of elevated CO₂.

Soil organic matter can be conceptually defined as a series of fractions that comprise a continuum based on decomposition rate (Sanford and Smith, 1972; Paul and Clark, 1996). Long-term mineralization (250–500 d) studies have been used to estimate the active and slow fraction of C and N in soil (Juma and Paul, 1984) and have shown promise for predicting available plant and crop nutrients in soil (Stanford and Smith, 1972; Rice and Havlin, 1994). Typically the slow fraction is composed of 5 to 40% of the SOM pool, and is relatively easy to define based on kinetic parameters obtained during the incubation (Stanford and Smith, 1972). The C and N left in soil after incubation is considered biologically more recalcitrant. Furthermore, separating the PMC and PMN from the more recalcitrant SOM can increase the power of analysis to detect changes in soil C and N (Cambardella and Elliot, 1992; Leavitt et al., 1996).

Though we do not know the exact ¹³C signature of the CO₂ used to double the CO₂ concentration from year to year, data presented in this paper, and from several other years suggests depleted ¹³C occurred from 1992 through 1995 (J. Jastrow, personal communication, 2001). This ¹³C altered the plant ¹³C signature of the plants, and the subsequent inputs of C to the soil. Since C₄ grasses constitute the majority of net primary productivity (NPP) in tallgrass prairie (Freeman, 1998), the C₃ plant contribution to the ¹³C signature was expected to be minimal. Changes in plant species composition over the 8-yr experiment were minor (Owensby et al., 1999).

Our objectives were to measure changes in the so-called active, slow, and passive pools by measuring microbial, potentially mineralizable, and residual soil C and N to evaluate the stability of new C under elevated CO₂. We expected that the majority of soil C inputs under elevated CO₂ (Williams et al., 2000) would occur in the potentially mineralizable pools of C and N. The ¹³C signal derived from elevated CO₂ would reside primarily in the potentially mineralizable slow fraction, whereas smaller alterations, if detectable, would occur in the passive fraction.

	MATERIALS AND METHODS

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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 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]. Members of the sedge family made up 5 to 10% of the composition. Principal forbs included ironweed (Vernonia baldwinii var. interior (Small) Schub.), western ragweed (Ambrosia psilostachya DC.), Louisiana sagewort (Artemisia 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, superactive, mesic 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 the field in early May 1989. Treatments were ambient CO₂—no chamber (NC), ambient CO₂—chamber (AC), and two times ambient CO₂—chamber (EC). Each treatment was replicated three times using a randomized complete block design. Carbon dioxide and air mixtures were supplied constantly from early April to late October. The overall experimental design is explained in detail by Owensby et al. (1993).

Laboratory Experiments
Soil was collected in August (0 to 5 cm) and October (5 to 15 cm) of 1996. Approximately 20 samples of soil (0- to 5-cm depth) were randomly collected from within the experimental plots using a 1.5-cm diam. soil core. Soil from the 5- to 15-cm depth was derived from 25-cm diam. polyvinyl chloride (PVC) cores inserted into the whole plots in April 1996 and utilized for an isotope tracer study (Williams et al., 2001). Because of this, we will make our comparisons between treatments, and will not compare differences due to depth.

To aid removal of large roots and homogenize the soil, all samples were sieved to pass a 4-mm mesh. Soil samples (25 g) were added to 980-mL mason jars and were analyzed for microbial biomass C using the fumigation–incubation procedure of Jenkinson and Powlson (1976). The mass of microbial biomass C was estimated using a k_c of 0.41, as suggest by Voroney and Paul (1984). Assuming that the fumigated and unfumigated samples represent their respective pools and are homogeneously mixed, we calculated ¹³C of microbial C using the following equation:

[1]

where _Fum represents the ¹³CO₂ of the C respired from a fumigated sample, _Unfum represents the ¹³CO₂ of the C respired from an unfumigated sample, and P_Unfum represents the proportion of C in the unfumigated compared with the fumigated sample. The ¹³C content of the CO₂ of the fumigated and unfumigated controls were measured on a continuous flow Europa 20/20 Isotope Ratio Mass Spectrometer (Europa, Crewe, UK) using a purge needle attached to an autosampler at a flow rate of 80 mL min^–1. The gas sample containing the CO₂ was first passed through a water trap containing Mg(ClO₄)₂, and then further separated and purified using a Porapak Q (Alltech Assoc., Inc., Deerfield, IL) (0.318 cm by 2 m; 80–100 mesh) column (25°C) before introduction into the mass spectrometer. Gas samples were tested periodically for high levels of N₂O using an ⁶³Ni electron capture detector with a similar Porapak Q column. All samples contained from 28 to 80 µg C per 10 mL injection. Two jars without soil were also used to account for background levels of CO₂. Background levels were between 410 to 490 µL CO₂ L^–1. We were not equipped to accurately measure the isotopic signature of these low concentrations by isotope ratio mass spectrometry (IRMS), so we assumed that the ¹³C-Vienna PeeDee Belemnite (VPDB) of the CO₂ was –8. Total analytical precision for the isotopic composition of CO₂ was 0.31 when using a standardized calcium carbonate mixture, which was converted to CO₂ using freshly prepared and degassed 1 M acetic acid. Calcium carbonate was standardized relative to National Bureau of Standards (NBS)-19 in both solid and gaseous forms, and stored with a water trap in an airtight desiccator.

Potentially mineralizable C and PMN incubations lasted up to 422 d, but all data could be fitted to a one-pool model (Stanford and Smith, 1972) by Day 290. Three PVC cores (5.08 cm diam., 10 cm height) per replicate plot were packed with field-moist sieved soil (approximately 100 g d.w.) and packed to a bulk density of 1 g cm^–3. Cores were preleached with eight separate additions of a 50-mL solution of 0.01 M CaCl₂ by placing the cores on a plastic buchner funnel sealed to a side-arm Erlenmeyer flask connected to a vacuum pump (0.033 MPa). The funnel contained a sealed cellulose filter (Millipore Corp., Bedford, MA) with a bubble point pressure of 0.0685 MPa and was filled with an approximately 25-mm thick bed of glass beads (29-µm mean particle size, Potters Industries, Inc., Parisppany, NJ). Approximately 1 h after the last CaCl₂ addition, the solution was weighed and a portion was poured into scintillation vials to prepare for inorganic N (NH⁺₄ and NO^–₃) measurement, then 50 mL of a free solution was added to each core. Calcium chloride extracts were analyzed colorimetrically 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). Carbon dioxide was measured periodically to assess PMC, but also to maintain O₂ levels above 19% in the incubation jars containing the soil. Details of the procedure used to estimate PMC and PMN are outlined in Omay et al. (1997). Respired CO₂ and its ¹³C signature were measured periodically as previously described. Gaps in the ¹³C data (Fig. 1) between 185 and 295 d of incubation were a result of problems with the IRMS. Carbon dioxide and inorganic N data from each replicate core were fitted to the one pool model and then averaged to represent the mean amount of potentially mineralizable C and N for that replicate plot.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Isotope signature of CO2 produced during long-term incubation of soil at the (A.) 0- to 5- and (B.) 5- to 15-cm depths. Standard errors are shown. Each value is the mean of three replications. Note differences in "Days of Incubation" between A and B.

Biologically recalcitrant (passive) C and N were determined by subtracting the mass of PMC and PMN from the total soil C and N, respectively. To estimate if some of the C_new was present into the recalcitrant pools we applied a calculated and a measured method. The measured method simply utilized the ¹³C values of the soil before and after the 290-d incubation. The ¹³CO₂ data was only available for the first 150-d of incubation at the 0- to 5-cm depth. The calculated method was estimated with the following equation:

[2]

for each treatment and depth; _r was the ¹³C value of the recalcitrant soil pool, M_t was the mass of total C, _t is the ¹³C value of the total soil pool, M_i was the mass of CO₂–C produced during each incubation interval, and _i was ¹³C value at each incubation interval. An incubation interval was defined as the number of days between ¹³C measurements (Fig. 1). The mass of C and ¹³C for each interval was taken as the midpoint (or average) between the two data points. It was considered that some of the C_new found in the recalcitrant pool may have been transferred from the potentially mineralizable pool, and this possibility is further communicated in the discussion.

Data were analyzed separately and conjointly by Proc Mixed (SAS Institute, 1996). Assumptions for normality were assessed, and all data presented met normality criteria. The model class statements were replication, treatment, and depth. The least significant difference (LSD) mean separation tests were used to determine where significant differences occurred. All results were considered significantly different at P < 0.10.

	RESULTS

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Potentially mineralizable C was enhanced in EC relative to the average of AC and NC by 19% (0–5 cm) and 24% (5–15 cm). Potentially mineralizable N was significantly enhanced by 14% at the 0- to 5-cm depth in EC relative to AC, but decreased by 12% in EC relative to NC (Table 1). At the 5- to 15-cm depth PMN was not significantly different among treatments.

View this table: [in this window] [in a new window]   Table 2. 13C values for each soil C pool directly after the conclusion of the 8-yr experiment or after 290-d laboratory incubation (35°C). Each value is the mean of three replicates.

During long-term incubation respired ¹³C-CO₂ was slightly depleted in EC relative to AC and NC (0- to 5-cm depth) throughout most of the 150-d incubation (Fig. 1a). At the 5- to 15-cm depth, the ¹³C-CO₂ respired was significantly depleted in EC relative to AC and NC until Day 301 (Fig. 1b). After Day 301, differences in ¹³C among treatments only occurred at Day 377. Before incubation (5- to 15-cm depth), approximately 70% of the new C in the PMC pool could be accounted for by microbial biomass.

Total amount of passive soil C was not significantly different among treatments at either depth (Table 3). Passive soil N was enhanced at the 5- to 15-cm depth in EC compared with AC and NC, but no differences were detected at the 0- to 5-cm depth.

	DISCUSSION

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Microbial activity is related to decomposition rates in soil, and during a typical tallgrass prairie-growing season, greater water contents generally translate into enhanced rates of microbial activity (Williams et al., 2000). Williams et al. (2000) showed a strong positive relationship between greater soil water and microbial activity in elevated compared with ambient conditions. But apparently, increases in plant inputs have outpaced increases in decomposition rates. A reduction in soil temperature is a factor that might contribute to C accrual in the PMC fraction.

No change in potentially mineralizable N was found at the 5- to 15-cm depth, but both elevated CO₂ and chambers significantly influenced PMN stocks in the surface 5-cm. This result suggests that chambers cause a reduction in PMN at the 0- to 5-cm depth, while elevated CO₂ can partially offset this loss (Table 1). Yet, CO₂ enrichment resulted in greater recalcitrant N but not greater PMN at the 5- to 15-cm depth, suggesting movement of N from the slow pool to a less easily mineralizable passive form. It is at the 5- to 15-cm depth that we see differences due to CO₂ enrichment in both total and PMC.

From the perspective of potential ecosystem N losses, typical annual rates (Blair et al., 1998; Williams, 1998; Sotomayor, 1996) of denitrification, associative N-fixation, and NH₃ volatilization from tallgrass prairie all range between 1 and 2 Kg N ha^–1 yr^–1. The difference in N content between EC and AC at the 5- to 15-cm depth was approximately 200 Kg N ha^–1. Only a small proportion of the greater soil N can be accounted for by changes in ecosystem N flux. Relatively large increases in N₂–fixation synchronized with dramatic reductions in denitrification and NH₃ volatilization can at best, account for 10% of the greater soil N derived from CO₂ enrichment under rates estimated under ambient conditions. It is unknown what effect enhanced C could have on these processes, although N fixation would be the most likely process affected by enhanced C availability.

If flux rates cannot account for the altered N pools, internal cycling within the prairie ecosystem might explain the dramatic change in soil N. Greater N stocks due to CO₂ enrichment might be partly explained by the high degree of N limitation in the tallgrass prairie, and mechanisms that prairie plants utilize to cope with low levels of available N. Williams et al. (2001) conducted an experiment to test whether plants and soil microorganisms altered their stocks of an added ¹⁵N tracer as a result of CO₂ enrichment. That study suggested that plant-microbial competition was enhanced because of CO₂ enrichment, and that microorganisms may have reduced plant available N in the soil surface. As a result of this reduction in available N, plants may alter root architecture that favors subsurface root growth and exploration for N in elevated compared with ambient CO₂ (Owensby et al., 1999). Greater N translocated to aboveground biomass from deeper soil depths could ultimately be stored in roots and rhizomes near the soil surface, and would eventually turnover and become a part of the surface soil N pool. Up to 40 Kg N ha^–1 yr^–1 is mineralized from organic sources in the top 15-cm of soil during a typical growing season (Blair, 1997) and plants assimilate a similar amount of N.

With the available data, we can provide only indirect explanations of how CO₂ enrichment could foster an increase in soil N. The high degree of N limitation inherent in tallgrass prairie (Owensby et al., 1999) could be further exacerbated because of greater microbial N immobilization. Greater immobilization is driven by the increased root C inputs, and suggests that C availability appears to be driving the dynamics of soil N.

Though we cannot make quantitative assessments of C flow from roots to soil pools, a qualitative assessment indicates that relatively depleted ¹³C in the whole soil of the elevated CO₂ treatment implies the presence of recently (<8yr) deposited new soil C (C_new). Relatively depleted CO₂ was supplied through a portion of the experiment as indicated by clippings of green leaf tissue at various intervals throughout the experiment. In elevated CO₂, root biomass collected during 1996 was depleted 7 to 8 compared with ambient conditions. Because root inputs provide a large portion of C supplied to soil microorganisms, it was expected that microbial biomass ¹³C values should closely resemble those found in roots. This was the case in the surface 5-cm of soil and for the two ambient treatments at the 5- to 15-cm depth. Relatively undepleted microbial ¹³C found in the elevated CO₂ treatment at the 5- to 15-cm depth suggests either slow turnover of C derived from roots or in a consistent source of ¹³C throughout the experiment. Some of the microbial C could be derived from older soil C, but the large degree of difference between the ¹³C of roots and microbial biomass would suggest that older soil C sources were not the primary contributor to the ¹³C of the microbial biomass. That is, for our value of microbial ¹³C, it would take a large amount of C flow from older more resistant soil pools in addition to a reduction in C flow from roots to the microbial biomass.

The difference in ¹³C of the CO₂ evolved from the PMC fraction was relatively consistent throughout the 290-d incubation. Yet, the CO₂ evolved during the incubation only differed in the elevated compared with the ambient CO₂–chambers by 1.5 to 2.5 at both the 0- to 5- (data not shown), and 5- to 15- cm depths (Fig. 1). This suggests either variation in ¹³C label during the 8-yr experiment or mineralization of older C sources during the laboratory incubation. During sample preparation for incubation the soil was sieved and homogenized, which may provide microorganisms with access to physically protected older soil C and thus relatively enriched ¹³C.

We attempted to measure a passive fraction of soil ¹³C by subtraction of PMC from the whole soil using a calculated (Eq. [2]) and measured methodology (Table 2). Though both sets of data closely resemble one another, only the calculated data indicate that a significant amount of C_new resides in the passive fraction. Both of these two estimates nevertheless indicate that most of the new C resides in the passive fraction of the whole soil. Assuming that our methods truly represent various SOM fractions, the amount of new C residing in the passive relative to the slow fraction appears considerably higher than expected for a pool with a MRT of centuries to millennia. This is particularly true given the 7 to 8 difference in ¹³C of the new root input measured during the final year of the experiment. The most likely mechanisms for this new C incorporation into the passive soil fraction is derived from a relatively high rate of turnover and C flow from plant roots in situ, redistribution and occlusion during sieving, or a combination of the two influences. In contrast with the isotopic data, measurement of PMC suggest that most of the increase in C due to CO₂ enrichment resides in the slow not the passive SOM pool.

The 60% increase in root growth during the 8-yr study was the likely catalyst for the greater potentially mineralizable soil C pools in the enriched CO₂ treatment. Though PMC is an active fraction and its size is prone to changes in environmental conditions, it confirms that C can accrue in soils under elevated CO₂. Our study suggests that C accrual into the relatively slow pool is possible due to elevated CO₂. We can only hypothesize the mechanism of greater soil N that occurs with CO₂ enrichment, and with current estimates of ecosystem N flux, we can explain just a small proportion the increase in soil N. Nitrogen accrual might be best explained through a mechanism of redistribution whereby soil N in roots is moved from the subsurface to surface because of enhanced N plant demand and enhanced associative N fixation. The increase in size of various soil N pools does indicate that N dynamics may be fundamentally altered in concert with, or directly due to C accrual. The total amount of new C increased under elevated CO₂ was 4 Mg C ha^–1 over the 8-yr period for an annual rate of change of 0.5 Mg ha^–1 yr^–1.

Received for publication July 9, 2001.

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