International Journal of AgronomyVolume 2012 (2012), Article ID 817343, 5 pagesdoi:10.1155/2012/817343

Research Article

Nitrogen and Carbon Cycling in a Grassland Community Ecosystem as Affected by Elevated Atmospheric

H. A. Torbert,¹ H. W. Polley,² and H. B. Johnson²

¹National Soil Dynamics Laboratory, USDA Agricultural Research Service, 411 S. Donahue, Auburn, AL 36832, USA²Grassland Soil and Water Research Laboratory, USDA Agricultural Research Service, 808 E Blackland Road, Temple, TX 76502, USA

Received 4 January 2012; Accepted 7 February 2012

Academic Editor: Dexter B. Watts

Copyright © 2012 H. A. Torbert et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Increasing global atmospheric carbon dioxide (CO₂) concentration has led to concerns regarding its potential effects on terrestrial ecosystems and the long-term storage of carbon (C) and nitrogen (N) in soil. This study examined responses to elevated CO₂ in a grass ecosystem invaded with a leguminous shrub Acacia farnesiana (L.) Willd (Huisache). Seedlings of Acacia along with grass species were grown for 13 months at CO₂ concentrations of 385 (ambient), 690, and 980 μmol mol⁻¹. Elevated CO₂ increased both C and N inputs from plant growth which would result in higher soil C from litter fall, root turnover, and excretions. Results from the incubation indicated an initial (20 days) decrease in N mineralization which resulted in no change in C mineralization. However, after 40 and 60 days, an increase in both C and N mineralization was observed. These increases would indicate that increases in soil C storage may not occur in grass ecosystems that are invaded with Acacia over the long term.

1. Introduction

The rise of CO₂ in the atmosphere is well documented [1]; what has not been documented are the sinks for this C, with an estimated unknown sink of  g C yr⁻¹ arising from the global C balance [2]. Carbon dioxide is a prime chemical input to the metabolism of higher plants and has a major role in governing plant-water relations and water use efficiency. The increased growth of most plants under higher levels of CO₂ [3–6] has prompted recent speculation on the ability of terrestrial ecosystems to sequester C [7]. However, the fate of C within ecosystems is affected by a biological chain of events which includes competition between plants. The ability of terrestrial ecosystems to sequester C will depend on the cycling of C among the various biomass and soil C pools and on the residence time of C in these pools.

The rate of C mineralization during decomposition of residue derived from plants grown under elevated CO₂ has not been resolved. It has been theorized that the commonly observed increase in plant C : N ratio under elevated CO₂ could lead to slower residue decomposition resulting in increased soil C storage and reduction in available N for plant production [8]. However, slower decomposition of leaf litter due to elevated CO₂ is not supported by the literature on litter quality [9]. Others have suggested that increased biomass might enhance microbial activity, resulting in a “priming effect” thereby leading to no increase in C storage [10]. Alternatively, microbial preference for easily decomposable plant material produced under CO₂-enriched conditions could reduce the turnover of more resistant organic material, thereby increasing soil C [11, 12]. Observations from field and laboratory studies indicate that with elevated atmospheric CO₂, N may limit the rate of plant residue decomposition and slow the release of N from decomposing plant material [13]. This indicates that understanding N cycling as affected by elevated CO₂ is fundamental to understanding the potential for soil C storage on a global scale.

It has also been speculated that changes in elevated CO₂ could impact the competitiveness between plant species, especially between native and invasive plant species. A study by Runion et al. [14] indicated that elevated atmospheric CO₂ increased biomass production in a longleaf ecosystem compared to ambient CO₂ conditions. However, close examination of biomass data for each species in this plant community indicated that only longleaf pine showed a significant positive response to elevated atmospheric CO₂ treatment. In grasslands, invasion by woody legumes such as Acacia can alter hydrology, nutrient accumulation and cycling, and C sequestration. The rate and magnitude of these changes are likely to be sensitive to the effects of atmospheric CO₂ enrichment on growth and water and N dynamics of leguminous shrubs. Polley et al. [15] indicated that, at the highest CO₂ concentration studied, biomass production of grass-tree communities increased more than 2.5-fold as a result of increased leaf photosynthetic rates, leaf area, and N₂ fixation.

Previous research, considering the effect of elevated CO₂ on the decomposition of individual plant parts and monoculture plant systems has indicated that increased soil C storage could occur [16, 17]. However, these past studies did not consider the impact of increased biomass input and the changes in soil brought about by differing responses of plant species. The objective of this study was to determine the impact of atmospheric CO₂ enrichment on potential soil C and N mineralization in a grassland community.

2. Materials and Methods

A study on the effects of elevated CO₂ concentrations on the productivity, water use, and N dynamics of the grassland invader Acacia farnesiana (L.) Willd (Huisache) was conducted by Polley et al. [15]. To study the impact of atmospheric CO₂ enrichment on potential soil C and N mineralization within a grassland community, an incubation study was conducted on soil collected from this study. Descriptions of the study site and the model ecosystem have been previously reported [15]. Briefly, seeds of Acacia were planted in fine sandy loam soil (Udic Paleustalfs [18], in wheeled, 380-liter containers (0.9 m deep and 0.65 m on each side)). Three Acacia plants were established in each container.

Three individuals each of the C₃ grass Stipa leucotricha Trin. and Rupr. (Texas winter grass) and the C₄ grass Schizachyrium scoparium (Michx.) Nash. (little bluestem) were also established from tillers in each container to provide a competitive environment for Acacia growth. Three 380-liter containers of the species mix were maintained for 13 months at average CO₂ concentrations of 385 (ambient), 690, or 980 μmol mol⁻¹ in air-conditioned greenhouse bays.

Carbon dioxide gas was injected into the greenhouse bays as necessary to maintain the desired concentrations. The CO₂ concentration and dewpoint temperature of air in each bay were measured at 4 min intervals with an infrared gas analyzer (Model 6262, Li-Cor, Inc., Lincoln, NE). The CO₂ readings were corrected for atmospheric pressure using a pressure indicator (Model DPI 260, Druck Inc., New Fairfield, CT). The CO₂ concentration of air averaged 385, 690, and 980 μmol mol⁻¹ for the three CO₂ treatments. Air temperature was measured with 25 mm diameter thermocouples, and bay temperature was changed seasonally to approximate outdoor temperature by manually adjusting thermostatic controls.

At the conclusion of the study (13 months), soil samples were collected from 0–20, 20–50, and 50–80 cm depth increments for use in the incubation study. Soil subsamples were dried (55°C) and ground to pass a 0.15 mm sieve. Soil inorganic N (NO₂-N + NO₃-N and NH₄-N) was extracted with 2 M KCl and measured by standard colorimetric procedures using a Technicon Autoanalyzer 3 (Bran + Luebbe, Buffalo Grove, IL). Soil subsamples were weighed (25 g dry weight basis) and placed in plastic containers. Deionized water was added to adjust soil water content to an equivalent of −20 kPa at a bulk density of 1.3 mg m⁻³. The containers were placed in sealed glass mason jars with 10 mL of water for humidity control; each jar also had a vial containing 10 mL of 1 M NaOH which functioned as a CO₂ trap. The jars were incubated in the dark at 25°C and removed after 20 and 60 days for inorganic N determination. Carbon dioxide in the NaOH traps was determined by titrating the excess base with 1 M HCl in the presence of BaCl₂ after 20, 40, and 60 days. Potential C mineralization was the difference between CO2 -C captured in sample traps and in blanks. Potential N mineralization was the difference between final and initial inorganic N contents. The C mineralization divided by total C (initial soil C concentration) was used to calculate C turnover. Statistical analyses were performed using the mixed procedure of SAS [19], and means were separated using least significant difference at an a priori 0.10 probability level.

3. Results and Discussion

Increasing atmospheric CO₂ concentration to 980 μmol mol⁻¹ increased biomass production of the woody legume Acacia more than 2.5-fold during the initial year of growth due to increased leaf photosynthetic rates, leaf area, and N₂ fixation [15]. The increase in biomass at elevated CO₂ required no more water than was consumed by the shrubs grown near ambient CO₂ concentration. As a result, apparent water use efficiency and biomass production increased by similar relative amounts. Elevated CO₂ greatly increased N₂ fixation by Acacia, with the total N fixed per plant at the highest CO₂ concentration being 1.5 times greater compared to ambient CO₂ conditions. This positive effect of CO₂ on N₂ fixation by Acacia during its initial year of growth is generally consistent with that reported for woody legumes [20, 21]. Nitrogen fixation promoted by elevated CO₂ would be expected to provide additional N to a grassland ecosystem due to the N addition to the soil following litter fall, root turnover, and exudation. These are important factors by which increasing atmospheric CO₂ concentration can modify the impact of woody legumes on grasslands. Studies have shown that plant matter grown under elevated CO₂ can greatly impact N mineralization [16, 17]. This incubation study was conducted to determine the impact of the elevated CO₂ concentration on soil N mineralization in a grassland system.

After 20 days of incubation of the 0–20 cm depth soil, N mineralization was significantly reduced by elevated CO₂, with N mineralization being 95% higher at ambient CO₂ compared to the 980 μmol mol⁻¹ treatment level (Table 1). During this same measuring period, soil C mineralization and C turnover were not significantly impacted. This indicated that changes under elevated CO₂ initially reduced N mineralization rates. While greater plant inputs were entering the system, with greater total N from the N₂ fixation, the quality of the plant residues were such that N mineralization was being limited. This reduction in inorganic N in soil solution in turn limited C mineralization, resulting in no difference in C respiration or C turnover at the 20-day sampling period (Tables 2 and 3). This is consistent with studies by Torbert et al. [17] which indicated that the impact of elevated atmospheric CO₂ concentration on plant residue quality may be more important than the impact on plant residue quantity in determining C cycling in soil. They observed that the potential effect of elevated atmospheric CO₂ concentration on C storage in agroecosystems will be dependent on the crop species grown, where N cycling within the plant/soil system would likely be the controlling factor for C storage in these systems. Likewise, these results agree with work from a free-air CO₂ enrichment (FACE) study utilizing cotton (Gossypium hirsutum L.) [22] and wheat (Triticum aestivum L.) [23], which provided evidence for increased soil C storage even though differences between the two species (cotton and wheat) were noted.

Table 1: Effect of atmospheric CO₂ concentration on soil N mineralization during a 60-day soil incubation at three soil depth increments†.

CO₂ level
0–20 days
0–60 days

Depth

0–20 cm
20–50 cm
50–80 cm
0–20 cm
20–50 cm
50–80 cm

(μmol mol⁻¹)
(mg kg⁻¹)

385
6.86 a
1.91 a
1.37 a
7.51 a
4.01 a
4.54 a

690
2.91 b
0.59 a
0.69 a
7.85 b
3.86 a
4.89 a

980
3.52 c
2.91 a
0.46 a
10.04 c
3.30 a
3.89 a

†Values represent means of three replications. Means within a column for each sampling date followed by the same letter do not differ significantly ().

Table 2: Effect of atmospheric CO₂ concentration on soil C mineralization during a 60-day soil incubation at three soil depth increments†.

CO₂ level
0–20 days
20–40 days
40–60 days

Depth

0–20 cm
20–50 cm
50–80 cm
0–20 cm
20–50 cm
50–80 cm
0–20 cm
20–50 cm
50–80 cm

mol mol⁻¹)
(mg kg⁻¹)

385
57.52 a
63.94 a
36.84 a
59.15 a
67.15 a
59.44 a
210.76 a
263.46 a
229.40 a

690
92.86 a
41.98 a
66.94 a
100.55 b
55.80 a
72.29 a
199.20 b
217.40 a
246.32 a

980
87.93 a
70.15 a
34.26 a
124.98 c
42.52 a
69.40 a
417.46 c
169.43 a
256.82 a

†Values represent means of three replications. Means within a column for each sampling date followed by the same letter do not differ significantly ().

Table 3: Effect of atmospheric CO₂ concentration on soil C turnover during a 60-day soil incubation at three soil depth increments†.

CO₂ level
0–20 days
20–40 days
40–60 days

Depth

0–20 cm
20–50 cm
50–80 cm
0–20 cm
20–50 cm
50–80 cm
0–20 cm
20–50 cm
50–80 cm

(μmol mol⁻¹)
(mg kg⁻¹)

385
1.01 a
1.12 a
0.65 a
0.85 a
1.18 a
1.04 a
3.70 a
4.62 a
4.02 a

690
1.63 a
0.74 a
1.17 a
1.75 b
0.98 a
1.27 a
3.49 b
3.81 a
4.32 a

980
1.54 a
1.23 a
5.34 a
2.19 c
0.75 a
1.22 a
2.19 c
2.97 a
4.51 a

†Values represent means of three replications. Means within a column for each sampling date followed by the same letter do not differ significantly ().

However, unlike the agroecosystem studies, limitations to N mineralization did not persist in this grass ecosystem with Acacia. At the 40-day sampling period, a significant increase was observed with C mineralization as atmospheric CO₂ was increased (Table 2). Likewise, an increase in the level of C turnover was observed with increasing atmospheric CO₂ levels. At the 60-day measurement period, C mineralization was also significantly increased at the 980 μmol mol⁻¹ compared to the ambient level as was C turnover (Tables 2 and 3). The C mineralization for the 980 μmol mol⁻¹ treatment was 98% higher than for the ambient treatment. At this sampling period, unlike the 20-day period, a significant increase in N mineralization was also observed (Table 1). This indicated that after 60 days, N was no longer limiting microbial decomposition of plant material. Since increased biomass inputs into the soil system were observed in this study [15] and the limitations due to N were no longer impacting the decomposition processes, C mineralization also increased.

No significant differences were observed for N mineralization or C mineralization at the 20–50 cm depth or the 50–80 cm depth (Tables 1 and 3). This indicated that most changes in the soil system can be expected to occur in the top 20 cm of the soil profile. This is the portion of the soil that would be most impacted from litter fall and could result in movement of N to grass species from N₂ fixation by the Acacia. While this study included a deep rooted species in the grass ecosystem, the greatest changes in C and N cycling occurred in the top of the soil profile where most plant rooting occurs.

It has been theorized that a reduction in N availability in ecosystems could reduce plant response to elevated CO₂ conditions [24–26]. The results from this incubation study indicate that N mineralization was increased and therefore the potential for N limitations to reduce plant response to elevated CO₂ in this grass ecosystem might not occur. This would indicate that over the long term in a grass ecosystem invaded with woody legumes such as Acacia, N resources should not be limiting for either microbial decomposition of residues or plant growth in future elevated atmospheric CO₂ conditions compared to today’s conditions.

In a study to examine responses to elevated CO₂ in a typical regenerating longleaf pine-wiregrass community, Torbert et al. [27] reported, as in this study with mixed plant species, an increase in N mineralization with elevated CO₂. At least over the short term in a regenerating longleaf pine system, N resources should not be limiting for either microbial decomposition of residues or plant growth in future elevated atmospheric CO₂ conditions compared to today’s conditions. However, elevated CO₂ decreased soil C respiration and C turnover in the regenerating longleaf pine-wiregrass community, indicating long-term C sequestration would be likely. In the current study with grass species, an actual increase in soil C mineralization was observed with elevated CO₂. This was likely due to the large input of N due to increase in N₂ fixation by the Acacia into the plant/soil system. Polley et al. [15] reported total N accretion by Acacia increased with each increase in CO₂ concentration: 55% from 385 to 690 μmol mol⁻¹ CO₂, and another 29% from 690 to 980 μmol mol⁻¹ CO₂. The increased C mineralization indicated that soil C levels in the elevated CO₂ are being reduced compared to the ambient treatments.

These findings are in contrast to other research which reported that C sequestration is likely with increasing atmospheric CO₂. Low-N availability of soil frequently limits production on grasslands [23, 24] and may also limit the response of net primary production to changes in CO₂ concentration. However, with the Acacia included in the grass ecosystem, CO₂ enrichment caused large N inputs into the soil system and large increases in biomass (a 2.5-fold increase during the initial year of growth, [15]). Although C inputs to soil increased, C mineralization also increased. The results from this short-term study indicate that CO₂ may not increase soil C storage in this grassland ecosystem. Thus, long-term studies with elevated CO₂ will be necessary to clearly determine the impact of Acacia invasion in this grassland ecosystems on soil C storage.

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