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a Formerly at Natural Resource Ecology & Management Dep., Iowa State Univ
b Currently at Virginia Tech, College of Natural Resources, 324 Cheatham Hall, Blacksburg, VA 24061
c Tennessee Valley Authority, Knoxville, TN. 37902-1499
* Corresponding author (jmkelly{at}vt.edu)
ABSTRACT
Concerns about global warming and discussions of possible mitigation measures have generated a need for information on changes in soil C over time. The objective of this study was to determine if there was a change in soil C concentration in an aggrading oak forest over a 26-yr interval. Using permanently identified points on the Camp Branch Experimental Watershed, soil samples were first collected in July of 1976 and archived. During July of 2002, 11 points covering 6 of the 18 soil series present on the watershed were resampled. The series chosen represent a range in topographic positions and forest cover types. In both 1976 and 2002, a bucket auger was used to collect samples at depth intervals of 0 to 10, 10 to 30, and 30 to 50 cm. Both sample sets were analyzed in 2002 using a loss on combustion technique to determine organic C concentration. A bootstrapping data analysis indicated an increase (95% confidence interval) in the concentration of C in the 0- to 10-cm depth. No change in C concentration occurred in the 10- to 30- or 30- to 50-cm samples. Average soil C concentration in the 0- to 10-cm samples increased from a mean of 20.8 g kg–1 in 1976 to 35.9 g kg–1 in 2002. Among soil series, concentrations ranged from 9.5 to 28.9 g kg–1 in 1976 and 22.1 to 64.7 g kg–1 in 2002. Although the sample numbers are limited, results indicate that average soil C concentration in the top 10 cm of the mineral soil increased by 73% at this site over a 26-yr interval.
THERE IS A GROWING NEED to increase our understanding of the amounts and patterns of C storage in soils. This need is driven in part by concerns about global warming and the role that forest soils may play in the long-term accumulation and sequestration of atmospheric C (Guo and Gifford, 2002). Since a large portion of terrestrial C stocks are in the soil, particularly in forest soils (Post et al., 1990), answering many of the science questions being considered in this context is highly dependent on understanding long-term changes in forest soil C. Attempts to quantify change using traditional soil sampling approaches have been inconclusive in part due to differences in analytical techniques and the heterogeneous nature of forest soil C distribution. Gaudinski and Trumbore (2003) suggest that biometric methods that calculate C accumulation through repeated measures over extended time intervals might help to address these limitations. While Yanai and colleagues (2003) concluded that detecting change in forest floor C was best accomplished through the use of paired designs and that the chance of detecting a significant change was enhanced by disturbance and the passage of time. Similarly, Richter et al. (1999) suggest that a paired sampling approach in a relatively stone free mineral soil might have a good chance of detecting change, especially if the interval between samples is on the order of decades.
The objective of our study was to determine if mineral soil C concentration changed over 26 yr in the oak dominated forest of the Camp Branch Watershed. Camp Branch was the site of a series of nutrient cycling studies during the period 1975 to 1990, the results of which have been summarized in Kelly (1984)(1988), Johnson et al., (1986)(1988), Kelly and Meagher (1986), and Cronan et al. (1990).
Materials and Methods
Site Description
Soil samples used for this study were collected from the Camp Branch Experimental Watershed (35°38' N lat.; 85°18' W long.), which is located within the boundaries of Fall Creek Falls State Park on the Cumberland Plateau in Central Tennessee. The upland vegetation of the central Cumberland Plateau, once dominated by white oak (Quercus alba L.), has been replaced by less productive second growth stands dominated by scarlet oak (Q. coccinea L.), chestnut oak (Q. prinus L.), white oak, and post oak (Q. stellata L.) mixed with Virginia pine (Pinus virginiana L.), shortleaf pine (P. echinata L.) and red maple (Acer rubrum L.) (Ramseur and Kelly, 1981). Past disturbances associated with agriculture (primarily woodland grazing) and timber harvest have contributed to the diversity of species and age classes found within the current forest. However, there have been no known disturbances on the watershed since the Park was established in the late 1930s.
Soil Sampling and Analysis
In 1976, a 100 x 100 m sampling grid was established across the 94 ha in the watershed and each grid intersection marked with a steel fence post. In July of 1976 samples of the mineral soil were collected 1-m south of each grid intersection using a 9-cm wide by 22-cm long stainless steel bucket auger. After removing the litter layer, mineral soil samples were collected from the following depth intervals: 0 to 10, 10 to 30, and 30 to 50 cm. Samples were oven dried at 105°C for 48 h, passed through a 5-mm sieve to remove roots, and then ground with a mortar and pestle to pass a 2-mm sieve. A 250-g reference subsample was taken from the sample collected at each depth–location combination, sealed in a glass jar, and stored in cardboard boxes in a building subject to normal fluctuations in ambient temperature.
During July of 2002, samples were collected from 11 of the permanently marked locations sampled in 1976. Sampling sites were chosen to provide a broad range of soil series, topographic positions, and forest cover types (Table 1). New soil samples were collected 1 m southeast of the grid intersection and were prepared for analysis using the same procedures used in 1976. Samples from both 1976 and 2002 were oven dried before being analyzed in duplicate for soil organic C concentration using a Model NA 1500 N/C/S Analyzer (Carlo-Erba Strumatazione, Milan, Italy). A total of 64 samples were analyzed (2 yr x 11 locations x 3 depths for all locations except Wallen-Ramsey only two depths) in duplicate. Variation between duplicates across all samples was quite low with an average standard error of only 6.2% of the mean. No charcoal was observed in any of the samples and it is assumed that carbonates were not present in these highly leached sandstone based soils.
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Results and Discussion
The average soil C concentration based on the raw data from all 0- to 10-cm samples increased by 73% (15.1 g kg–1) during the 26-yr period between 1976 and 2002 (Table 2). The observed values for individual series ranged in magnitude from a low of 24% for the Clarkrange series to a high of 311% for the Wallen–Ramsey complex (Table 2). Results from the bootstrap analysis of the data indicate this increase in the C concentration across all samples in the 0- to 10-cm depth (Table 3) was significant. The bootstrap analysis indicated no change in B horizon concentrations at either the 10- to 30- or 30- to 50-cm depths (Table 3).
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Results from previous evaluations of changes in mineral soil organic C concentration as a function of time in deciduous forest stands in the southeastern USA have reported increases and decreases as well as no change. For example, Knoepp and Swank (1997) found a 21 and 41% decrease, respectively, in A and B horizon soil C concentration when beginning and ending values are compared over a 17-yr interval on a south-facing site on the Coweeta Watershed. However, it is noteworthy that A horizon observations made at various intervals over the 17-yr Coweeta study suggest a pattern of increasing C concentration during the interval from 1977 through 1983 and then a decline through 1994. If the 1983 high point is used to calculate change, concentration increased by 30% during the first 5 yr and then declined by 45% in the subsequent 11 yr. Carbon concentration in the A horizon of a north facing soil, also from Coweeta, declined by 37% during a 20-yr interval (1970–1990) between samples, while the B horizon values remained essentially unchanged (Knoepp and Swank, 1997). Trettin et al. (1999) resampled eight soils differentiated on the basis of soil parent material or topographic position on two occasions at Walker Branch. However, no change in soil C concentration was observed after intervals of either 10 or 21 yr.
In the context of the regional studies just discussed, the Camp Branch 0- to 10-cm interval changes are substantial increases. However, Richter et al. (1999), working on a sandy coastal plain soil in South Carolina found a C increase of approximately 40% in the top 7.5 cm of the mineral soil over 29 yr (1968–1997). While surface mineral soil C concentration in this aggrading South Carolina pine plantation increased, there was no change at other depths. The latter is consistent with observations on the Camp Branch soils. We have no reason to doubt the results of our analysis unless there is some unknown artifact associated with the re-analysis of archived samples. While our sample size is small, the variances around the means for individual soil series where we had more than one sample are quite low due to our ability to accurately return to the same reference points used for the 1976 samples.
Nevertheless, certain questions remain. For example, Walker Branch and Camp Branch have similar land use histories and have been free of external disturbance for approximately the same 60- to 70-yr time period. However, Walker Branch, which has a higher level of standing biomass, 234 vs. 118 Mg ha–1 at Camp Branch, (Johnson et al., 1988) exhibited no change in mineral soil C. One possible mechanistic explanation for this difference, although highly speculative, is the difference in N deposition between the two sites and the role this may play in soil C to N ratios. Berg (2000) and Berg et al. (2001) report that lowering the C to N ratio via N deposition leads to more humus formation and thus slows the loss of C from the soil. An earlier study of N inputs at the Camp Branch and Walker Branch sites found net N retention at Camp Branch exceeded that at Walker Branch by slightly more than 8% per year (Kelly and Meagher, 1986). Whether the cumulative effect of this difference would be enough to tip the balance in favor of C accumulation is unknown. Unfortunately, the negative effect of oven drying soil at 105°C on soil N estimates (Bremner, 1996) precludes a valid C to N ratio calculation from our samples. It is also worthwhile to note that visual observations at the time of the 2002 sampling suggest that there has been a substantial increase in the shrub layer across the Camp Branch watershed. This additional source of soil C through the mortality of ephemeral roots and easily decomposable leaf litter may also play a role in increasing the C level in the surface mineral soil. Also in simplest terms, it may be possible that the Camp Branch forest is still aggrading. In summary, even though sample numbers are not sufficient to make unequivocal statements concerning C accumulation, the results are certainly provocative and suggest a substantial capacity for C accumulation in forest soils on sites with a similar climate and land use history.
ACKNOWLEDGMENTS
Support for this project was provided by the Tennessee Valley Authority and the Iowa State University Agriculture and Home Economics Experiment Station, Ames, IA. Project No. 3905 and supported in part by McIntire-Stennis and State of Iowa funds. The authors express their appreciation to Cory Heilmann and Phil Dixon for assistance with statistical analysis.
Received for publication July 14, 2004.
REFERENCES
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