Published in Soil Sci. Soc. Am. J. 67:1594-1601 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SYMPOSIUM
Changes in Ecosystem Carbon and Nitrogen in a Loblolly Pine Plantation over the First 18 Years
D. W. Johnson*,a,
D. E. Todd, Jr.b and
V. R. Tolbertb
a Environmental and Resource Sciences, Fleischmann Agriculture Bldg/370, University of Nevada, Reno, NV 89557
b Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Building 1059, Oak Ridge, TN 37831-6422
* Corresponding author (dwj{at}cabnr.unr.edu).
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ABSTRACT
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Eighteen years after the establishment of a loblolly pine (Pinus taeda L.) plantation, ecosystem C content had approximately tripled (from 54 to 161 Mg C ha-1) primarily because of increases in tree biomass. Ninety-three percent of the net ecosystem C accumulated in biomass (100 Mg C ha-1) and 6% of net ecosystem C accumulated in the forest floor (13 Mg C ha-1). No statistically significant changes in soil C were found. Growth responses to fertilization noted in Year 4 were no longer statistically significant in Year 18. Nitrogen accumulation in aboveground biomass and forest floor were approximately equal (averaging approximately 270 kg N ha-1 each) and could have come from a combination of atmospheric deposition, soil N mineralization, and, in the treated plots, fertilizer input. No statistically significant changes in soil N content were found. The results of this study are similar to those from a previous study in a loblolly pine plantation in South Carolina but contrast with those in nearby deciduous forests where substantial changes in soil C and N over similar time periods have been noted.
Abbreviations: DBH, diameter at breast height • GLM, General Linear Model • NERP, National Experimental Research Park • WBC, Walkley-Black C
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INTRODUCTION
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ON A GLOBAL SCALE, soils contain two to three times as much C as either terrestrial vegetation or the atmosphere, and fluxes into and out of soils exceed those of fossil fuel emissions by an order of magnitude (Post et al., 1990; Schimel 1995; Schlesinger, 1997). Thus, it would seem that soils could provide a substantial sink for C, offsetting fossil fuel CO2 emissions. The potential for C sequestration in soils is controversial; however, some scientists see great potential for soil C sequestration with proper management (e.g., Lal et al., 1998) whereas others are skeptical (Post and Kwon, 2000; Schlesinger, 1990, 1997). The field evidence for soil C change has produced conflicting results. Conversion of forest or grassland to agriculture is known to cause large losses in soil C (Mann, 1986; Post and Kwon, 2000) that could presumably re-accumulate if the sites are allowed to revert to native vegetation. In some cases, large re-accumulations of C in soils have been noted following reversion of agricultural soils to forest (e.g., Jenkinson, 1991), but in other cases, little or no re-accumulation has been found (e.g., Compton and Boone, 2000; Compton et al., 2000; Richter et al., 1999). In an extensive literature review, Post and Kwon (2000) found large variations in both the rates and the duration of soil C accumulation after agricultural abandonment (including both positive and negative changes). They noted that the average values of soil C re-accumulation in their review (0.33–0.34 Mg C ha-1 yr-1) was very similar to rates previously calculated by Schlesinger (1990), who contended that the potential for C accumulation on a global scale is low. A recent literature review by Johnson and Curtis (2001) indicated that forest harvesting followed by reforestation caused little or no change in soil C on average, regardless of the intensity of harvest. Considerable variation in responses of individual sites was noted however, ranging from large net losses (e.g., Turner and Lambert, 2000) to large net gains (Johnson and Todd, 1998), even over relatively short time periods (Knoepp and Swank, 1997).
Thus, it seems that the potential for soil C change is large in certain sites and under certain conditions, yet the available data (which was certainly not randomly collected) indicate that average soil C changes based on the literature are small thus far. Clearly, the database on soil C change needs to be expanded. Within the Oak Ridge National Environmental Research Park (NERP), Tennessee, we have had the opportunity to investigate changes in soil and ecosystem C and N in deciduous forest stands within a relatively small geographic area (2–4 km apart) with similar climate and soils (Ultisols derived from dolomite). Studies in mature deciduous forests on Walker Branch Watershed over a 21-yr period (1972–1993) showed either stable or declining soil C and N (Trettin et al., 1999). In contrast, soils in a nearby naturally regenerating deciduous forest showed inexplicably large increases in soil C and N over a15-yr period (1980–1995) following harvesting (Johnson and Todd, 1998). In this paper, we add to this database on soil change and report the results of an 18-yr (1982–2000) resampling of a loblolly pine plantation located within 2 to 3 km of the previous two studies. As in the harvesting study, native vegetation was cleared, but in this case, the site was planted with loblolly pine instead of being allowed to naturally regenerate to deciduous species. Based on the results of the harvesting study, we hypothesized that soils in this loblolly pine plantation would accumulate both C and N. Alternatively, if species is a more important factor than location, we might have expected to see little or no change in soil C and N, as in the study of Richter et al. (1999) in South Carolina.
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MATERIALS AND METHODS
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The site is on the Oak Ridge NERP near Oak Ridge, TN. Soils at the site are the Armuchee series, fine, mixed, semiactive, thermic Inceptic Hapludults derived from mixed parent materials of dolomite and shale, and are highly eroded. Before the establishment of the plantation, the predominant vegetation at the site consisted of secondary hardwood-pine scrub forest resulting from agricultural abandonment. After the harvest of all salvageable timber and pulpwood in 1981, the slash was windrowed and the soil was disked and machine planted with loblolly pine seedlings on a 2 by 2 m spacing. After planting, nine 20 by 20 m plots were established and randomly assigned the following treatments (each in triplicate plots): (i) annual applications of 100 kg ha-1 of N as urea, applied in March of 1982, 1983, and 1984 (for a total of 300 kg ha-1); (ii) quarterly applications of 25 kg ha-1 of N as urea, applied in March, June, September, and December of 1982, 1983, and 1984 (for a total of 300 kg ha-1), and (iii) control, no fertilization. Seedling biomass was estimated from diameter tallies and biomass regressions based on selected harvesting during 1982, 1983, and 1984 (Johnson and Todd, 1988). Soils in each plot were sampled by depth (0–20 and 20–40 cm, three replicate cores per plot along a diagonal transect) in February 1982 (just after plantation and before fertilization) and in February 1986 (4 yr after plantation and 13 mo after the last fertilization). Unfortunately, litter was not sampled in 1986, but field observations indicated that accumulations were small and patchy (the seedlings had not yet begun to shed a significant amount of needles). The 1982 soils were analyzed for C by the Walkley-Black method (Nelson and Sommers, 1996) and total N by Kjeldahl digestion (Bremner, 1996) at A&L Agricultural Labs, Memphis, TN in 1982. The 1986 soils were analyzed for C with a LECO analyzer (LECO Corp., St. Joseph, MO) and N by Kjeldahl digestion in 1986.
In May 2000, the plots were relocated (all corner stakes and centers were found). Vegetation in 2000 consisted largely of closed-canopy overstocked loblolly pine, which was undergoing bark beetle (Dendroctonus frontalis Zimmerman) attack at the time. The understory consisted mainly of yellow-poplar (Liriodendron tulipifera L.) with occasional sweetgum (Liquidambar styraciflua L.), red maple (Acer rubrum L.), and various oaks (Quercus sp.), all of which constituted <8% of total aboveground biomass. Ground cover was either absent or negligible. Diameter at breast height (DBH) of all trees within each plot were measured. Seven trees were harvested and weighed by component (foliage, branch, bole) for the purpose of comparing with biomass regression equations published by Van Lear et al. (1984); Ralston (1973)(as given by Ter-Mikaelian and Korzukhin, 1997); and Naidu et al. (1998). The best fit was found for the equations for suppressed loblolly pine trees provided by Naidu et al. (1998) (Fig. 1)
. Stump mass was estimated with equations from Harris et al. (1973). Tree components (foliage, branch, and bole) from this destructive harvest were subsampled (clipping at various heights in the crown for foliage and twigs, disks removed by chain saw at several heights for bole) and analyzed for C and N with a Perkin-Elmer 2400 CHN Analyzer (Perkin-Elmer Corp., Wellesly, NJ) at the Desert Research Institute, Reno, NV. Understory biomass was estimated with regression equations from nearby Walker Branch Watershed (Harris et al., 1973) and N contents were estimated using N concentration values for these species at the same site (Johnson and Van Hook, 1989).
Within each plot, litter was sampled by horizon (Oi, Oe, and Oa; with miscellaneous pieces of wood cones, bark included in separate category) within 0.25 m2 rings at three locations along a diagonal transect, as in the previous soil samplings. After removal of the litter, soils at each sampling point were sampled by depth (0–20 and 20–40 cm) with a bucket auger. Bulk density was also measured at each sampling point and depth with a coring device. Thus, in 2000 as in 1982 and 1986, three replicate soil samples from each of three replicate plots were obtained from each treatment (for a total of nine samples per treatment). Soils from both the 2000 and the 1986 samplings were analyzed simultaneously for C and N by Perkin-Elmer 2400 CHN Analyzer at the Desert Research Institute, Reno, NV. Re-analysis of the 1986 soils revealed no significant bias between the 1986 and 2000 analyses: the slopes of the regression equations were not significantly different from 1 when the intercept was set to 0 (Fig. 2)
. Unfortunately, soils from the 1982 sampling could not be found for re-analysis. Previous quality assurance tests comparing Walkley-Black C (WBC) and CHN analyses indicated that original WBC values from A&L Labs were higher than modern CHN analyses on the same soils, whereas re-analysis of old soils for WBC showed good correlation to CHN analyses (Johnson and Todd, 1998). These results suggested either significant laboratory differences or possibly changes in soils during storage, but no bias associated with methodology could be pinpointed.
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Fig. 2. Comparison of original LECO-C and CHN-C (top) and comparison of original kjeldahl N and CHN-N (bottom) analyses on 1986 soils in the loblolly pine site, Oak Ridge National Environmental Research Park, Tennessee.
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Statistical analyses were performed using the General Linear Model (GLM) in DataDesk software (Data Description, Inc; Ithaca, NY) (Velleman, 1997). Within GLM, analysis of variance was used for overall treatment effects and Bonferonni post-hoc tests for differences between the fertilizer treatments, the effects of year, and, for 1986, the effects of analysis (LECO in 1986 and CHN in 2000).
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RESULTS
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Changes in Vegetation
As reported previously (Johnson and Todd, 1988), the annual fertilization treatment produced a statistically significant, 55% growth response compared with the control treatment as of Year 4 (in 1986) (Fig. 3A)
. The quarterly fertilizer treatment produced no significant growth response. Fifteen years after the last fertilization (in 2000), total biomass in the annual fertilizer treatment was only 14% greater than in the control treatment (and loblolly pine biomass in the annual fertilizer treatment was only 8% greater than in the control treatment) and the differences were no longer statistically significant (Fig. 3B). No statistically significant treatment effects were found on the amount of mortality or hardwood understory invasion in 2000. Of the original stocking level of 2500 loblolly pine seedlings ha-1, an average of 1786 trees ha-1, or 71% were still standing (live or recently dead from southern pine beetle). Average basal was 57 m2 ha-1, 53 m2 ha-1 of which was loblolly pine.
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Fig. 3. Tree biomass in (A) 1986 and (B) 2000 in the loblolly pine site. Error bars denote standard errors of total biomass and letters denote significant differences (p < 0.05) using Bonferonni post-hoc tests.
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Changes in Soil C and N
There were no statistically significant fertilizer treatment effects on soil C, N, or C/N ratio in any horizon at any time, and thus the data among treatments was combined for the purpose of analyzing changes over time (Fig. 4–6)
. There were no statistically significant differences in soil C or N among years or analysis method (for the 1986 soils) in the 0- to 20-cm depth (Fig. 4 and 5). Soil C/N ratio in the 0- to 20-cm depth was greater in 1982 (Walkey-Black/Kjeldahl) than in the 1986 LECO/Kjeldahl analyses, however (Fig. 6). In the 20- to 40-cm depth, the 1982 Walkley-Black C analyses were significantly greater than the 1986 LECO and 2000 CHN C analyses (Fig. 4). There were no differences in soil N among years or analyses method (for the 1986 soils) in the 20- to 40-cm depth, but, as in the surface soils, C/N ratio was significantly greater in the 1982 (Walkley-Black/Kjeldahl) than in the 1986 LECO/Kjeldahl analyses (Fig. 5 and 6). Because the 1982 soils could not be found for re-analysis, we cannot conclude that these differences, while statistically significant, represent changes in the soil in the field rather than laboratory bias. As noted above, we have previously found that values for old WBC analyses were significantly higher than both new WBC and new CHN C analyses on the same soils (Johnson and Todd, 1998). We could not determine whether these differences were due to laboratory bias or changes in samples during storage, but suspect laboratory bias.
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Fig. 4. Soil C analyses on 1982 soils by Walkley-Black in 1982 (1982 WBC), on 1986 soils by LECO in 1986 (1986 LECO-C), on 1986 soils by Perkin-Elmer CHN analyzer in 2000 (1986 CHN-C), and on 2000 soils by Perkin-Elmer CHN analyzer in 2000 (2000 CHN-C) in the loblolly pine site. Error bars denote standard errors and letters denote significant differences (p < 0.05) using Bonferonni post-hoc tests.
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Fig. 6. Soil C/N ratio on 1982 soils by Walkley-Black and Kjeldahl digestion in 1982 (1982 WBC/KJN), on 1986 soils by LECO and Kjeldahl digestion in 1986, (1986 LECO/KJN), on 1986 soils by Perkin-Elmer CHN analyzer in 2000 (1986 CHN), and on 2000 soils by Perkin-Elmer CHN analyzer in 2000 (2000 CHN) in the loblolly pine site. Error bars denote standard errors and letters denote significant differences (p < 0.05) using Bonferonni post-hoc tests.
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Fig. 5. Soil N analyses on 1982 soils by Kjeldahl digestion in 1982 (1982 KJN), on 1986 soils by Kjeldahl digestion in 1986 (1986 KJN), on 1986 soils by Perkin-Elmer CHN analyzer in 2000 (1986 CHN-N), and on 2000 soils by Perkin-Elmer CHN analyzer in 2000 (2000 CHN-N) in the loblolly pine site. Error bars denote standard errors and letters denote significant differences (p < 0.05) using Bonferonni post-hoc tests.
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Ecosystem C and N Contents
Estimated ecosystem C and N contents (excluding lateral roots and including soil to a depth of 40 cm) are shown in Fig. 7
. Lateral roots were not sampled in this study and are excluded from the analysis; according to Harris et al. (1977), they would be roughly equivalent to stump biomass, which in this study would constitute 12 to 14% of total aboveground biomass. Van Lear et al. (1995) found that roots constituted 24% of below-stump C in a loblolly pine stand in the Piedmont of South Carolina. Comprehensive bulk density samples were not taken in 1982 or 1986, and so soil C and N contents for these years had to be estimated by multiplying the bulk density values for 2000 by the concentrations for each year. For 1986, the LECO analyses were used because (i) there was one missing sample in the re-analysis of the 1986 soils by CHN in 2000, and (ii) there were no statistically significant differences in C analysis between the 1986 LECO and 2000 CHN analyses performed on the 1986 soils.
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Fig. 7. Carbon (A) and N (B) contents in the loblolly pine plots in 1982, 1986, and 2000. Error bars denote standard errors and letters denote significant differences (p < 0.05) using Bonferonni post-hoc tests.
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Between 1982 and 2000, vegetation (excluding roots) accumulated 100 Mg C ha-1, litter accumulated 13 Mg C ha-1, and the soils showed no significant change. Given the number of samples and variation observed in this study, soil C changes of more than 9 Mg ha-1 should have been detectable. Net ecosystem C increased by an average of 107 Mg ha-1 (somewhat lower than the sum of vegetation and litter change because of slight, non-significant decreases in soil C). During the same period, vegetation (excluding roots) accumulated 270 kg N ha-1, litter accumulated 270 kg N ha-1, and soils showed no significant change. Given the number of samples and variation observed in this study, soil N changes of <500 kg ha-1 should have been detectable. Total ecosystem N content increased by an average of 560 kg N ha-1 (due to the increases in vegetation and forest floor N) but this increase was not statistically significant (Fig. 7).
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DISCUSSION
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The hypothesis that soil C and N would increase over time in this loblolly pine plantation was not supported. Instead, the results of this study are similar to those found by Richter et al. (1999) in resampling a site 40 yr after conversion from cotton field to a loblolly pine plantation. Richter et al. (1999) found that the regenerating forest ecosystem was a strong C sink, but 80% of the new C was sequestered in the trees, 20% in the forest floor, and <1% in the mineral soil. In this study, we also found that the regenerating forest was a strong sink and that 94% of the new C was sequestered in the trees, 6% in the forest floor, and no significant change in the soil. This does not necessarily imply that soil C is inert, however; Richter et al. (1999) found significant increases in C in surface (0–7.5 cm) soils and decreases in subsurface (15–35 and 35–60 cm) soils. Richter et al. (1999) also found through the analysis of bomb 14C that the mineral soil contained new C, but that it was turning over very rapidly. In this study, we found no differences in the direction of change (or lack of change) between soil depths. The apparent decline in soil C and C/N ratio between 1982 and 1986 could represent a delayed response to clearing of the native vegetation and site preparation; however, these changes cannot be verified because of changes in methodology and potential biases between methods and labs, as noted previously.
These results contrast with those from the nearby deciduous forest sites sampled over similar time intervals on the Oak Ridge NERP. Trettin et al. (1999) found that soil C and N contents in a series of long-term sampling plots with various soil types and vegetation cover on Walker Branch Watershed were either stable or declining over a 21-yr period (1972–1993). Cases where soil C declines were noted were offset by increases in vegetation C so that ecosystem C remained relatively stable. In cases where soil N declines were found, the N losses exceeded N gains in vegetation and indicated a net N loss from the ecosystem that was larger than the measured N leaching rates. Reanalysis of old soils by new methods and quality assurance checks were positive and could not explain the observed results. A similar situation for N was also found in long-term resampling at Coweeta Hydrologic Watershed in North Carolina, where observed soil N declines could not be explained by measured leaching rates (Knoepp and Swank, 1997).
The closest comparison to be made within the Oak Ridge NERP is that between this loblolly pine study and the whole-tree harvesting study in a nearby deciduous forest (Johnson and Todd, 1998). In both these cases, the forests were regenerating from nearly bare soil (minimal logging residues). In the whole-tree harvesting study, we found large statistically significant net gains of both C and N in soils over the 15-yr period (1980–1995) following harvest (Table 1). Net ecosystem C accumulation in the whole-tree harvesting study (3.3 Mg C ha-1 yr-1) was approximately equally due to aboveground biomass (1.7 Mg C ha-1 yr-1) and mineral soil (1.8 Mg C ha-1 yr-1) with little or no accumulation in the forest floor (Table 1). Net ecosystem N accumulation (87 kg N ha-1 yr-1) was far lower in the vegetation (5 kg N ha-1 yr-1) than in soil (83 kg N ha-1 yr-1), and could not be reconciled with any known sources of N input (Johnson and Todd, 1998). Despite strong suspicions that sampling or laboratory bias was responsible for these apparent increases in soil C and N, extensive quality assurance checks revealed no problems (Johnson and Todd, 1998). In the current loblolly pine study, net ecosystem C accumulation (6.4 Mg C ha-1 yr-1) was approximately twice that in the whole-tree harvesting study and occurred mostly in the vegetation (5.4 Mg C ha-1 yr-1) with a minor amount in the forest floor (0.8 Mg C ha-1 yr-1), and virtually no change in the soil (0.2 Mg C ha-1 yr-1) (Table 1). In the loblolly pine study, the net ecosystem N accumulation (20 kg N ha-1 yr-1) was easily within the error bounds of known N inputs (deposition and fertilizer), and occurred about equally within the vegetation (13 kg N ha-1 yr-1) and forest floor (15 kg N ha-1 yr-1). Although the changes in soil N were not statistically significant, they were nevertheless of nearly the same magnitude (-8 kg N ha-1 yr-1) as the changes in the vegetation and forest floor. As noted above, differences of <500 kg N ha-1 (28 kg N ha-1 yr-1) in the soil would not have been detectable in the loblolly pine soil.
This comparison illustrates the importance of C/N ratio in considering potential C sequestration in vegetation and soils. The C/N ratios in vegetation at the whole-tree harvest and loblolly pine sites were 327 and 379, respectively, whereas the C/N ratios in the forest floor were 39 and 54, respectively; and C/N ratios in the soil ranged from 15 to 21 (Table 1). Thus, sequestering C in soils or the forest floor requires considerably more N than sequestering it in vegetation, and in cases where high soil C sequestration rates are observed, they may require an explanation for inexplicably high rates of soil N accumulation.
Finally, the attenuation of fertilizer growth response from an initial 55 to 14% over a 14-yr period of time shows that early fertilization had no lasting effect on growth at this site. Miller (1981) argues that we fertilize the tree, not the site, and this has certainly been proven true in this case: fertilizer caused only short-term (<2yr) increases in soil N availability (Johnson and Todd, 1988), as is commonly observed elsewhere (e.g., Morrison and Foster, 1977; Worsnop and Will, 1980). Miller (1981) also argues that N fertilization effectively advances the stage of stand development, thereby facilitating a long-term growth response even after any residual effects of fertilizer on soil available N have disappeared. The results of this study do not support that hypothesis, probably because fertilization in this case was initiated at too early a stage and should have been continued for a few years longer.
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SUMMARY AND CONCLUSIONS
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- The hypothesis that soils in the loblolly pine plantation sampled in this study would accumulate C and N was not supported. Although net ecosystem C accumulation was substantial (6.4 Mg C ha-1 yr-1), 94% of this was sequestered in the trees, 7% in the forest floor, and none in the soil.
- The amount of N sequestered within the ecosystem during this period (20 kg N ha-1 yr-1) was within the error bounds of known inputs via atmospheric deposition, fertilization (in the fertilized plots), and changes in soil N (net N mineralization) that could have occurred without being detected.
- Growth responses to annual fertilization over the first 3 yr were not sustained at Year 18.
- The results of this study are similar to those in a loblolly pine plantation on abandoned agricultural land in South Carolina (Richter et al., 1999) but contrast with those of a nearby deciduous forest where large amounts of C and N accumulated in soils over a 15-yr period of forest regeneration after harvest.
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ACKNOWLEDGMENTS
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This research was supported by the Department of Energy's Agenda 2020 program and the Nevada Agricultural Experiment Station. We thank Valerie Yturiaga for CHN analyses.
Received for publication January 7, 2002.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
