Published online 27 August 2007
Published in Soil Sci Soc Am J 71:1636-1638 (2007)
DOI: 10.2136/sssaj2005.0309
© 2007 Soil Science Society of America
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FOREST, RANGE & WILDLAND SOILS
Possible Mechanisms Leading to a Delay in Carbon Stock Recovery after Land Use Change
Hirotsugu Araia,*,
Naoko Tokuchib and
Keisuke Kobac,d
a Laboratory of Silviculture, Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto Univ., Kyoto 606-8502, Japan
b Field Science Education and Research Center, Kyoto Univ., Kyoto 606-8502, Japan
c Dep. of Environmental Science & Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Inst. of Technology, Yokohama 226-8502, Japan
d Tokyo Univ. of Agriculture and Technology, Saiwai-cho 3-5-8, Fuchu-city, Tokyo, 183-8509, Japan
* Corresponding author (hiroarai{at}kais.kyoto-u.ac.jp).
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ABSTRACT
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Changes in land use sometimes lead to soil C loss, and a long time may be required for the C stock to recover to initial levels. Thus, it is important to evaluate the mechanisms related to accumulation of newly input C following land use changes. In this study, we sought to determine the signature of newly input C in the soil profile after land use change. We used stable and radioactive C isotopes with soil fractionation methods in a C3 coniferous plantation converted from C4 grassland in Japan. The difference in 
13C values between the surface litter and the soil organic carbon (SOC) below the litter was 5
or greater; this large isotopic difference was attributed to rapid decomposition in the litter layer and preservation of C derived from the previous C4 vegetation. Most SOC 
14C values were negative throughout the soil profile, suggesting that most of the SOC in the soil profile was recalcitrant and had been preserved for a long time. Only the surface sand values were slightly positive. These results suggest that most newly input C is consumed at the soil surface. The low ability of these soils to preserve newly input C is one factor in the slow recovery of soil C.
Abbreviations: SOC, soil organic carbon
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INTRODUCTION
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Changes in land use, which frequently result in changes in vegetation, greatly impact SOC stocks (Guo and Gifford, 2002). Paul et al. (2002) observed, however, that during the first 10 yr following afforestation, C stocks were sometimes reduced from initial levels, and about 40 yr were needed for recovery because of the slow accumulation of C. Thuille and Schulze (2006) suggested that at least 80 yr were required for initial stock levels to recover after afforestation in Thuringia and the Alps. The factors and processes related to SOC stock dynamics were summarized by Post and Kwon (2000). Although changes in land use affect many factors that regulate SOC dynamics (e.g., physical and biological conditions in the soil and vegetation type; Post and Kwon, 2000), it is not completely clear how these factors affect C stocks and dynamics.
A stable C isotope (natural abundance of 13C, expressed as 13C) can be used to study SOC dynamics in forest soils (Balesdent et al., 1988). The 13C values of C3 plants commonly range from –30 to –22, whereas the values of C4 plants range from –15 to –9 (O'Leary, 1995). This large isotopic difference between C3 and C4 plants is useful in calculating short-term C turnover following vegetation change (Henderson et al., 2004). Radiocarbon has also been used to study long-term SOC turnover (e.g., Huang et al., 1999). Moreover, atmospheric nuclear tests produced large amounts of 14C (bomb 14C) during the 1950s and early 1960s, resulting in an increase in atmospheric 14C content (Manning et al., 1990). Because the SOC supplied before these nuclear tests should contain no bomb 14C, the signature of bomb 14C in soils can be attributed to the organic C derived from recent vegetation. This bomb 14C signature allows us to estimate the soil C cycle on shorter time scales (e.g., annual to decadal scales; Trumbore et al., 1989; Richter et al., 1999; Krull et al., 2005). For precise detection of bomb 14C in the soil profile, it is important to distinguish 14C activity that is not derived from bomb 14C to avoid mixing "younger" and "older" C. The mean residence time of C differs among particle-size fractions (sand, silt, and clay) (Christensen, 2001); the sand fraction is expected to contain relatively young SOC with higher bomb 14C content than other fractions (Baldock et al., 1992). Therefore, it is assumed that among the three fractions, the sand fraction is the most suitable for detecting a bomb 14C signature at any depth interval. Besides size fractions, Trumbore et al. (1989) and Trumbore and Zheng (1996) reported that the residues after chemical fractionation (acid–alkali–acid treatment) contained refractory and 14C-depleted SOC.
In this study, we chose a 51-yr-old Cryptomeria japonica D. Don (C3 plant) plantation that was planted at about the same time as atmospheric 14C content began to increase (Manning et al., 1990). Therefore, we hypothesized that most of the bomb 14C in the soil profile was derived from the current C. japonica. Before plantation establishment, the area had been used for harvesting Miscanthus sinensis Andress (a C4 plant). Thus, from the distinct isotopic signatures of the two vegetation types, the accumulation of SOC that is derived from the current vegetation during the past 51 yr can be evaluated using both the 14C and 13C values of SOC. The objective of this study was to detect the signatures of new C input into the soil during the past 51 yr from C isotope data using physical or chemical separation methods. We also attempted to identify mechanisms related to slow C stock recovery after land use change.
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MATERIALS AND METHODS
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The study site was a 51-yr-old C. japonica (Japanese cedar) plantation about 1000 m in elevation at the Wakayama Forest Research Station, Kyoto University, Wakayama Prefecture, Japan (34°4' N, 135°31' E). Mean annual precipitation and temperature values from 1971 to 2000 were 2647 mm and 12.3°C, respectively. At similar elevations, deciduous broadleaf forests probably dominated this area since 8000 yr BP (Takahara, 1998). For at least 30 yr before the establishment of the plantation, this area served as a commonly held community plot that was used for harvesting M. sinensis as a roofing material, according to foresters working in this area. Miscanthus sinensis is a perennial grass species with high plant biomass that is widely distributed throughout Japan (Yazaki et al., 2004). The soil at the study site was classified as a Dystrochrept with silt loam texture.
Soil samples were taken from a single soil profile (100 cm wide and 120 cm deep) in October 2002. The samples were collected at depth intervals of 10 cm to a depth of 120 cm in the profile (?300 g soil per sample), except from the layer at 100- to 110-cm depth, which contained many rocks. The litter layer (composed mostly of C. japonica litter) was thin (less than ?2 cm). Fresh M. sinensis tissue was also collected near the sampling site. The samples were air dried, sieved (<2 mm), and visible root fragments were removed by hand. The soil texture was determined with H2O2 (30%). Soil pH (1:2.5 in H2O) was also measured. Table 1 shows the soil particle-size distribution and pH data. Silt occupied about 70% of the overall profile, and sand comprised about 20 to 30% (Table 1). The pH of the soil profile ranged from 4.7 to 5.5 (Table 1).
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Table 1. The soil properties (pH and texture), soil organic carbon (SOC) concentrations, and 13C values at each depth interval.
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To comprehensively investigate the distribution of bomb 14C in this soil profile, 14C activity was analyzed in the sand fraction collected from depths of 0 to 10, 30 to 40, 50 to 60, and 70 to 80 cm. In addition, the silt fraction from 50 to 60 cm was also used for 14C analysis to compare the 14C signal between the sand and silt fractions. To evaluate older SOC, samples with chemical fractionation (10–20- and 30–40-cm depths) were also prepared for 14C analysis.
For chemical and particle-size fractionation, unground soil from each depth interval was divided into two subsamples. One subsample was used to obtain the sand and silt fractions by dry sieving (63 µm) and gravity sedimentation based on the methods of Trumbore and Zheng (1996), while the other subsample was used for chemical fractionation (Trumbore and Zheng, 1996; Lee et al., 2000). The chemically fractionated samples, as well as the silt fraction, were freeze-dried for further analysis. Seven fractionated samples were graphitized under Fe catalysis according to Kitagawa et al. (1993). The graphite samples were sent to the Rafter Radiocarbon Laboratory for 14C analysis. Measured 14C activity was corrected for isotopic fractionation and expressed as 14C (Stuiver and Polach, 1977). A positive 14C value indicated the existence of bomb 14C. For these results, the system error component was conservatively estimated at 0.18%.
The ground soil and litter samples were analyzed for C concentration and for stable isotope ratios using an isotope ratio mass spectrometer (Delta S, Thermo Finnigan, Bremen, Germany) coupled with an elemental analyzer (Carlo Erba NA 1500, Carlo Erba Instruments, Milan, Italy). The bulk soil samples were analyzed without removal of inorganic C because of the low pH (Table 1). Analytical precision with the running standard (DL-alanine) was ±0.15 (as the SD).
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RESULTS AND DISCUSSION
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The 13C values for all SOC were enriched (ranging between –23.1 and –20.6, Table 1) compared with the litter (–28.4 ± 0.19). Many studies have shown that the 13C value at the soil surface is comparable to that of the current aboveground vegetation (e.g., Quideau et al., 2003), even when a vegetation change has recently occurred (e.g., Connin et al., 1997), as the C is derived from current aboveground vegetation and has accumulated at the surface. The remarkable difference in 13C values between the litter layer and surface soil, however, and the high 13C throughout the soil profile (Table 1) observed in this study suggest a low contribution of the current C. japonica-derived C to total SOC. In addition, this area was covered by a C4 grass (M. sinensis; 13C value = –10.9) before the establishment of the C. japonica plantation, so it is likely that the old C4–derived C contributed greatly (and relatively constantly) to total SOC in all horizons.
Most 14C values were negative throughout the soil profile regardless of differences in fraction and depth interval (Table 2). Only the surface sand fraction had a slightly positive 14C value (Table 2). This suggests that there was little bomb 14C below the surface. Although it is possible that much older SOC masked the bomb 14C signature, the results clearly indicate that old (14C-depleted) C has been preserved throughout the soil profile at this site.
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Table 2. The 14C and 13C data for each depth interval. The values in parentheses show the analytical error as the SE.
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The results of 13C and 14C analysis of SOC suggest only slight accumulation of new C derived from the current C. japonica vegetation at the soil surface, and there was little new C below the surface. Furthermore, most of the SOC at this site was composed of a mixture of SOC derived from the previous C4 grass and from much older C3 plants, which has been preserved for a long time. This suggests that new C derived from the current vegetation is lost from the soil surface. Factors related to the low accumulation of new C are rapid decomposition in the litter layer (Nadelhoffer and Fry, 1988), preferential respiration of new C because of its high decomposability (Hagedorn et al., 2003), low flux into the recalcitrant SOC fractions (Lichter et al., 2005), and priming effects at the surface (Fontaine et al., 2004). Although we cannot provide definitive information as to which factors are responsible for the low accumulation of new C at our site, a low rate of new C accumulation may increase the time needed for recovery of initial C stock levels after land use change.
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ACKNOWLEDGMENTS
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We especially thank Dr. N. Matsuo and Dr. F. Hyodo for their support in isotope measurements and Mr. K. Fukushima and Mr. R. Harada for their support in sampling. We also thank Dr. T. Shimamura, Dr. L. Koyama, and Ms. A. Ogawa for their important comments on early versions of the manuscript. This study was financially supported by the Institute of Humanity and Nature, Ministry of Education and Science, Japan (no. 15310029 and 15380105).
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication September 19, 2005.
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ABSTRACT
INTRODUCTION
