Published online 3 August 2006
Published in Soil Sci Soc Am J 70:1541-1546 (2006)
DOI: 10.2136/sssaj2005.0119
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Soil & Water Management & Conservation
Carbon Turnover and Carbon-13 Natural Abundance under Land Use Change in Semiarid Savanna Soils of La Pampa, Argentina
A. Zacha,
H. Tiessena,* and
E. Noellemeyerb
a Institute for Tropical Agronomy, Universität Göttingen, Grisebachstr. 6, 37077 Göttingen, Germany. H. Tiessen, current address: InterAmerican Institute for Global Change Research, c/o INPE, Ave. dos Astronautas 1758, São José dos Campos, sp. 12227-010, Brazil
b Facultad de Agronomía, Universidad Nacional de La Pampa, 6300 Santa Rosa, La Pampa, Argentina
* Corresponding author (htiessen{at}dir.iai.int)
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ABSTRACT
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Half-lives of 50 to 100 yr have been reported for native soil C in temperate Mollisols taken under cultivation, whereas some C fractions have shown mean residence times of hundreds of years. In the Tropics, C turnover is much more rapid. This lower C stability may be attributable to climate or to a mineral suite with low C stabilization potential typical of many tropical soils. We present data on the mineralization and accretion pattern of organic C under land use conversion in illitic loessial loams of the semiarid, warm temperate Province of La Pampa, Argentina. These soils should have a low C stabilization potential, and the semiarid temperate climate should provide for relatively slow turnover compared with tropical conditions. Using 13C-natural abundance and soil fractionation, we determined soil C derived from C3 and C4 plants under land use changes between native Calden savannah, pastures, and arable fields, specifically: (i) the conversion of C3 natural savannah to agriculture that includes (C4) maize in the rotation, (ii) the conversion of a 40-yr-old C4 pasture into arable land under C3 crops, and (iii) the reclamation of highly degraded C3 cultivated land with C4 pastures. Although none of the land cover changes represented 100% conversions between plants of C4 and C3 metabolism, C turnover could still be determined. All sites provided evidence for rapid C losses with C half-lives just above 10 yr and no evidence for long-term stabilized C in any soil fractions. Soil under long-term pasture or the natural vegetation of the region, with initial C contents between 24 and 33 mg C g–1 bulk soil, lost 33 to 57% of this original bulk soil C within 12 to 18 yr of continuous cultivation. On degraded soils under restoration with C4 pasture, C accretion was also rapid but leveled off well below the original C levels. Theses results provide evidence for potentially irreversible soil degradation and corroborate local practical experience that these temperate, coarse-textured, illitic soils are highly susceptible to degradation and should be managed carefully.
Abbreviations: INTA, Instituto Nacional de Tecnología Agropecuaria • OM, organic matter
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INTRODUCTION
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LAND USE CHANGE affects soil carbon (C) contents through mineralization and release of CO2 to the atmosphere (Veldkamp and Weitz, 1994) and by sequestration of CO2 by new vegetation and deposition of plant residues in the soil. Soil organic C budgets indicate the net results of mineralization and sequestration on soil C content but do not provide actual rates of C turnover. Actual turnover rates can be obtained using natural 13C abundance when a change between C3 and C4 vegetation occurs.
As a result of discrimination against the heavier isotope, the 
13C signature of C3 plants is significantly more negative than that of C4 plants. Values for C3 plant material usually range from –20 to –35
(average, –26), whereas C4 plants exhibit values ranging from –9 to –17 (average, –12) (Krull and Skjemstad, 2003). This difference in 13C has been used to evaluate C turnover when land cover changed completely from C3 to C4 vegetation or vice versa (Veldkamp and Weitz, 1994). However, the 13C technique can also be applied where a C3/C4 mixed vegetation is substituted by a C3- or C4-dominated vegetation. This was shown by Henderson et al. (2004) for a grazed, C3/C4 mixed Canadian prairie, which had been previously converted into arable land 70 yr prior to abandonment and was naturally revegetated with predominantly C3 grasses after abandonment. Cambardella and Elliott (1992) studied organic matter (OM) changes under wheat monoculture on a previously mixed prairie that had a 13C signature of –21.4 (i.e., somewhere between that of C3 and C4 plants).
Much of the published data on C turnover in tropical soils show faster C turnover rates than those of temperate regions (Trumbore, 1993, Tiessen et al., 1994, Shang and Tiessen, 1997, Solomon et al., 2000). Solomon et al. (2000) found soil C losses of 56% in less than 15 yr of cultivation of a semiarid tropical woodland in Tanzania. For North American cultivated grassland soils of the Great Plains, average C losses of 30 to 50% after 50 to 80 yr were measured (Tiessen and Stewart, 1983). The higher temperatures of the tropics or the lower C stabilization potential of the mineral suite of tropical soils might be responsible for the lower C stability. The present study examines the C loss and sequestration on land use change in the warm temperate semiarid Argentinean La Pampa province. Organic matter loss is of regional concern as an important factor of soil degradation. In addition, the silty, illitic loess soils of La Pampa provide a mineralogy with a lower C stabilization potential than the smectitic soils of the North American plains, where many of the previous studies were conducted. Analysis of C isotope composition in bulk soils and size fractions were used to characterize soil C stability. One objective of the study was to examine the influence of mineralogy and climate on soil C stability by comparing these Argentinean with other temperate soils and with results from the tropics. A second objective was to provide information on the resilience of the C pool in these important agricultural soils under different land uses.
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MATERIALS AND METHODS
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Study Sites
The study region is located in the central Argentinean temperate, semiarid province of La Pampa. Eleven sites were sampled in 2003 in the northeastern provincial departments of Conhelo, Capital, and Toay. Mean annual rainfall is around 500 mm, with a peak in summer (December through February) and lowest amounts in winter (June through August). With an average annual temperature near 12°C, hot summers and cool winters, evapotranspiration usually exceeds 800 mm. All soils under study are loamy Entic Haplustolls on loess parent material. The dominant mineralogic composition of soils and sediments in the region is volcaniclastic with fresh particles of volcanic rocks, pyroxenes, clinopyroxenes, plagioclases, and volcanic glass shards. Illite is the dominant clay mineral (Zárate, 2003). The texture classes of the study sites are loam (Conhelo) and sandy loam (Capital, Toay), with the highest sand contents at Toay (Table 1). The pHs are near neutral, and top soils are carbonate free (Instituto Nacional de Tecnología Agropecuaria [INTA], 1980).
Four sites sampled in the Department Conhelo (S 35°56'/W 64°40') were converted from a naturally C3-dominated savannah ("Caldenal") to arable land with a mixed C3/C4 crop rotation (Table 2). CaldenA and CaldenB sites represent the natural vegetation with a dense (C3) grass cover, scattered Caldén (Prosopis caldenia) trees, and few shrubs and bushes. Agr18 and Agr12 are cultivated sites adjacent to CaldenA and CaldenB, respectively.
Agr18 has been under continuous cultivation with a crop pasture rotation since 1985: 2–3 yr of winter pasture of (C3) rye (Secale cereale L.) or (C3) oats (Avena sativa L.) combined with a summer crop of (C4) maize (Zea mays L.) alternated with 3 to 5 yr of continuous (C3) alfalfa (Medicago sativa L.). The year of sampling (2003) was the first year of a rye pasture. Agr12 had been converted in 1991 and is cultivated using the same cropping sequence as Agr18. The year of sampling was the third year of alfalfa.
In the Capital department (S 36°34'/W 64°15'), a 40-yr-old (C4) Eragrostis curvula pasture had been converted to arable land. These sites belong to the experimental station of INTA, Anguil. The Pasture40 site represents the remaining 40-yr-old permanent C4 pasture that had replaced native C3 vegetation. During the pasture period, the site was used for cattle grazing at a low stocking rate. The Til2 site had been part of the same 40-yr-old pasture but was plowed up in 2001 and planted with (C3) sunflower (Helianthus annuus L.) and rye. The Til13 site, also taken out of Pasture40, had been under continuous agriculture for 13 yr before sampling. After conversion, it was initially planted with rye and (C4) millet (Pennisetum americanum L.), alternately. Since 1994, the site has been cultivated only with C3 plants in a crop pasture rotation (Table 2). The fields are frequently tilled with disk and moldboard plows.
At Toay (S 36°48'/W 65°08'), soils were highly degraded from many years of (C3) cropping (Table 2) and had been seeded to (C4) E. curvula pastures to restore them. The selected pastures had been under E. curvula for 4, 7, 11, and 14 yr at the time of sampling. Accordingly, they are referred to as Pasture4, Pasture7, Pasture11, and Pasture14. These planted pastures are dominated by the C4 grass, but some C3 weeds, mainly Medicago minima and Centaurea solistialis, occur in association with E. curvula. Stocking rates are above the recommended 0.3 cow units per ha for cattle grazing in this region (Frank et al., 2003).
Experimental Design
Each of the 11 treatments consists of one field with its distinct cropping history (Table 2). The fields vary in size from 25 ha at INTA to 50 ha at Conhelo and up to 100 ha at Toay. From each site, six soil samples were taken on a linear transect, with 10 m between sampling points. Because preexisting unreplicated fields were sampled, this pseudo-replication weakens statistical pairwise comparisons. We therefore use all sites to evaluate C turnover processes and draw conclusions based only on results corroborated by all 11 sites.
Soil Sampling and Analyses
The 0- to 6-cm soil layer was collected in October 2003 using a bulk density cylinder of 471 cm3. The soil from the cylinder was air-dried and weighed to provide bulk density. Samples were sieved through a 2-mm mesh before fractionation and analyses. Roots and coarse organic debris were removed. Subsamples of the soil and fractions were used for 13C measurement. Soil texture was determined with a Bouyoucus (1962) hydrometer.
Soil fractions were separated from three of the six samples from each site by wet-sieving into three different fraction sizes (>100 µm, 50–100 µm, <50 µm). Fifty grams of air-dried soil was suspended in 200 mL distilled water and shaken with three glass beads for 6 h at 120 rotations per minute. Fractions were separated by washing them through stacked 50- and 100-µm mesh sieves with a stream of water. The smallest fraction (<50 µm) was sampled by collecting the first 500 to 600 mL of the soil suspension that passed the sieves. All fractions were dried at 60°C. This routine fractionation is used in La Pampa to evaluate soil aggregation (and thus degradation). In most of our samples, it resulted in coarse fractions practically devoid of aggregates that only contained sand and coarse OM. Because the finest fraction is only partially sampled, subsequent C analyses are reported only on a fraction (not whole-soil) basis to characterize fractions in terms of C stability and turnover.
13C Measurements
Dried subsamples of soil and soil fractions were ground and weighed into tin capsules (5–10 mg soil) and analyzed for 13C and 12C by mass spectrometry (using a Porapak PQS and Finnigan MAT 252 with a ConFloII interface) (Werner et al., 1999). The ConFloII-interface provided simultaneous C and N measurements. Carbon isotope ratios were expressed as 13C values () according to the following formula:
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The ratio of soil C derived from the new and previous vegetation was calculated using the following equation (e.g., Shang and Tiessen 2000):
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where is the 13C isotope ratio of the soil or soil fraction at sampling, 0 is the 13C signature of the soil or fraction under the previous vegetation, and 1 is the 13C value of the new plant material added after land use change. The quantity (mg C g–1 soil or fraction) of newly added C (C1) and old plant-derived C (C0) of bulk soil and soil fractions can then be calculated as
 | [3] |
respectively, where C* is the total C concentration of the soil or fraction. 13C signatures of C3 crops have been documented between –26.7 and –27.9 (Balesdent et al., 1988, Balesdent et al., 1990, Cambardella and Elliott, 1992). Allowing for a slight enrichment during residue decomposition and soil OM formation (Balesdent et al., 1990), we assumed an average 13C signature of –26.5 resulting from a mix of C3 crops. This value is similar to the soil values found for the Calden savanna.
Data were analyzed by one-way ANOVA and comparison of means was by Fisher's LSD at P < 0.05 using Statgraphics plus 3.1. Carbon mineralization was fitted to linear regressions. The half-life of organic C of bulk soil and soil fractions was estimated based on the assumption of a linear decay of organic carbon in the soil. There was no evidence that an exponential function improved the results.
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RESULTS
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13C Abundances
At Conhelo, after conversion from natural vegetation to arable land, 12 yr of cultivation (Agr12) that included regular maize crops in the rotation resulted in significant differences in 13C signature of bulk soil and soil fractions (Table 3). Differences after 18 yr of agriculture (Agr18) were significant only in the smaller size fractions (50–100 and < 50 µm). The coarse fraction probably contained a large proportion of residues from the 3 yr of (C3) alfalfa grown just before sampling.
Pasture40 at INTA with a 13C of –16.2 is predominantly covered by the C4 grass E. curvula L. (13C of the plant material: –13.0). No significant differences in 13C signature are seen after only 2 yr of cultivation (Til2) of the pasture. Cultivation with C3 crops (13C: –26.5) for 13 yr (Til13), of which 11 were exclusively with C3 crops, altered the isotopic ratios of topsoil and size fractions toward a significant depletion in 13C (Table 3). The C4 pasture sequence at Toay showed continuously increasing 13C signatures with time of pasture on degraded soils (Table 3).
Carbon Budget and Carbon Turnover under Land Use Changes
The highest C contents can be found under the natural Caldén savanna (Table 4). Continuous cultivation resulted in considerably lower topsoil C contents. Twelve and 18 yr (Agr12, Agr18) of cultivation resulted in similar bulk soil C reductions near 56% (Table 5). Thus, the soils lost half their C content in 10–16 yr. The greatest losses occurred from the coarse fraction > 100 µm, which showed net losses of about 75% of the initial C. The 50- to 100-µm fraction had a 60% lower C content after 12 yr (Agr12), whereas net losses from the smallest fraction were nonsignificant at both sites (Table 5).
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Table 4. C content of whole soil (mg C g–1 soil) and soil fractions (mg C g–1 fraction) at Conhelo, Capital, and Toay.
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Table 5. Total (mg C g–1 soil or fraction) and percent C-loss and totals and proportions of maize-derived C4-C (13C: –12.8) and of C3-C in whole soil and soil fractions after conversion from natural vegetation to arable land at Conhelo.
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Agr12 showed a 13C signature, indicating the presence of maize C4-C in the bulk soil (Table 3). The C balance resulting from the shift from C3 to mixed C3/C4 vegetation can therefore be calculated using a 13C for maize leaves and roots of –12.8 (Balesdent, 1996) and a similar 13C for the C3 Calden and C3 crops of –26.5. Only 8.9 mg C g–1 bulk soil are C3 derived after 12 yr of cropping, which is a reduction of 70% from the CaldenB value (28.6 mg g–1 soil). Meanwhile, 
30% of the C is new maize-derived C4-C. This indicates a half-life of the original C of < 10 yr.
Conversion of C4 Pasture40 to arable land caused a 33% reduction in total topsoil C content after 13 yr of conventional tillage (Til13) (Tables 4 and 6). The greatest loss of 58% occurred from the coarsest fraction > 100 µm. Two years of millet at the beginning of cultivation of Til13 will have added some C4 material after the plowing of the pasture, but the effect will be small relative to the plowing in of the pasture residues. Using Eq. [2] and the difference between the measured pasture soil 13C (–16.2) and the expected soil 13C for complete conversion to incoming C3 material (–26.5), 52% new C3-C can be calculated for the bulk soil. The amount of remaining pasture C is 7.6 mg g–1, or 32% of the original C (23.7 mg g–1 soil). The resulting half-life of the old C is slightly less than 10 yr.
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Table 6. Total (mg C g–1 soil or fraction) and percent C-loss and totals and proportions of crop-derived C3-C (13C: –26.5) and C4-C of whole soil and soil fractions 13 yr after land use conversion (pasture to arable land) at Capital.
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At Toay, there is no reference site corresponding to the original degraded fields. Net changes in C content can therefore not be calculated. However, the introduction of C4 pastures on long-term C3 crop lands provides a clear shift from C3 to C4 vegetation (Table 7). One can estimate a 13C signature for the degraded soil at Toay (13C near –26) based on the 13C value of the native Calden savanna and the average for C3 crops. The bulk soil C3-C decreases linearly from the year 4 to the year 14 levels (Fig. 1
, Table 7). Within that period, a half-life of 11 yr is calculated. This estimate is independent of estimates for the initial and final C contents associated with the landuse conversion and is relatively robust with respect to the assumptions for initial and final values.
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Fig. 1. Change of original C3-derived carbon (mg C g–1 bulk soil) over time after land use conversion from degraded (C3) land into C4pastures at Toay.
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The proportion of C4-C of bulk soil at Toay accounted for 39% after 4 yr of E. curvula introduction, increasing to 68% after 14 yr (Table 7). The accumulation of bulk soil C4-C with time after pasture introduction (Table 7) is nonlinear, leveling off 6 mg C g–1 bulk soil (Fig. 2
). This leveling-off represents a weak C sequestration from the soil reclamation effort. The data point to a strong hysteresis between C loss and C sequestration once soils have been degraded to the degree seen at Toay. This may be an indication for a threshold of hard-to-reverse degradation that should be avoided, especially on more sandy soils, such as those found at Toay.
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Fig. 2. Sequestration of pasture-derived C4-C (mg C g–1 bulk soil) with time of pasture introduction on original C3 vegetation at Toay.
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DISCUSSION
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For a North American grassland loam soil, Tiessen and Stewart (1983) reported 34% C loss after 60 yr and 52% loss after 90 yr of wheat fallow rotations. In a sandy soil, C loss amounted to 51% after 65 yr. Losses from the >50-µm fractions were greater, and losses from the finer fractions were correspondingly less (Tiessen and Stewart, 1983; Cambardella and Elliott, 1992). Balesdent et al. (1988) found 50% of the original C in the stable OM pool 100 yr after conversion of native prairie into continuous agriculture. In the soil OM fraction (>50) µm, Cambardella and Elliott (1992) found 69% of the native grassland C after 20 yr of wheat cultivation under temperate conditions and concluded that grassland-derived C turned over much more slowly than newly added wheat residues. Numerous studies from the North American Great Plains have shown C losses of around 50% under cultivation of 50 to 100 yr and the presence of highly stabilized C in some of the silt- and clay-associated OM fractions. Carbon dating of such soils has shown mean C ages for mineral-associated OM of several hundred and at times over 1000 yr (Martell and Paul, 1974). Arrouays and Pelissier (1994) found a lower C half-life of about 35 yr in a temperate humic loamy forest soil under cultivation. In contrast, the long-term pastures and natural soils in La Pampa with initial C contents between 24 (Pasture40) and 33 mg g–1 bulk soil (CaldenA) showed an average C half-life of only 10 yr under continuous cultivation.
Soil carbon in the semiarid soils of La Pampa is much less stable than that of North American grasslands and is less stable than that of the more humid French forest soils. Half-lives near 10 yr are more comparable to some of the tropical data in the literature. In a semiarid tropical woodland soil in Tanzania, for instance, 15 yr of cultivation resulted in soil C losses of 56% (Solomon et al., 2000). The rapid mineralization under tropical conditions has been attributed to higher temperatures or a different mineral suite that may not stabilize C to the same degree (Shang and Tiessen, 1998).
At the Capital site, where pasture-derived C4-C is replaced by the introduction of C3 crops, soil size fractions lost 38 to 61% of their former C4-C after 10 to 13 yr of cultivation (Table 6), indicating that, unlike the North American soils, they contain no stable soil carbon fraction. The predominance of silts with relatively low sorption potential for OM and of illitic clays with a lower swelling (and sorption) potential than the smectites of many North American sites in these soils of La Pampa could explain the low stabilization of C, pointing to a similar effect of the predominantly low-activity clay suite of many tropical soils.
On the other hand, C sequestration can also be rapid. At Toay, most of the new C4-C is found in the fraction <50 µm, accounting for 65% of C in that fraction after only 4 yr and for 81% after 14 yr of C4 vegetation (Table 7). This rapid accretion quickly levels off (Fig. 2), indicating a reduced C binding capacity of this degraded soil. The long-term cropping history of Toay (Table 2) using conventional tillage has resulted in a reduced soil structural stability (Quiroga et al., 1999) and C stabilization capacity.
The data presented here provide evidence that the soils of La Pampa have low C stability and a low capacity for C restoration after intensive agriculture, even on restoration efforts with pasture implantation. The management of these soils must use sustainable cultivation practices, no-till and/or crop-pasture rotations, low-impact grazing management, and the return of adequate amounts of plant residues, strategies also essential in the management of many tropical soils.
Received for publication April 11, 2005.
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ABSTRACT
INTRODUCTION
