Soil Biology & Biochemistry 39 (2007) 1173–1180
Thermal stability of soil organic matter pools and their d
13
C values after
C
3
–C
4
vegetation change
Maxim Dorodnikov
a,c,
Ã
, Andreas Fangmeier
a
, Yakov Kuzyakov
b
a
Institute of Landscape and Plant Ecology (320), University of Hohenheim, August-v.-Hartmann-Str. 3, 70599 Stuttgart, Germany
b
Department of Agroecosystem Research, University of Bayreuth, 95440 Bayreuth, Germany
c
Institute of Physico-Chemical and Biological Problems in Soil Science, RAS, Institutskaya 2, 142290 Puschino, Russian Federation
Received 15 October 2006; received in revised form 6 December 2006; accepted 11 December 2006
Available online 22 January 2007
Abstract
Carbon isotopic composition of soils subjected to C
3
–C
4
vegetation change is a suitable tool for the estimation of C turnover in soil
organic matter (SOM) pools. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal
stability. Soil samples from a field plot with 10.5 years of cultivation of the C
4
plant Miscanthus  gigantheus and from a reference plot
under C
3
grassland vegetation were analysed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). According
to differential weight losses (dTG) and energy release or consumption (DSC), five SOM pools with increasing thermal stability were
distinguished: (I) 20–190 1C, (II) 190–310 1C, (III) 310–390 1C, (IV) 390–480 1C, and (V) 480–1000 1C. Their d
13
C values were analysed by
EA-IRMS. The weight losses in pool I were connected with water evaporation, since no significant C losses were measured and d
13
C
values remained unchanged. The d
13
C of pools II and III in soil samples under Miscanthus were closer to the d
13
C of the Miscanthus
plant tissues (À11.8%) compared to the thermally stable SOM pool V (À19.5%). The portion of the Miscanthus-derived C
4
-C in total
SOM in 0–5 cm reached 55.4% in the 10.5 years. The C
4
-C contribution in pool II was 60% and decreased down to 6% in pool V. The
mean residence times (MRT) of SOM pools II, III, and IV were similar (11.6, 12.2, and 15.4 years, respectively), while pool V had a MRT
of 163 years. Therefore, we concluded that the biological availability of thermal labile SOM pools (o480 1C) was higher, than that of the
thermal stable pool decomposed above 480 1C. However, the increase of SOM stability with rising temperature was not gradual.
Therefore, the applicability of the TG-DSC for the separation of SOM pools with different biological availability is limited.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: C
3
–C
4
vegetation change; Miscanthus  gigantheus; Differential scanning calorimetry; Thermogravimetry; TG-DSC; Thermal stability; d
13
C;
Soil organic matter
1. Introduction
Carbon (C) isotopic composition of soil organic matter
(SOM) after C
3
–C
4
vegetation change (and vice versa) has
been frequently used in the last decade to estimate C
turnover rates in soil and the incorporation of new C in
various SOM pools (Balesdent and Mariotti, 1987; Volkoff
and Cerri, 1987; Ludwig et al., 2003; John et al., 2003,
2005; Kristiansen et al., 2005). This approach is based on
the different stable isotope composition (represented as
d
13
C value) of residues from plants with C
3
and C
4
photosynthesis (Farquhar et al., 1989; Ehleringer and
Cerling, 2002). After C
3
–C
4
vegetation change the d
13
C
value of SOM starts to change slowly from the original
d
13
C value, which is closer to that of C
3
vegetation, to a
new steady-state d
13
C value, which is closer to that of C
4
vegetation. When the new steady state is not reached (as in
most studies) and the period after the C
3
–C
4
vegetation
change is known, the contribution of the new C
4
-derived C
to the total SOM can be calculated. Based on this C
4
-C
contribution and the period after vegetation change, the
SOM turnover rates can be roughly estimated (Balesdent
and Mariotti, 1996).
ARTICLE IN PRESS
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doi:10.1016/j.soilbio.2006.12.025
Ã
Corresponding author. Present address: Department of Agroecosys-
tem Research, University of Bayreuth, 95440 Bayreuth, Germany.
Tel.: +49 921552305.
E-mail address: maxim.dorodnikov@uni-bayreuth.de
(M. Dorodnikov).