Buurman, P.

Roscoe, R.

Vasconcellos, C.A.

Velthorst, E.

NARCIS 

Soil organic matter dynamics in density and particle-size fractions as revealed by the 13C/12C isotopic ratio in a cerrado oxisol

Wageningen Yield 

SOIL ORGANIC MATTER DYNAMICS IN DENSITY AND PARTICLE-SIZE 
FRACTIONS AS REVEALED BY THE U C/"C ISOTOPIC RATIO IN A 
CERRADO OXISOL 
ROSCOE R.(1). BUURMAN, P.,(1) VELTHORST, E.(1) & VASCONCELLOS, C A . 0 ) 
(1>WagenJngen University, Dep. Environmental Sciences, Lab. Soil Science &Geology, 
P.O. Box 37,6700 AA, Wageningen, The Netherlands; 
(2)
 EMBRAPA-CNPMS, P.O. Box 151, 35701-970, Sete Lagoas-MG, Brazil. 
1. INTRODUCTION 
The Cerrados region occupies an area of 207 million hectares (24.42% of the Brazilian territory), 
localized mostly in the Mid-West region, and small areas fringing other regions such as Northeast 
and North regions. It is localized between the parallel 5°50' to 21°26'. The Cerrados region 
contributes significantly to the Brazilian agricultural and forestry product. Due to the inadequate 
management systems adopted, these soils are easily degraded leading to decrease in crop yields, 
losses of organic matte and nutrients, structure degradation, erosion, high runoff, sedimentation and 
contamination of the water. 
Soil organic matter dynamics under Cerrado ecosystems are still unclear. Losses of up to 80% of 
the total soil organic matter (SOM) in 5 years of soybeans were already reported, showing the risks 
of degradation of the Cerrado soils under cultivation (Silva et al., 1994). However, the losses of 
carbon in cultivated pastures can be minimal and, in some cases, the content of SOM can be even 
higher than in the natural ecosystem (Corazza et al., 1999). 
The isotopic composition of the carbon can be successfully used for SOM dynamic studies 
(Balesdent and Mariotti, 1996). The 13C/,3C ratio widely differs between plants that follow the C3-
and C4-photosynthetic pathways, providing a natural tracer when a C3 vegetation is substituted by a 
C4 one (Balesdent and Mariotti, 1996). 
In order to better understand the impact of converting native Cerrado into pasture, we studied the 
dynamics of different physically separated SOM pools in different depths of a Cerrado's Oxisol 
(Typic Haplustox), under natural conditions and after 23 years of cultivated pasture (Brachiaria 
s pp.) via the replacement of the native C (Crderived) by pasture C (C4-derived). 
2. MATERIAL AND METHODS 
The study was carried out at the Maize and Sorghum National Research Centre - Brazilian Institute 
of Agricultural Research (EMBRAPA-CPMS), Sete Lagoas-MG, Brazil. The area is located at 
latitude 19°26'S and longitude 44°10' W, and has a mean altitude of 730 m. The mean annual 
temperature is 22.1°C, and annual precipitation is 1340 mm, with a dry season from April to 
September. Two representative sites were selected in a homogeneous Dark Red Latosol (Typic 
Haplustox) soil unit Site 1 was a reserve of native "cerrado sensu-stricto" vegetation, dominated 
by Cj-plants (Roscoe et al, 2000). Site 2 was an adjacent cultivated pasture, where the native 
Cerrado was replaced by Brachiaria spp. 23 year before sampling. In the last year, however, the 
pasture was plowed (10 cm) and cultivated with millet The possible influence of the millet 
237 
cropping could be higher in the upper soil layer (Ap horizon). At the sampling time (two months 
after harvest), Brachiaria spp. was covering about 60% of the area. Though the millet material is 
also C^-derived, we neglected that influence and considered the last year as equivalent to the 
previous ones. Duplicated profiles were described per site and sampled until lm depth. 
SOM was separated after density and particle-size fractionation of soil. Air-dried soil samples (20 g 
of soil < 2 mm) of the Ah, AB|, AB2 and Bw2 horizons (Site 1), and Ap, AB, and Bw2 horizons 
(Site 2) were fractionated in triplicate. Low-density organic matter (LDOM) was separated by 
flotation using a Nal solution (1.7 g cm' ) after end-over-end shaking, getting the free-LDOM 
fraction, and after sonication dispersion (260 - 270 J ml"1), getting the occluded-LDOM fraction. 
The heavy fraction was then separated into sand (2000 - 50 urn) silt (50 - 2 um) and clay (< 2 um). 
Organic Carbon (OC) was analyzed for all the samples and fractions in an Interscience Elemental 
Analyser EA1180. I3C abundance was determined after the conversion of total C to C02, purified 
by CuO and Ag, in a VG/SDRA 9 Mass Spectrometer at the Centre for Isotope Research at the 
University of Groningen, the Netherlands. Results were expressed as 613C (%o). For the 
calculations, we used a mass balance equation (1), according to Balesdent and Mariora' (1996): 
TOC x S13CT = SOCo x Ô13Cc3 + SOCC4 x 5l3Cc4 (1) 
Where: TOC = total soil organic carbon; SOCo = soil organic carbon derived from C3 vegetation; 
SOCC4 = soil organic carbon derived from C4 vegetation; 513CT = 613C of total soil organic carbon; 
5l3Cc3 = 513C of soil organic carbon derived from C3 vegetation; ô13Cot = ÔBC of soil organic 
carbon derived from C4 vegetation. 
The reference values for Cj-derived carbon were the value observed for the Cerrado plot. The 
reference 8I3C value for the C^-derived carbon (C from pasture) was calculated for each depth and 
OC fraction using the S13C of the pasture litter (-13,52 ± 0.07%o) and the same ,3C proportional 
discrimination (D(%)) during the decomposition observed in the C3 reference profile. The D(%) was 
obtained by: D(%) = ( 5 l 3 C a ^ , 3 C f a e ^ M 0 0 / 8 u C W Where 8uCHaa is the 513C of the cerrado litter 
(-27.84 ± 0.50%o) and 5liCucaon is the Ö13C of each fraction at a given depth-
Linear and non-linear models were fitted to the data of 513C and OC contents as a function of 
depth. The least square criterion was used to select the best-fitted model for each of these two 
parameters in total soil and in different fractions. Fitted curves for Cerrado and pasture were 
compared by the F test at 5% of probability according to Snedecor & Cochran (1967). 
3. RESULTS 
No significant differences in OC contents were observed between the fitted models for Cerrado and 
pasture in the fractions: occluded-LDOM (Figure IC), sand (Figure ID), and silt (Figure IE). Then 
the average model for the two sites could describe the behavior of OC contents with depth. On the 
other hand, for total soil (Figure 1A), free-LDOM (Figure IB), and clay (Figure IF), significant 
differences were observed between the models using the F test at the level of 5% of probability 
(Snedecor & Cochran, 1967). However, considering the 95% confidence interval of the differences 
of the predicted values, only in the first 2 cm of the Cerrado topsoil significantly higher values of 
OC for total soil and free-LDOM were observed. In the clay fraction, the difference was not 
significant through out the profile. In general, the differences between the sites were close to zero, 
suggesting no significant losses of carbon after 23 years of cultivated pasture. 
No significant trend in S,3C values with depth was observed in the Cerrado plot for total soil, free-
LDOM, sand, silt, and clay. The average and standard deviation were respectively -24.87 ± 0.72%o, 
-27.19 ± 0.41%o, -25.52 ± 0.62%o, -25.55 ± 0.57%o, and -24,25 ± 0.68%«,. For the occluded-LDOM, 
613C increased logarithmically with depth till 50 cm depth, according to the equation 813C = 0.28 
Ln(depth) - 27.81 (R2 = 0.76***). In the pasture, plot the values of 8,3C were, in general, 
significantly higher than the Cerrado plot through out the profile for all fractions. A decreasing 
trend in the values was also observed for all the fractions (Table 1). 
238 
OC (g dmJ) OC (g dmJ) OC (g dm") 
0 ÎO «3 60 0 5 10 IS 20 0 02 OA 06 0« 
-Thrash 
6* 
o. 
4p> 
a. 
Si 
0-
10 
30 
30' 
40 
30 
60-
70. 
ao-
90 
m-
77 
/ / A 
n 
| 
J »C*rr*do 
ï y-»»«.-«125" 
I R*»0.» 
J • Pi»tur» 
I y .a008 . Ä n w ' 
f R*«0.94 
«Carodo 
aPmrtun 
y •21741« -> CWTWlO 
• P*Mur» 
°C*mdo 
y - -0 03«2i»13J 
« • • I U I 
• Futur« 
r--«oooV« 
R"«M2 
FJgnre 1. Total organic 
carbon (OC) contents in 
the Cerrado and pasture, 
as a function of depth: 
(A) for total soil; (B) for 
free-LDOM (low density 
organic matter); (C) for 
occluded-LDOM; (D) in 
sand (2000-0.05 urn); 
(E) in silt (0,05-2 um); 
and (F) in the clay 
fraction (<2 um). In A, 
B, and F, the two lines 
represent the equation 
fitted for data from 
Cerrado and pasture 
separately. In C, D, and 
E the line represents the 
equation fitted for data 
from Cerrado and 
pasture together. 
The percentage of carbon from the pasture present in the total soil and in each of the fractions was 
calculated by equation (1), using the integrated predicted values of OC and 513C. In each horizon, 
the OC from pasture substituted about the same quantity of Cerrado's OC in the total soil and in all 
the fractions, but occluded-LDOM (Table 2). In Ap, the free-LDOM substitution also tended to be 
slightly higher man in the other fractions. The occluded-LDOM showed the lowest replacement 
among all the fractions and horizons. For all the fractions the replacement of Cerrado's OC tended 
to decrease with depth. 
Table 1» Models describing the changes in SI3C with depth in the pasture plot 
Fraction Model R^ 
Free-LDOM 
Occluded-LDOM 
Sand (2000-50um) 
Silt (50-2unx) 
Clay (< 2um) 
Total soil 
Y - - 1.405 Ln(deptfa) - 18.6 
Y - - 0.834 Ln(deptn) - 23.7 
Y = -0.027 (depth)-21.1 
Y = - 0.032 (depth)- 20.3 
Y - -0.025 (depth)-19.8 
Y = -0.022 (depth)-20.1 
0.73 
0.97 
0.83 
0.70 
0.41* 
0.40' 
4. DISCUSSION AND CONCLUSIONS 
After 23 years of pasture, the levels of organic carbon in the studied soil did not change 
significantly in the first 100 cm. The high biomass production (especially roots) of the tropical C,-
grass Brachiaria spp. (Corazza et al. 1999) was probably able to maintain more or less the same 
supply of OC to the soil. Similar studies in Cerrado have shown that cultivated pastures not only 
can maintain the OC contents of the soil, but may also increase it (Carazza et al., 1999). 
239 
Table 2. Percentage of the total organic carbon in the whole soil and in the different fractions originated from 
the pasture after 23 years. The values were integrated for each horizon from the combination of (he models 
describing OC and SUC distribution with depth. 
Horizon* Free- Occluded- Sand Silt Clay Whole 
LDQM LDOM (20O0-50u.m) (S0-2um) (< 2pm) Soil 
Ap (0-9cm) 
AB,(10-25cm) 
AB2 (26-46cm) 
Bw, (47-75cm) 
Bw2 (76- 100cm) 
47 
32 
25 
20 
16 
17 
6 
1 
-
-
33 
30 
26 
21 
16 
y0 . 
39 
36 
31 
25 
19 
35 
34 
31 
24 
19 
36 
33 
29 
26 
22 
* depth of the horizon in Site 2 (pasture). 
In general, the high replacement of OC from Cerrado by pasture material showed that the carbon 
dynamics m this soil was relatively fast, regardless to its high clay content (>700 g kg"1). The OC 
half-life in the total soil and fractions (except occluded-LDOM) was in average 34 year in the Ap 
horizon, increasing to 85 years in Bw2. These findings were in accordance with previous studies 
suggesting a fast turnover time of SOM in tropical Oxisols (Shang and Tiessen, 1997). 
In contrast with our results, higher turnover times for free-LDOM than for heavy fractions (sand, 
silt, and clay) were usually reported in literature (Golchin et al., 1995). However, we observed 
abundant fragments of charcoal in the free-LDOM throughout the pasture profile, with a S13C of-
27.94 ± 0.50%o. The; charcoal probably was responsible for a substantial part of the remaining 
carbon from Cerrado in this fraction. 
The occhided-LDOM showed the lowest replacement of Cerrado derived.carbon among the soil 
fractions. This fraction was removed after aggregate disruption, suggesting that the aggregation 
was somehow protecting OC from decomposition (Golchin et al., 1995). Although the contents of 
carbon in this fraction in relation to total soil was very small (Figure 1), it might give a rough 
estimation of the aggregate turnover and processes of OC protection. A better understanding of the 
occluded-LDOM would provide useful information about aggregation processes and management 
impact in Oxisols. 
5. REFERENCES 
Balesdent, J. & Mariotti, A. 19%. Measurement of soil organic matter turnover using 13C natural abundance. 
In: Boutton, T.W. & Yamasalci, S. Mass spectrometry of soil. Marcel Dekker, New York. pp. 47-82. 
Corazza, EJ., Silva, J.E., Resck, D.V.S. & Gomes, AC. 1999. Comportamento de diferentes sistemas de 
manejo como fonte ou deposito de carbono em relacao a vegetacao de cerrado. Re vista Brasileira de 
Ciencias do Solo, 23:425-432. 
Golchin, A, Oades, J.M., Skjemstad, J.O. & Clarke, P. 1995. Structural and dynamic properties of soil 
organic matter as reflected by 13C natural abundance, pyrolysis mass spectrometry and solid-state 13C 
NMR spectroscopy in density fractions of an Oxisol under forest and pature. Australian Journal of Soil 
Research, 33: 59-76. 
Roscoe, R., Buurman, P., Velthorst, EJ. & Pereira, J A.A. 2000. Effects of fire on soil organic matter in a 
"Cerrado sensu-stricto" from Southeast Brazil as revealed by changes in 5,JC. Geoderma, (in press). 
Shang, C. & Tiessen, H.1997. Organic matter lability in a tropical Oxisol: evidence from shifting cultivation, 
chemical oxidation, particle size, density, and magnetic fractionations. Soil Science, 162(11): 795-807. 
Silva; JJL, Lemainski, J. & Resck, D.V.S. 1994. Perdas de materia organica e suas relacoes com a 
capacidade de troca cationica em solos da regiao de cerrados do oeste baiano. Revista Brasileira de 
Cieocia do Solo, 18: 541-547. 
Snedecor, G.W. & Cochran, W.G. 1967. Analysis of covariance: comparison of regression lines. In: 
Snedeeor, G.W. & Cochran, W.G. (Eds.) Statistical methods, 6* ed., Ames, Iowa, The State University 
Press. P. 432-436. 
240 


Soil organic matter dynamics in density and particle-size fractions as revealed by the 13C/12C isotopic ratio in a cerrado oxisol

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