www.elsevier.com/locate/catena

Catena 58 (2004) 125–150

Monitoring soil organic carbon erosion with d13Cand d15N on experimental field plots in the

Venezuelan Andes

Boris Bellangera,*, Sylvain Huona,*, Fernando Velasquezb,Vincent Vallesc,1, Cyril Girardina, Andre Mariottia

aUMR 7618 (UPMC- INRA- CNRS) BIOMCO, Universite Pierre et Marie Curie (UPMC),

Case 120, 4 place Jussieu, 75252 Paris cedex 05, FrancebUnidad PLANINDECH, Centro de Ecologia de Bocono. Universidad de los Andes. Bocono,

Estado Trujillo, Venezuelac INRA Science du Sol Domaine St Paul, Site Agroparc, 84914 Avignon Cedex 9, France

Received 12 March 2003; received in revised form 10 December 2003; accepted 12 March 2004

Abstract

The question of discriminating sources of organic matter in suspended sediment transported by

streams can be addressed by using total organic carbon concentration and stable isotope (d13C, d15N)measurements when constant fluxes of organic matter supply can be assumed. However, little is

known about the dynamics of organic matter release during soil erosion and the temporal stability of

its isotopic composition. In this study, we have monitored soil organic carbon loss and water runoff

using natural rainfall events during the 1998 rainy season on three experimental field plots with

different vegetation covers (bare, maize and coffee plots), set up on slopes of a tropical mountainous

watershed in NW Venezuela (Bocono watershed, 08j57V–09j31VN, 70j02V–70j34VW). Runoff

and soil organic carbon losses were considerably higher for the bare field plot than for the cultivated

field plots (270 and 2.2–21.0 kg C ha� 1, respectively), and these findings are consistent with other

soil erosion experiments. Total organic carbon concentrations, total nitrogen concentrations and the

d13C and d15N composition of fine ( < 50 Am) size suspended sediments remain constant for high

runoff periods (>30 ml s� 1) and ‘‘high’’ suspended matter concentrations (>0.5 g l� 1), closely

reflecting the composition of soil organic matter from which they originate and integrating the high

variability of organic carbon in top soil horizons. Because runoff and soil organic carbon loss are

closely linked during most of the water flow (at the time scales covered by this study), the

0341-8162/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.catena.2004.03.002

* Corresponding authors. Tel.: +33-1-44-27-51-74; fax: +33-1-44-27-41-64.

E-mail addresses: bbellan@ccr.jussieu.fr (B. Bellanger), huon@ccr.jussieu.fr (S. Huon).1 Present address: Laboratoire Chimie et Environnement. Case 29, Universite de Provence (Aix-Marseille I),

3 Place Victor Hugo. 13331 Marseille Cedex 3, France.

B. Bellanger et al. / Catena 58 (2004) 125–150126

contribution of soil organic matter, originating from soils with different compositions, to suspended

sediments in runoff can be derived using soil organic carbon erosion rates and stable (d13C and d15N)isotope measurements.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Soil erosion; Experimental field plots; Organic matter; Stable isotopes; Venezuela

1. Introduction

Organic matter plays a key role in soil physical and chemical properties such as

structural stability, porosity, nutrient availability, and ion-exchange capacity (e.g., Oades,

1986; Le Bissonnais, 1990; Tiessen et al., 1994; Lal et al., 1999; Roscoe et al., 2001).

Besides its major impact on soil quality, Soil Organic Matter (SOM) is the largest

terrestrial reservoir of organic carbon with an estimated total content of 1600 Pg C located

in the first meter, exceeding the terrestrial biosphere (560 Pg C) and atmosphere (750 Pg

C) storage capacities (Post et al., 1982; Eswaran et al., 1993; Sundquist, 1993).

Understanding linkages between carbon pools, in particular SOM, is of great interest

because atmospheric CO2 concentrations drastically increases as a result of anthropogenic

releases (e.g., Keeling et al., 1984; IPCC, 1990) and the behaviour of soils as a sink or

source for atmospheric CO2 is still debated (e.g., Ciais et al., 1995; Houghton et al., 1998).

Indeed, a rising atmospheric CO2 level coupled with global warming may stimulate

terrestrial photosynthesis (fertilisation effect) and enhance organic carbon sequestration in

soils (e.g., Houghton et al., 1993; Kirschbaum, 2000; Schlesinger and Andrews, 2000).

However, present-day deforestation and land use change question the efficiency of carbon

storage in soils, because enhanced water and tillage erosion may induce opposite trends

(e.g., Houghton, 1991; Lal, 1995; Houghton et al., 2000). This is particularly critical in

tropical regions that account for 30% of the total soil organic carbon reservoir (Milliman et

al., 1987; Ross, 1993; Dixon et al., 1994; Zech et al., 1997). Soil erosion leads to removal

and mineralization of organic matter and the global carbon flux generated into the

atmosphere may reach 1.14 Pg year� 1 (assuming that 20% of the carbon displaced is

easily decomposed; Lal, 1995).

Soil erosion budgets and models are based on empirical relationships between estimates

of the average organic carbon content of soils and that of suspended organic carbon

derived from river discharges (e.g., Esser and Kohlmaier, 1991; Meybeck, 1993, Ludwig

et al., 1996). However, they do not provide information on the nature and the origin of

eroded organic carbon and its further evolution in the hydrographic network. Some

refinement in soil organic carbon source identification can be achieved using d13Cmeasurements. Indeed, discrimination between SOM originating from vegetation covers

with different photosynthetic pathways (e.g., C3 and C4, Bender, 1968; O’Leary, 1981;

Schwartz et al., 1986; Balesdent et al., 1987; Farquhar et al., 1989; Mariotti and

Peterschmitt, 1994; Desjardins et al., 1994; Arrouays et al., 1995; Balesdent and Mariotti,

1996) or between autochthonous aquatic production and soil-derived detritus have been

successfully performed (e.g., Hedges et al., 1986; Wada et al., 1987; Ittekot, 1988; Mariotti

et al., 1991; Bird et al., 1992, 1994; Thornton and McManus, 1994; Onstad et al., 2000;

B. Bellanger et al. / Catena 58 (2004) 125–150 127

Masiello and Druffel, 2001). When coupled with d13C, d15N measurements may also

provide a constraint on the source of nitrogen (Mariotti et al., 1984) or highlight the

presence of lithic-derived organic matter components (e.g., Kao and Liu, 1996, 2000) that

may bias organic carbon budgets solely based on soil erosion processes (Meybeck, 1993).

These overall applications are based on the general assumption that constant organic

carbon fluxes are generated during soil erosion and that the isotopic composition of

displaced organic carbon does not vary with time. This question cannot be solely

addressed on the basis of organic carbon concentration measurements and organic carbon

pools identification because the distribution of organic matter in soils is not homogenous

(Eswaran et al., 1995). Moreover, the extent of soil organic carbon mobilisation in water

flows relies on several interacting factors described by the Universal Soil Loss Equation

(USLE, Wischmeier and Smith, 1978). The objective of this study is to monitor

simultaneously soil organic carbon loss and water runoff on experimental field plots with

different vegetation covers under natural rainfall conditions in order to: (1) better assess

the relationship between runoff and soil organic carbon release during soil erosion and (2)

examine the reliability of total organic carbon concentration and stable isotope (d13C,d15N) composition measurements in suspended sediments generated by soil erosion to

identify and monitor sources of soil organic matter in runoff waters.

2. Material and methods

2.1. Physiographic setting

The experimental field plots were set up on natural high slopes of the Rio Corojo sub-

catchment (25 km2) in the Bocono watershed (1620 km2). Located in NW Venezuela (Fig.

1, 08j57V–09j31VN, 70j02V–70j34VW), the Bocono watershed represents about 8% of

the Venezuelan Andes. The tropical climate conditions prevailing in the region are highly

influenced by altitude, which ranges from 200 to 4000 m asl in the watershed. Average

temperature decreases with altitude by 0.6 jC per 100 m. Precipitation also displays a

decreasing trend with altitude, from 1500 to 2500 mm in the low parts of the Bocono

watershed to less than 1000 mm in the Paramo regions, located 2700–4000 m asl

(Cornieles, 1998). About 87% of the total rainfall occurs during the rainy season, between

April and October, with a maximum supply in June and July (MARNR, 1991 in

Rodrıguez, 1999). Mean annual temperature, precipitation, and evapotranspiration at the

study area location in the Corojo sub-catchment (1200 m asl) are 21 jC, 1600, and 980

mm, respectively (Romulo Quintero, 1999). The main stream of the watershed is the

Bocono river whose discharge averages 77.65 m3 s� 1 (1952–1973 record; MARNR, 1991

in Rodrıguez, 1999) with a mean sediment load of 7 106 t year� 1 (Cornieles, 1998). In the

upper part of the Bocono watershed, flood events in the range 200–400 m3 s� 1 occur with

return periods of 2–10 years, respectively (1963–1990 record; Velasquez and Perez,

1990).

Ultisols are the most common soil types found on the Bocono watershed (Perez, 1997).

These soils are developed on a geological basement composed of Palaeozoic metasedi-

mentary rocks and Quaternary alluvial deposits in the southern part of the watershed,

Fig. 1. Location of the experimental field: (A) location of the Bocono Watershed in Venezuela, (B) physiographic

map of the Bocono watershed, (C) location of the field plots on the Corojo sub-catchment.

B. Bellanger et al. / Catena 58 (2004) 125–150128

whereas crystalline rock formations mainly outcrop on the northern part (Gonzales de

Juana et al., 1980; Pouyllau, 1989). About 42% of the watershed is occupied by rain forest

(500–1700 m asl) and dry mountainous forest (1500–2700 m asl). Paramo-type

vegetation and bush fallow stand for 17.5% of the area. The remaining land is devoted

to agricultural use (i.e., maize, coffee and living productions).

2.2. Experimental field plots

Soil loss and runoff monitoring experiments were carried out during the 1998 rainy

season under natural rainfall conditions. The main soil characteristics are displayed in Table

1. The plots (3� 10 m) were designed by the PLANINDECH group of the Centro de

Ecologia (Universitad de Los Andes, Bocono), using metal cut-off walls on three sides. The

down-slope boundaries are equipped with a collector system using covered drainpipes and

drums (e.g., Felipe-Morales et al., 1977; Ataroff and Monaterio, 1997). The runoff experi-

ments were conducted on cultivated land with different land use: a bare field plot, formerly

under maize andmaintainedmanually free of vegetation during the experiment, amaize field

plot (Zea mays), and a coffee field plot (Coffea arabica). Coffee is commonly cultivated

under a 4–5-m-high tree cover on high slopes at medium altitudes (500–1500 m asl, Ataroff

and Monaterio, 1997), whereas, maize fields are installed near stream channels.

Table 1

Characteristics for the three experimental field plots

Bare field Coffee field Maize field

Slope (%) 35 60 35

Sand 63–2000 Am (wt.%) 56 46 56

Silt 2–63 Am (wt.%) 18 26 20

Clay < 2 Am (wt.%) 26 28 24

Apparent density (g cm� 3) 1.83 1.72 1.86

Bulk organic matter (wt.%) 1.67 3.78 2.33

pH 5.4 5.5 5.7

B. Bellanger et al. / Catena 58 (2004) 125–150 129

2.3. Sample collection and analytical procedures

Runoff and sediment discharge calculations are based on weekly measurements of the

volume of water and soil in the collecting tanks, performed from April 15 to September 15.

Sample aliquots were also collected in the collecting tank. In addition, the June 14 flood

event was covered in detail with a sampling frequency of 5–30 min. Samples were air-dried

for transport, dispersed in deionised water using ultrasonic wave treatment for 10 min and

split into three grain size fractions using wet sieving ( < 50, 50–200 and >200 Am). Samples

were treated with 1 N HCl at room temperature in order to remove carbonates prior to

elemental and stable isotope carbon and nitrogen analyses, using a pH-controlled procedure,

known to have very little leaching effect on SOM (Midwood and Boutton, 1998; Huon et al.,

2002). Samples were thoroughly rinsed in deionised water to remove dissolved salts. The

solid residue was recovered by centrifugation (5000 rpm for 30 min), dried at 50 jC and

hand-ground with a mortar, following a procedure commonly used for stable isotope studies

of sedimentary organic matter (e.g., Schubert and Nielsen, 2000a). Due to the recovery

procedure employed, the fine ( < 50 Am) size fraction is composed of particles greater than

0.02 Am and smaller than 50 Am.

Total organic carbon (TOC), total nitrogen (TN, i.e., organic and inorganic nitrogen),

d13C and d15N isotope ratios were measured on the same sample aliquots for < 50 and 50–

200 Am size fractions by EA-IRMS (Carlo-Erba NA-1500 NC Elemental Analyser on line

with a Fisons Optima Isotope Ratio Mass Spectrometer). The d13C and d15N values are

reported in per mil (x) relative to Pee Dee Belemnite (PDB) standard and relative to air

N2, respectively. TOC and TN concentrations are reported in mg g� 1 of dry sample.

Analytical precision (F 1r) was F 0.1x and F 0.2x for d13C and d15N, respectively.The average precision on concentration measurements was F 0.1 mg C g� 1 for TOC and

F 0.05 mg N g� 1 for TN. Data reproducibility was checked by three replicate analysis of

selected samples and of a tyrosine laboratory standard (d13C =� 23.2F 0.1x and

d15N = 10.05F 0.30x).

3. Results

The data obtained in this study correspond to a 5-month experiment and therefore,

cannot be considered statistically meaningful in terms of long-term runoff and soil loss

B. Bellanger et al. / Catena 58 (2004) 125–150130

budgets. Indeed, assessment of these latter parameters for diagnostic and preventive

purposes requires at least a 5-year survey that is beyond the scope of this study (e.g.,

Lal, 1990; Starr et al., 2000).

3.1. Precipitation, runoff and soil loss data

Daily precipitation from April 15 to September 15 amounted to 897 mm, a third of

which fell in June (Fig. 2A). Plots of mean runoff or mean soil loss against mean

precipitation reveal different trends between cultivated and bare field plots (Fig. 2B,C).

Mean runoff and mean soil loss range from 0.1 to 27.1 mm day� 1 and from 0.01 to

2.60 kg day� 1, respectively, for the bare field plot. The values obtained for the coffee

field plot are much lower, in the range 0.02–0.9 mm day� 1 for runoff and 0.001 to

0.05 kg day� 1 for soil loss. We observed a threshold around 6 mm day� 1, above

Fig. 2. (A) Daily precipitation from April to September 1998 (at the Corojo station). (B) Relationships between

mean precipitation and mean runoff expressed in log– log scale. (C) Relationships between mean precipitation

and mean soil loss expressed in log– log scale. (D) Relationships between mean soil loss and mean runoff

expressed in log– log scale. Correlation coefficients are reported for each regression line.

B. Bellanger et al. / Catena 58 (2004) 125–150 131

which runoff, and to a lesser extent soil loss, increases significantly. According to the

regression coefficients calculated, meaningful correlations linking average precipitation,

runoff and soil loss are only obtained for the bare field plot experiment. The

relationship between mean soil loss and mean runoff is weakly supported in particular

for the coffee field plot (Fig. 2D). The bare field experiment provides an overall

runoff coefficient of 20% (the ratio of runoff to precipitation) and a cumulative soil

loss of 18 t ha� 1 in contrast to 1.3% and 0.5 t ha� 1 for the coffee field experiment

(Table 2).

The June 14 rainfall event lasted 6 h and amounted to 43.5 mm, i.e., one of the

highest precipitation amounts during the 1998 rainy season (Fig. 2A). According to the

available 30-year meteorological record (Rodrıguez, 1999), this rainfall event has a

return period of 1 year and thus, is representative of ‘‘intense’’ episodes on the Bocono

watershed. For this experiment simultaneous monitoring of runoff and associated

sediment was performed (Figs. 3A and 4A). Runoff and soil loss are in the same

range for the two cultivated plots but, much lower (l/30 and 1/120, respectively) than

for the bare field plot (Table 2), as previously observed for the 5-month experiment

and other long-term experiments (Roose, 1977; Lal, 1990). However, these ratios

increase if data covering the entire season are taken into account (Table 2). Total

suspended matter (TSM) concentrations roughly match the distribution of runoff

intensity, with maximum values up to 28.9 g 1� 1 for the bare field plot, 1.4 and

2.7 g 1� 1 for the coffee and maize field plots respectively. The grain size distribution

of suspended matter closely reflects that of the soil surface horizons for the bare field

plot but the cultivated plots mainly provide fine ( < 50 Am) size sediments (Table 3).

Some discrepancies between maximum runoff rates and maximum sediment discharges

are observed at both time scales (Fig. 5).

Table 2

Hydrological and soil data for the three experimental field plots

Bare field Coffee field Maize field

Exp. 1a Exp. 2b Exp. 1 Exp. 2 Exp. 1 Exp. 2

Precipitation (mm) 897 43.5 897 43.5 656 43.5

Runoff (mm) 182 27 12 0.9 3.9 0.8

Runoff coefficient (%)c 20 62 1.3 2.0 0.6 2.0

Total soil loss (kg ha� 1) 18,000 860 500 7 90 20

Total TOC loss (kg C ha� 1) 270 13 21 0.3 2.2 0.5

Total TN loss (kg N ha� 1) 27 1.3 1.85 0.03 0.2 0.05

d13C vs. PDB < 50 Am (x)d � 19.7 20.1 � 25.5 � 24.9 – �18.6

d15N vs. Air < 50 Am (x)d 6.1 5.8 4.0 5.3 – 6.1

TOC < 50 Am (mg C g� 1)d 35.3 31.6 67.4 44.7 – 25.1

TN < 50 Am (mg N g� 1)d 3.6 3.2 5.9 4.3 – 2.5

TOC/TN<50 Amd 9.9 9.9 11.4 10.4 – 10.0

a Five-month experiment (April 15 to September 15, 1998).b June 14 rainfall event.c Runoff/Precipitation ratio.d Mean weighted values for suspended organic matter.

Fig. 3. Plots of temporal records on the bare field plot during the June 14 rainfall event: (A) runoff and total

suspended matter concentration (TSM), (B) suspended matter concentration and TOC concentration in the fine

( < 50 Am) size fraction and (C) d13C and d15N compositions of fine suspended matter ( < 50 Am) The black

arrows refer to periods of minimum rainfall intensity. Time is in fractional (-decimal) hours. Error bars are

included in the size of the symbols.

B. Bellanger et al. / Catena 58 (2004) 125–150132

3.2. Soil organic matter data

3.2.1. Soil data

Reference soil surface horizons display the decreasing TOC–TN concentrations

and increasing d13C–d15N depth trends (Table 4) usually reported in soil studies

(e.g., Balesdent and Mariotti, 1996). The extent of these depth-related changes is

lower for the bare field plot than for the two cultivated ones. The d13C values of

SOM reflect the C3 and C4 photosynthetic pathways of coffee and maize covers, with

lower values in the coffee-bearing soil horizons (see further in the text). Soil organic

matter on the bare field plot, formerly under maize, keeps a maize-like d13Csignature. However, a slight d13C increase and TOC decrease is observed near the

surface ( < 10 cm, Table 4). d15N values in the coffee field plot are significantly

lower than for the two others field plots and increase with depth ( + 2.4x at 20 cm

depth with respect to surface), whereas they are almost constant for the bare and

maize field plots. Below 10 cm, all field plots display comparable fine-sized TOC and TN

concentrations. However, the coarser (50–200 Am) size fractions of the coffee soil

horizons are highly enriched in organic carbon and nitrogen compared to the other field

plots (Table 4).

3.2.2. Suspended matter data

Most of suspended sediment yield from the cultivated field plots is represented by

fine ( < 50 Am) size fractions (Table 3). Settling processes in water flows are reduced

with respect to coarse size material. Temporal variations in TOC, TN, d13C and d15Nvalues of suspended sediment are displayed in Figs. 3B,C and 6A,B, for the bare field

plot experiments. Elemental concentrations and stable isotopes values are equivalent for

both time scales. In contrast, more discrepancy is shown for the coffee field plot data

(Figs. 4B,C and 6C,D). TOC and TN concentrations are approximately double for the

5-month experiment compared with the single rainfall event. However, d13C and d15Nvalues remain similar in both cases. Due to very low runoff and sediment accumulation

in the outlet reservoir, too few data are available for the maize field plot. Results from

the maize field plot for the June 14 event are displayed in Fig. 4B and C. Elemental

concentrations and stable isotopes values do not show meaningful variations except for

d13C.In order to compare the two experiments for each field plot, the relationships

between organic matter data (TOC, TN, d13C, d15N) and suspended sediment concen-

trations are plotted in Fig. 7. All data superimpose following hyperbolic trends for the

bare field plot (Fig. 7A–D). For sediment concentrations greater than 0.5 g l� 1 nearly

constant values are obtained, with averages (F 1r): TOC= 31.1F1.4 mg C g� 1,

TN = 3.1F 0.2 mg N g� 1, d13C =� 20.0F 0.3x, d15N = 6.2F 0.3x (n = 14). When

runoff is low, soil loss mainly provides 13C-depleted and organic carbon-rich matter

whereas, 13C-enriched and organic carbon-poor matter predominates during periods of high

runoff and sediment output. In contrast, the relationships obtained for the coffee field plot are

more puzzling (Fig. 7E,F). TOC, TN, d13C and d15N values are independent of sediment

concentrations for the June 14 flood event and some variability is noticed for the 5-month

experiment.

B. Bellanger et al. / Catena 58 (2004) 125–150 133

B. Bellanger et al. / Catena 58 (2004) 125–150134

Table 3

Grain size distribution of soils and suspended sediment for the three experimental field plots (June 14 event)

Bare field Coffee field Maize field

Soila/sedimentb Soil/sediment Soil/sediment

[>200 Am] (wt.%) 31/37 40/4 38/1.5

[50–200 Am] (wt.%) 22/18 10/3 18/1.5

[ < 50 Am] (wt.%) 47/45 50/93 44/97

a Average concentrations in 0–20 cm soil horizons.b Mean weighted concentrations.

B. Bellanger et al. / Catena 58 (2004) 125–150 135

3.2.3. Comparison between soil and suspended matter data

The compositions of fine suspended organic matter falls in the range of the SOM

compositions for both field plots (Figs. 3, 4, 6 and Table 4) except for the TOC and TN

concentrations for the coffee field for the 5-month experiment. The overall TOC and TN

concentrations in suspended matter and soil samples of the three field plots are linearly

correlated with a zero-intercept very close to zero (Fig. 8). The TOC/TN ratios deduced

from these relationships average 10 for all fine fractions and 17 for coarser size fractions.

Additional relationships between organic matter data can be portrayed, in particular

between d13C and TOC concentrations (Fig. 9).

4. Interpretation and discussion

4.1. Relationship between runoff and soil erosion

Water runoff and soil loss evolve together for both the 5-month and the single rainfall

event experiments. However, a close match between sediment yield and runoff is only

displayed at the beginning and at the end of the rainy season in the 5-month experiments with

maximum deviations in June and July (Fig. 5A,C). These deviations are best explained by

the temporal succession of flushing effects (e.g., Martins and Probst, 1991; Laraque et al.,

1995) and soil crusting effects (e.g., Lal, 1990; Valentin and Bresson, 1992; Greene and

Ringrose-Voase, 1994). Indeed, crusting effects favour higher water runoff and lower

sediment yields, whereas destruction of surface crusts leads to concentrated flows.

Accordingly, runoff coefficient and soil loss increase at the beginning and at the end of

the rainy season (Fig. 5), whereas the rainiest months (June and July) provide a negative

trend connecting crusting (Fig. 5E, point b) and flushing effects (Fig. 5E, point a).

High-resolution monitoring of the June 14 event provides simultaneous soil loss and

runoff records. Transport and deposition processes are observed for the bare field plot, on

the basis of changing sediment transport rates (denudation rate in kg m� 2 time� 1) with

Fig. 4. (A) Relationships between runoff and total suspended matter (TSM) for the coffee and maize field plots

during the June 14 event. Plots of temporal records obtained for the coffee and maize field plots during the June

14 rainfall event: (B) TOC and TN concentrations in fine ( < 50 Am) size suspended matter, (C) d13C and d15Nvalues of fine ( < 50 Am) size suspended matter. Time is in fractional (-decimal) hours. Error bars are included in

the size of the symbols.

Fig. 5. Plots of temporal records of cumulative runoff and suspended matter: (A) 5-month data for the bare field

plot, (B) June 14 event data for the bare field plot, (C) 5-month data for the coffee field plot, (D) June 14 data for

the coffee field plot, (E) relationship between runoff coefficient and soil loss (data from the 5-month experiment

on the bare field plot). Points a and b are discussed in the text. Time is in fractional (-decimal) hours.

B. Bellanger et al. / Catena 58 (2004) 125–150136

respect to runoff (Fig. 5B). A decrease in runoff rate corresponds to a near-to-zero TSM

yield. Export of soil particles is the main process when the runoff rate is high. For the

coffee field plot, runoff and sediment yield increase at a similar rate without any evidence

of a step effect (Fig. 5D). Such an effect is pictured by change in the slope of the

cumulative runoff curve with respect to that of the cumulative TSM (Fig. 5A,B), indicating

Table 4

Soil organic matter data for the three experimental field plots

Land

use

Soil

depth

(cm)

d13C < 50 Amvs. PDB (x)

d15N < 50 Amvs. Air (x)

TOC

< 50 Am(mg C g� 1)

TN

< 50 Am(mg N g� 1)

TOC/TN

< 50 Amd13C 50–200 Amvs. PDB (x)

d15N50–200 Amvs. Air (x)

TOC

50–200 Am(mg C g� 1)

TN

50–200 Am(mg N g� 1)

TOC/TN

50–200

Am

Bare 0–2 � 18.5 7.1 31.3 2.9 10.6 � 20.5 4.7 8.8 0.7 12.5

2–6 � 19.3 6.8 32.7 3.0 10.8 � 22.9 4.7 6.3 0.5 13.1

6–10 � 19.3 6.7 31.4 3.1 10.1 � 23.1 4.8 5.3 0.4 13.0

10–15 � 18.9 6.4 29.8 2.7 11.0 � 22.8 4.8 4.3 0.3 13.1

15–20 � 17.9 6.7 26.1 2.3 11.4 � 21.7 5.0 2.6 0.2 12.8

Coffee 0–2 � 25.6 3.8 51.2 4.0 12.7 � 29.1 0.6 65.2 2.9 22.6

2–6 � 25.0 4.0 42.6 3.8 11.2 � 27.9 1.6 44.0 2.2 20.2

6–10 � 23.7 4.4 33.7 3.3 10.1 � 27.2 1.8 28.7 1.6 18.3

10–15 � 23.6 4.7 33.2 3.2 10.2 � 27.8 2.0 30.3 1.6 19.5

15–20 � 22.3 5.5 28.6 3.0 9.5 � 26.5 2.6 15.2 0.9 16.9

20–25 � 21.1 6.2 24.8 2.7 9.2 � 25.5 – 9.2 0.6 14.9

Maize 0–5 � 20.3 6.3 35.2 3.5 10.1 � 23.5 4.2 9.2 0.7 13.3

5–10 � 20.1 6.2 33.9 3.3 10.3 � 23.3 4.0 6.7 0.5 12.7

10–15 � 18.8 5.9 27.3 2.7 10.2 � 22.2 4.1 4.3 0.4 11.4

B.Bella

nger

etal./Caten

a58(2004)125–150

137

Fig. 6. Plots of temporal records from April to September 1998 for the suspended fine ( < 50 Am) size fraction:

TOC and TN concentrations for the bare field plot (A), and for the coffee field plot (C); d13C and d15N values for

the bare field plot (B), and for the coffee field plot (D). Average (F 1r) TOC and TN concentrations and d13Cand d15N are reported for each graph. Error bars are included in the size of the symbols.

B. Bellanger et al. / Catena 58 (2004) 125–150138

that no sediment yield may take place when runoff is still high. This different behaviour is

also reflected by the contrasting grain size distribution of the suspended sediments, as

previously outlined (Table 3). Sediments released on the coffee field plot comprise almost

exclusively fine ( < 50 Am) size particles, whereas coarser (>200 Am) size fractions are also

recovered from the bare field plot, in particular during runoff maxima (Fig. 10).

Enrichment of overland flow with fine size particles has already been described (e.g.,

Wan and El-Swaify, 1997; Hairsine et al., 1999). The runoff rates observed for the

cultivated field plots are rather low ( < 8 ml s� 1, Fig. 4) and cannot produce significant

flow detachment of soil particles. The resulting overland flow is associated with low

sediment concentrations ( < 3 g l� 1, Fig. 4) where fine size particles dominate (93–97% of

total sediment yield, Table 3). For the bare field plot, runoff rates range 2–200 ml s� 1 and

suspended sediment concentrations reach a maximum value of 30 g l� 1 (Fig. 3A). Under

these conditions, flow shear stress and stream power exceed the critical value required for

rill erosion (Wan and El-Swaify, 1997). The hyperbolic relationship between total

suspended matter concentration and fine ( < 50 Am) grain size percentage (Fig. 10)

indicates that both overland flow (approximately below 2 g l� 1) and rill erosion

(approximately above 2 g l� 1) processes took place in the bare field plot during the June

14 flood event.

Fig. 7. Plots of fine ( < 50 Am) size suspended matter concentration against (A) TOC concentration, (B) TN

concentration, (C) d13C and (D) d15N for the bare field plot; (E) TOC concentration, (F) TN concentration, (G)

d13C and (H) d15N values for the coffee field plot. Dashed rectangles represent the range of variations in topsoil

horizons (0–20 cm). Error bars are included in the size of the symbols.

B. Bellanger et al. / Catena 58 (2004) 125–150 139

The Universal Soil Loss Equation (USLE, Wischmeier and Smith, 1978) links soil

erosion to a function of six factors among which vegetation cover is the most

important. Indeed, vegetation reduces thoroughly soil erosion by reducing raindrop

impact and enhancing soil porosity (e.g., Lal, 1990). Our results clearly support a

major vegetation cover effect since runoff and soil loss for the bare field plot are

Fig. 8. Correlation of TOC concentrations vs. TN concentrations for all fine ( < 50 Am) size fractions and all coarse

(50–200 Am) size fractions of soil and suspended matter samples. Error bars are included in the size of the symbols.

B. Bellanger et al. / Catena 58 (2004) 125–150140

markedly superior to those obtained for the coffee field plot: by a factor 15–36,

respectively, for the 5-month experiment, and by a factor 30–120, respectively, during

the June 14 event (Table 2).

Fig. 9. Plots of d13C vs. TOC concentrations for all fine ( < 50 Am) size fractions of soils and suspended matter

samples (SS). Error bars are included in the size of the symbols.

Fig. 10. Plots of time vs. fine ( < 50 Am) and coarse (>200 Am) suspended matter concentrations for the bare field

plot (A) and the coffee field plot (B). (C) Plot of total suspended matter concentrations vs. fine ( < 50 Am) size

suspended matter percentages (wt.%) for the three experimental field plots. Time is in fractional/decimal hours.

Error bars are included in the size of the symbols.

B. Bellanger et al. / Catena 58 (2004) 125–150 141

4.2. Soil organic carbon loss during field erosion experiments

The increase of d13C values and the decrease of TOC concentrations with soil

depth (Table 4) are usually interpreted as reflecting mineralization processes of

SOM, either isotopic discrimination associated with microbial respiration/fermentation

or preferential removal of 13C-depleted components. Both processes leave a 13C-

enriched residue in the soil organic matter pool (Balesdent and Mariotti, 1996). To

a lesser extent, the decrease of atmospheric CO2–d13C during the past 150 years

may also account for lower values in top surface soil horizons. Increasing d15N and

decreasing TN trends with soil depth in the coffee field plot (Table 4) are also

related to selective mineralization of SOM, leading to TN depletion and 15N

enrichment in the residual organic matter pool (Mariotti et al., 1980; Tiessen et

al., 1984). The amplitude of the variations is thoroughly reduced in the top soil

horizons of the bare and maize field plots, most likely due to tillage homogeni-

sation of SOM before or during the experiments. The higher TOC concentrations

found for the coffee field plot, in particular for the coarse (50–200 Am) size

fractions, reflect enhanced concentration of litter debris with respect to soil organo-

mineral aggregates. This interpretation is supported by the composition obtained for

a litter sample that represents a high TOC content (300 mg C g� 1) and low d13C(� 30x) end-member component. The major difference between SOM data from

bare and maize field plots is a slight 13C and 15N enrichment of topsoil SOM on

the bare field plot (Table 4). This difference records some mineralization effect,

resulting from the clearance of the field plot formerly under maize. Surprisingly, the

coarse 50–200 Am size fractions of SOM for the maize field plot do not provide

enriched 13C compositions typical of C4-derived soil organic matter (i.e., Balesdent

and Mariotti, 1996). This deviation is best explained by rotating C3–C4 plant

cultivation cycles and/or by fallow periods, which involves soil colonization by C3

plants. The linear correlation between TOC and TN previously outlined, reflects a

low contribution of inorganic nitrogen (NH4+ sorption by clay minerals) and most of

nitrogen can be regarded as organic in nature (e.g., Schubert and Nielsen, 2000b).

The composition of organic matter in suspended sediment samples generally

reflects that of surface soil horizons for each field plot (Figs. 3 and 4 and Table 4).

However, higher TOC and TN concentrations can be found during the 5-month

experiment on the coffee field plot (Fig. 6). This deviation indicates that these

samples underwent some selective SOM sorting and may reflect a bias in sample

collection (settling of suspended particles in the storage tank). However, TOC / TN,

d13C and d15N values are still consistent with SOM composition and this possible

bias does not change our interpretation of organic matter signature. When expressed

as a function of total suspended sediment load (Fig. 7), organic matter data follow

hyperbolic trends towards constant values that reflect approximately topsoil organic

matter composition. Deviation of suspended organic matter d13C and TOC from

SOM average values only accounts for a few percents of TOC loss during the

single rainfall experiment. In addition, the average weighted d13C and TOC values

obtained for the fine fractions of suspended organic matter (� 20.1x and 31.6 mg

C g� 1, respectively; Table 2) match SOM composition. Accordingly, the variations

B. Bellanger et al. / Catena 58 (2004) 125–150142

B. Bellanger et al. / Catena 58 (2004) 125–150 143

observed in the suspended organic matter record obtained in the framework of these

experiments should not be interpreted as reflecting some temporal evolution of SOM

towards an equilibrium state, following removal of the vegetation cover. The validity

of this assumption, of course, only refers to the time scales covered by this study.

Suspended organic matter compositions are rather a function of sediment load that

is monitored by runoff intensity. In contrast to the bare field plot, suspended

organic matter values (TOC, TN, d15N and d13C) for the cultivated field plots are

rather constant during the single rainfall event and in the range of SOM

compositions (Fig. 4).

The overall suspended matter values are spread out on hyperbolic mixing curves

that connect a TOC-depleted and 13C-enriched soil end-member on one side and a

TOC-enriched and 13C-depleted litter end-member on the other (Fig. 9). This

observation is corroborated by well-defined linear relationships between d13C and

the inverse of TOC concentration (r2 = 0.81 for bare field plot sediments, r2 = 0.88

for coffee field plot sediments, correlations not shown) that support the mixing

assumption (e.g., Balesdent and Mariotti, 1996). Distinct SOM pools are thus

involved in soil organic carbon erosion on the field plots. Cultivated lands (coffee

and maize in our study) should theoretically provide less TOC (in mg l� 1) to

stream flow when compared to bare soils but enriched-TOC detritus (in mg g� 1).

Accordingly, local input of terrestrial plant organic carbon in rivers can be

identified on the basis of higher organic carbon contents with respect to allochth-

onous soil organic matter derived from soil horizons (e.g., Hedges et al., 1986).

Moreover, because organic matter originating from deeper soil horizons is charac-

terized by 13C-enriched compositions when compared to vegetation cover, a more13C-depleted composition is also expected from local plant detritus inputs. On the

whole, a good match between the composition of fine ( < 50 Am) size organic

matter in soils and suspended particles can be inferred from these soil erosion

experiments and suspended organic particles integrate the high spatial variability of

organic matter composition in surface soil horizons (Eswaran et al., 1995; Hopkins

et al., 1998).

During the rainy season, soil loss reaches 18 t ha� 1 for the bare field plot and 0.5 t

ha� 1 for the coffee field plot (Table 2). These values can be assumed as yearly rates

because soil erosion mainly takes place during the rainy season. These results are

consistent with other studies in the tropics. Fritsch (1992) reported a multiplicative

increase of soil loss of 20–30 after deforestation, providing soil erosion rates in the range

8–20 t ha� 1 year� 1. Soil organic carbon releases are 270, 21 and 2.1 kg C ha� 1, for the

bare, coffee and maize field plots, respectively. Considering that the Bocono watershed is

composed approximately by 10% of bare/uncultivated soils, 30% of cultivated soils

(maize and other living productions) and 60% of soils under coffee or forest (Cornieles,

1998), we estimated a total soil organic carbon loss of 4 t C km�2 year�1. The particulate

organic carbon flux of 1.59 t C km� 2 year� 1 assigned by Ludwig et al. (1996) to the

Orinoco basin falls within the range of our estimates. As most of suspended sediment

supply to the Orinoco originates from the Andean watersheds (Meade et al., 1983), a

particulate organic carbon flux higher than the average flux calculated for the Orinoco

river watershed is also expected.

4.3. Reliability of organic carbon concentration and stable isotope measurements in

suspended sediments to monitor sources of soil organic matter in runoff waters

Quantitative discrimination between sources of dissolved compounds using geochem-

ical or stable isotope signatures and end-member mixing equations are widely used in

stream flow studies (e.g., Sklash and Farvolden, 1979; Bazemore et al., 1994; Ribolzi et

al., 1996; Ladouche et al., 2001). The relative stability of TOC, TN, d13C and d15N values

of suspended organic matter shown by these soil erosion experiments emphasizes the

possibility of extending these methods to solid components (e.g., Cai et al., 1988). The

validity of this potential use is constrained by several requirements that can be summarised

in the following: (1) the tracer is conservative at the scale of the study; (2) the composition

of the contributing end-members are known and differ sufficiently from each other and; (3)

the concentration of each tracer in each reservoir is constant. Although suspended organic

matter may be involved in a number of biogeochemical processes in rivers (e.g., Hedges et

al., 1986; Mook and Tan, 1991), constant TOC, TN, d13C and d15N values above a given

suspended sediment load (>0.5 g 1� 1 in this study) suggest a first-order conservative

behaviour during the transfer of SOM to water flow at the scale of the flood event. This

behavior is still validated by the 5-month continuous record with some minor bias. The

conditions listed above are apparently fulfilled during our experiments. However, because

soil shielding by vegetation controls the grain size distribution of the suspended particles

generated during soil erosion, only fine sized fractions ( < 50 Am in this study) are

expected to provide organic carbon concentration and stable (d13C and d15N) isotope

compositions that accurately match the composition of soil organic matter in the source

areas. Indeed, the extent of soil organic carbon transport outside the field plot (30 m2 in

this study) to the stream main channel is controlled by local topography. Temporary

sorting and deposition effects due to local changes in slope may occur before runoff waters

reach the main stream. Conservative transport can only be verified for fine ( < 50 Am) size

particles, at least during periods of sustained runoff as shown in these experiments for

different soil covers. Accordingly, the contribution of several sources of soil organic matter

with contrasted compositions can be monitored in runoff flows for fine-sized suspended

matter load using end-member mixing equations. A potential application of these results is

pictured below.

Using the estimates of organic carbon loss made for the bare and the coffee field plots

(270 and 21 kg C ha� 1, respectively, Table 2) and the average d13C value for suspended

sediment originating from the two field plots (� 19.7x and � 25.5x, respectively, Table

2), we can calculate, for fine size suspended sediments, the amount of organic carbon (Ct)

theoretically released by soil erosion for various bare land and coffee land surface ratios

and the corresponding d13Ct composition using the mixing equations:

Ct ¼ fb � Cb þ fc � Cc ð1Þ

d13Ct � Ct ¼ d13Cb � fb � Cb þ d13Cc � fc � Cc ð2Þ

where fb, Cb, d13Cb and fc, CC, d13Cc represent the fraction, specific organic carbon loss

and d13C of suspended sediments corresponding to the bare and coffee land surfaces,

B. Bellanger et al. / Catena 58 (2004) 125–150144

Fig. 11. Plots of simulated bare land surface fraction vs. organic carbon loss and d13C of suspended sediments for

an ideal mixture of organic carbon originating from bare soil and soil under coffee end-members. Data are

calculated using Eqs. (1) and (2) displayed in the text.

B. Bellanger et al. / Catena 58 (2004) 125–150 145

respectively (with fb + fc = 1). For an ideal two components mixture (bare vs. coffee-

cultivated soil surfaces), organic carbon losses are linearly correlated with the contribution

of land surfaces whereas, the d13C signature of suspended sediments is mainly controlled

by the high soil erosion rate on the bare field even for low surface contributions

(hyperbolic trend in Fig. 11). Accordingly, knowledge of specific erosion rates for each

source of soil organic matter pool may provide information on the relative contribution of

the bare and coffee land surfaces in the suspended fine size organic carbon load.

5. Conclusions

Monitoring soil organic carbon loss and water runoff using natural rainfall events on

experimental field plots provides information on the dynamics of organic carbon release

and on the reliability of suspended organic matter samples to reflect soil organic matter

composition. According to the results derived from a 5-month and a single rainfall event

experiments on field plots with different vegetation covers (bare soil, maize and coffee)

established on natural slopes in the Venezuelan Andes, several conclusions can be

drawn:

(1) Suspended matter export is mainly controlled by runoff fluxes with some discrepancy

during the rainy season, possibly induced by soil surface conditions. Runoff and soil

losses are greatly enhanced for the bare field plot with respect to the cultivated field

plots (i.e., 18 and 0.5 t ha� 1 for bare and coffee field plots, respectively). Soil

shielding by vegetation or tillage also generates a sorting effect that reduces grain size

in suspended matter flows.

B. Bellanger et al. / Catena 58 (2004) 125–150146

(2) During periods of ‘‘high’’ runoff (>30 ml s� 1) and ‘‘high’’ suspended matter

concentration (>0.5 g l � 1), erosion of bare soils provides fine grained ( < 50 Am)

organic carbon concentrations that closely reflect those of top soil horizons and

integrates soil spatial variability. Temporal stability of total organic carbon

concentration, total nitrogen concentration, d13C and d15N is only verified for fine

size suspended organic matter. In contrast, when runoff and sediment output are low,

suspended organic matter is mainly composed of litter-derived debris that deviate

from average surface soil organic carbon composition. Suspended organic matter

composition is apparently independent of runoff intensity for the two cultivated field

plots.

(3) The temporal stability of total organic carbon concentration and stable isotope (d13C,d15N) composition of fine sized suspended sediments generated by soil erosion

indicate that these measurements can be used to derive quantitative information on the

contribution of soil organic matter sources with different compositions (i.e., using end-

member mixing equations). If top soil organic matter compositions are known it may

also be possible to estimate (i.e., for simple two component mixtures), specific soil

erosion rates using suspended sediments data in stream flows. However, due to the

sorting and deposition effects that take place during suspended sediment transport by

runoff, reliable estimates of the contribution of soils with contrasted organic matter

compositions (i.e., covered with C3–C4 plants) can only be accurately derived from

fine sized fractions ( < 50 Am in this study).

Acknowledgements

This work was supported by the French INSU (Program 97 PROSE 53) and by the

Venezuelan MARNR. The authors wish to thank M. Cornieles-Valles, A. Moreau D. and

Colonel R. Mena Nava (Instituto Geografico de Venezuela Simon Bolivar) for their long-

term hospitality and support. We also benefited from technical help from G. Perez, R.

Castro, J. Romulo Quintero, A. Alfonso Rodrıguez (Centro de Ecologıa-Bocono). We are

also very grateful to M. Grably and G. Bardoux (LBI-Paris) for their help. H. Bendjoudi

(UPMC-Paris), C. Aubaud (GIS-Paris) and three anonymous reviewers kindly helped us

with their comments and suggestions to improve a former draft of this manuscript.

References

Arrouays, D., Balesdent, J., Mariotti, A., Girardin, C., 1995. Modelling organic carbon turnover in cleared

temperate forest soils converted to maize cropping by using 13C natural abundance measurements. Plant Soil

173, 191–196.

Ataroff, M., Monaterio, M., 1997. Soil erosion under different management of coffee plantations in the Ven-

ezuelan Andes. Soil Technol. 11, 95–108.

Balesdent, J., Mariotti, A., 1996. Measurement of soil organic matter turnover using 13C natural abundance. In:

Boutton, T.W., Yamasaki, S. (Eds.), Mass Spectrometry of Soils. Marcel Dekker, New York, pp. 83–111.

Balesdent, J., Mariotti, A., Guillet, B., 1987. Natural 13C abundance as a tracer for studies of soil organic

matter dynamics. Soil Biol. Biochem. 19, 25–30.

B. Bellanger et al. / Catena 58 (2004) 125–150 147

Bazemore, D.E., Eshleman, K.N., Hollenbeck, K.J., 1994. The role of soil water in streamflow generation in

a forested headwater catchment: synthesis of natural tracer and hydrometric evidence. J. Hydrol. 162,

47–75.

Bender, M.M., 1968. Mass spectrometric studies of 13C variations in corn and other grasses. Radiocarbon 10,

468–472.

Bird, M.I., Fyfe, W.S., Pinheiro-Dick, D., Chivas, A.R., 1992. Carbon isotope indicators of catchment vegetation

in the Brazilian Amazon. Global Biogeochem. Cycles 6 (3), 293–306.

Bird, M.I., Giresse, P., Chivas, A.R., 1994. Effect of forest and savannah vegetation on the carbon-isotope

composition of sediments from the Sanaga River, Cameroon. Limnol. Oceanogr. 39 (8), 1845–1854.

Cai, D.L., Tan, F.C., Edmond, J.M., 1988. Sources and transport of particulate organic carbon in the Amazon

River and estuary. Estuar. Coast. Shelf Res. 26, 1–14.

Ciais, P., Tans, P.P., Trolier, M., White, J.W.C., Francey, R.J., 1995. A large northern hemisphere terrestrial CO2

sink indicated by the 13C/12C ratio of atmospheric CO2. Science 269, 1098–1102.

Cornieles, M., 1998. Etude et modelisation des transferts d’eau, d’elements dissous et particulaires dans un bassin

versant torrentiel. Cas du Rio Bocono dans les Andes venezueliennes. Unpublished Doct. thesis Univ.

Avignon (France). 183 pp.

Desjardins, T., Andreux, F., Volkoff, B., Cerri, C.C., 1994. Organic carbon and 13C contents in soils and soil size-

fractions, and their changes due to deforestation and pasture installation in eastern Amazonia. Geoderma 61,

103–118.

Dixon, R.K., Brown, S., Houghton, R.A., Solomon, A.M., Trexler, M.C., Wisniewski, J., 1994. Carbon pools and

flux of global forest ecosystems. Science 263, 185–190.

Esser, G., Kohlmaier, G.H., 1991. Modelling terrestrial sources of nitrogen, phosphorus, sulphur and organic

carbon to rivers. In: Degens, E.T., Kempe, S., Richey, J.E. (Eds.), Biogeochemistry of Major World Rivers.

Scope UNEP, vol. 42. Wiley, New York, pp. 297–320.

Eswaran, H., Van Den Berg, E., Reich, P., 1993. Organic carbon in soils of the world. Soil Sci. Soc. Am. J. 57,

192–194.

Eswaran, H., Berg, E.V.D., Reich, P., Kimble, J., 1995. Global soil carbon resources. In: Lal, R., Kimble, J.,

Levine, E., Stewart, B.A. (Eds.), Soils and Global Change. Advances in Soil Science. Lewis Pub., Boca Raton,

pp. 27–43.

Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., 1989. Carbon isotope discrimination and photosynthesis. Annu.

Rev. Plant Physiol. Plant Mol. Biol. 40, 503–537.

Felipe-Morales, C., Meyer, R., Alegre, C., Vitorelli, C., 1977. Determination of erosion and runoff under various

cultivation systems in the Santa Amahuancayo region: I. Preliminary results of the 1974–1975 and 1975–

1976 seasons. An. Cient. U.N. 15 (1–4), 75–84.

Fritsch, J.M., 1992. Les effets du defrichement de la foret amazonienne et de la mise en culture sur l’hydrologie

de petits bassins versants. Operation ECEREX en Guyane franc�aise Doct. thesis, Univ. Montpellier II,

ORSTOM eds, Paris, 392 pp.

Gonzales de Juana, C., Ithurralde, A., Picart-Cadillat, X., 1980. Geologia de Venezuela y De Sus Cuencas

Petroleras. FONINVES, Caracas. 1031 pp.

Greene, R.S.B., Ringrose-Voase, A.J., 1994. Micromorphological and hydraulic properties of surface crusts

formed on a red earth soil in the semi-arid rangelands of eastern Australia. In: Ringrose-Voase, A.J., Hum-

phreys, G.S. (Eds.), Soil Micromorphology. Studies in Management and Genesis. Dev. Soil Sci., vol. 22.

Elsevier, Amsterdam, pp. 763–776.

Hairsine, P.B., Sander, G.C., Rose, C.W., Parlange, J.Y., Hogarth, W.L., Lisle, I., Rouhipour, H., 1999. Unsteady

soil erosion due to rainfall impact: a model of sediment sorting on the hillslope. J. Hydrol. 220, 115–128.

Hedges, J.I., Clark, W.A., Quay, P.D., Richey, J.E., Devol, A.H., Santos, U.M., 1986. Composition and fluxes of

particulate organic material in the Amazon river. Limnol. Oceanogr. 31 (4), 717–738.

Hopkins, D.W., Wheatley, R.E., Robinson, D., 1998. Stable isotope studies of soil nitrogen. In: Griffiths, H.

(Ed.), Stable: Isotopes, Integration Of Biological, Ecological and Geochemical Processes. Bios Scientific

Pub., Oxford, pp. 75–88.

Houghton, R.A., 1991. Tropical deforestation and atmospheric carbon dioxide. Clim. change 19, 99–118.

Houghton, R.A., Unruh, J.D., Lefebvre, P.A., 1993. Current land cover in the tropics and its potential for

sequestering carbon. Global Biogeochem. Cycles 7 (2), 305–320.

B. Bellanger et al. / Catena 58 (2004) 125–150148

Houghton, R.A., Davidson, E.A., Woodwell, G.M., 1998. Missing sinks, feedbacks, and understanding the role

of terrestrial ecosystems in the global carbon balance. Global Biogeochem. Cycles 12 (1), 25–34.

Houghton, R.A., Skole, D.L., Nobre, C.A., Hackler, J.L., Lawrence, K.T., Chomentowski, W.H., 2000. Annual

fluxes of carbon from deforestation and regrowth in the Brazilian Amazon. Nature 403, 301–304.

Huon, S., Grousset, F.E., Burdloff, D., Bardoux, G., Mariotti, A., 2002. Sources of fine-sized organic matter in

North Atlantic Heinrich layers: d13C and d15N tracers. Geochim. Cosmochim. Acta 66 (2), 223–239.

IPCC (Intergovernmental Panel on Climate Change), 1990. In: Houghton, J.T., Jenkins, G.J., Ephraums, J.J.

(Eds.), Scientific assessment of climate change-report of working group 1. Cambridge University Press,

Cambridge, UK, 365 pp.

Ittekot, V., 1988. Global trends in the nature of organic matter in suspensions. Nature 332, 436–438.

Kao, S.J., Liu, K.K., 1996. Particulate organic carbon export from a subtropical mountainous river (Lanyang Hsi)

in Taiwan. Limnol. Oceanogr. 41 (8), 1749–1757.

Kao, S.J., Liu, K.K., 2000. Stable carbon and nitrogen isotope systematics in a human-disturbed watershed

(Lanyang Hsi) in Taiwan and the estimation of biogenic particulate organic carbon and nitrogen fluxes.

Global Biochem. Cycles 14 (1), 189–198.

Keeling, C.D., Carter, A.F., Mook, W.G., 1984. Seasonal, latitudinal and secular variations in the abundance and

isotopic ratios of atmospheric carbon dioxide: 2. Results from oceanographic cruises in the tropical Pacific

Ocean. J. Geophys. Res. 89, 4615–4628.

Kirschbaum, M.U.F., 2000. Will changes in soil organic carbon act as a positive or negative feedback on global

warming? Biogeochemistry 48 (1), 21–51.

Ladouche, B., Probst, A., Viville, D., Idir, S., Baque, D., Loubet, M., Probst, J.-L., Bariac, T., 2001. Hydrograph

separation using isotopic, chemical and hydrological approaches (Strengbach catchment, France). J. Hydrol.

242, 255–274.

Lal, R., 1990. Soil Erosion in the Tropics: Principles and Management. McGraw-Hill, New York. 580 pp.

Lal, R., 1995. Global soil erosion by water and carbon dynamics. In: Lal, R., Kimble, J., Levine, E., Stewart,

B.A. (Eds.), Soils and Global Change. Advances in Soil Science. Lewis Pub., Boca Raton, pp. 131–142.

Lal, R., Mokma, D., Lowery, B., 1999. Relation between soil quality and soil erosion. In: Lal, R., Kimble, J.,

Levine, E., Stewart, B.A. (Eds.), Soil Quality and Soil Erosion. CRC Press, Boca Raton, pp. 237–259.

Laraque, A., Bricquet, J.P., Olivry, J.C., Berthelot, M., 1995. Transports solides et dissous du fleuve Congo (bilan

de six annees d’observations). In: Olivry, J.C., Boulegue, J. (Eds.), Grands Bassins Fluviaux Peri-Atlantiques:

Congo, Niger, Amazone, Actes Colloque PEGI-Paris. ORSTOM editions, Paris, pp. 133–145.

Le Bissonnais, Y., 1990. Experimental study and modelling of soil surface crusting processes. Catena. Suppl.,

vol. 17, pp. 13–28.

Ludwig, W., Probst, J.-L., Kempe, S., 1996. Predicting the oceanic input of organic carbon by continental

erosion. Global Biogeochem. Cycles 10 (1), 23–41.

Mariotti, A., Peterschmitt, E., 1994. Forest savanna ecotone dynamics in India as revealed by carbon isotope

ratios of soil organic matter. Oecologia 97, 475–480.

Mariotti, A., Pierre, D., Vedy, J.C., Bruckert, S., 1980. The abundance of natural nitrogen-15 in the organic matter

of soils along an altitudinal gradient (Chablais, Haute-Savoie, France). Catena 7, 293–300.

Mariotti, A., Lancelot, C., Billen, G., 1984. Natural isotopic composition of nitrogen as a tracer of origin for

suspended organic matter in the Scheldt estuary. Geochim. Cosmochim. Acta 48, 549–555.

Mariotti, A., Gadel, F., Giresse, P., Mouzeo, K., 1991. Carbon isotope composition and geochemistry of partic-

ulate organic matter in the Congo River (Central Africa): application to the study of Quaternary sediments off

the mouth of the river. Chem. Geol., Isot. Geosci. Sect. 86, 345–357.

Martins, O., Probst, J.-L., 1991. Biogeochemistry of major African rivers: carbon and mineral transport. In:

Degens, E.T., Kempe, S., Richey, J.E. (Eds.), Biogeochemistry of Major World Rivers. SCOPE, vol. 42.

Wiley, New York, pp. 127–155.

Masiello, C.A., Druffel, E.R.M., 2001. Carbon isotope geochemistry of the Santa Clara River. Global Biogeo-

chem. Cycles 15 (2), 407–416.

Meade, R.H., Nordin Jr., C.F., Hernandez, D.P., Mejia, A., Godoy, J.M.P., 1983. Sediment and water discharge

in Rio Orinoco, Venezuela and Colombia. Proceedings of the Second International Symposium on River

Sedimentation. 11–16 October, 1983, Nanjing, China Water Resources and Electric Power Press, Beijing,

pp. 1134–1144.

B. Bellanger et al. / Catena 58 (2004) 125–150 149

Meybeck, M., 1993. Riverine transport of atmospheric carbon: sources, global typology and budget. Water Air

Soil Pollut. 70, 443–463.

Midwood, A.J., Boutton, T.W., 1998. Soil carbonate decomposition by acid has little effect on d13C of organic

matter. Soil Biol. Biochem. 30 (10/11), 1301–1307.

Milliman, J.D., Quin, Y.S., Ren, M.E., Saito, Y., 1987. Man’s influence on the erosion and transport of sediments

by Asian rivers: the Yellow River (Huanghe) example. J. Geol. 95, 751–762.

Mook, W.C., Tan, F.C., 1991. Stable carbon isotopes in rivers and estuaries. In: Degens, E.T., Kempe, S., Richey,

J.E. (Eds.), Biogeochemistry of Major World Rivers. SCOPE, vol. 42. Wiley, New York, pp. 245–264.

Oades, J.M., 1986. The retention of organic matter in soils. Biogeochemistry 5, 35–70.

O’Leary, M.H., 1981. Carbon isotope fractionation in plants. Phytochemistry 20 (4), 553–567.

Onstad, G.D., Canfield, D.E., Quay, P.D., Hedges, J.I., 2000. Source of particulate organic matter in rivers from

the continental USA: lignin phenol and stable carbon isotope compositions. Geochim. Cosmochim. Acta 64

(20), 3539–3546.

Perez, G.J., 1997. Validacion de metodos de muestreo no convencionales de sedimentos en suspension. Caso

cuenca alta Rıo Bocono. Unpublished thesis Ing. Forestal. Universidad de Los Andes, Merida (Venezuela).

96 pp.

Post, W.M., Emanuel, W.R., Zinke, P.J., Stangenberger, A.G., 1982. Soil organic carbon pools and world life

zones. Nature 298, 156–159.

Pouyllau, M., 1989. Geomorphologie du Venezuela: carte au 1/4000000 et bibliographic selective. Venezuela,

environnements et changements. Unpublished report Univ. Toulouse (France).

Ribolzi, O., Valles, V., Bariac, T., 1996. Comparison of hydrograph deconvolutions using residual alkalinity,

chloride and oxygen 18 as hydrochemical tracers. Water Resour. Res. 32 (4), 1051–1059.

Rodrıguez, A.A., 1999. Aplicacion preliminar de tecnicas de diseno hidrologico a parcelas de erosion del

municipio Bocono, estado Trujillo. Unpublished report. Universidad de los Andes. Bocono. 62 pp.

Romulo Quintero J., 1999. Determinacion y analisis del coeficiente de escorrentıa en parcelas de erosion del

sector la Corojo. Bocono, estado Trujillo. Unpublished report. Universidad de los Andes. Bocono. 100 pp.

Roose, E., 1977. Erosion et ruissellement en Afrique de l’ouest: vingt annees de mesures en petites parcelles

experimentales. Paris, Orstom, coll. ‘‘Trav et Doc’’ 78, 108 pp.

Roscoe, R., Buurman, P., Velthorst, E.J., Vasconcellos, C.A., 2001. Soil organic matter dynamics in density and

particle size fractions as revealed by the 13C/12C isotopic ratio in a Cerrado’s oxisol. Geoderma 104, 185–202.

Ross, S.M., 1993. Organic matter in tropical soils: current conditions, concerns and prospects for conservation.

Prog. Phys. Geogr. 17 (3), 265–305.

Schlesinger, W.H., Andrews, J.A., 2000. Soil respiration and the global carbon cycle. Controls on soil respiration:

implications for climate change. Biogeochemistry 48 (1), 7–20.

Schubert, C.J., Nielsen, B., 2000a. Effects of decarbonation treatments on d13C values in marine sediments.

Mar. Chem. 72, 55–59.

Schubert, C.J., Nielsen, B., 2000b. Nitrogen and carbon isotopic composition of marine and terrestrial organic

matter in Arctic Ocean sediments: implications for nutrient utilization and organic matter composition. Deep-

Sea Res. 48, 789–810.

Schwartz, D., Mariotti, A., Lanfranchi, R., Guillet, B., 1986. 13C/12C ratios of soil organic matter as indicators of

vegetation changes in the Congo. Geoderma 39, 97–103.

Sklash, M.G., Farvolden, R.N., 1979. The role of groundwater in storm runoff. J. Hydrol. 43, 45–65.

Starr, G.C., Lal, R., Malone, R., Hothem, D., Owens, L., Kimble, J., 2000. Modeling soil carbon transported by

water erosion processes. Land Degrad. Dev. 11, 83–91.

Sundquist, E.T., 1993. The global carbon budget. Science 259, 934–941.

Thornton, S.F., McManus, J., 1994. Application of organic carbon and nitrogen stable isotope and C/N Ratios as

source indicators of organic matter provenance in estuaries systems: evidence from the Tay estuary, Scotland.

Estuar. Coast. Shelf Sci. 38, 219–233.

Tiessen, H., Karamanos, R.E., Stewart, J.W.B., Selles, F., 1984. Natural nitrogen-15 abundance as an indicator of

soil organic matter transformations in native and cultivated soils. Soil Sci. Soc. Am. J. 55, 312–315.

Tiessen, H., Cuevas, E., Chacon, P., 1994. The role of organic matter in sustaining soil fertility. Nature 371, 783.

Valentin, C., Bresson, L.M., 1992. Morphology, genesis and classification of surface crusts in loamy and sandy

soils. Geoderma 55, 225–245.

B. Bellanger et al. / Catena 58 (2004) 125–150150

Velasquez, F., Perez, G., 1990. Analisis preliminar de la creciente del 05/04/90. Rıo Bocono en la Cavita. Internal

report. Centro de Ecologıa de Bocono. 21 pp.

Wada, E., Minagawa,M., Mizutani, H., Tsuji, T., Imaizumi, R., Karasawa, K., 1987. Biogeochemical studies on the

transport of organic matter along the Otsuchi river watershed, Japan. Estuar. Coast. Shelf Sci. 25, 321–336.

Wan, Y., El-Swaify, S.A., 1997. Flow-induced transport and enrichment of erosional sediment from a well-

aggregated and uniformly-textured Oxisol. Geoderma 75, 251–265.

Wischmeier, W.H., Smith, D.D., 1978. Predicting Rainfall Erosion Losses: A Guide to Erosion Planning.

Agriculture Handbook 537 United States Department of Agriculture, Washington, DC.

Zech, W., Senesi, N., Guggenberger, G., Kaiser, K., Lehmann, J., Miano, T.M., Miltner, A., Schroth, G., 1997.

Factors controlling humification and mineralization of soil organic matter in the tropics. Geoderma 79,

117–161.