Castillo, A.

Schuller, P.

Trumper, R.E.

Horizon / Pleins textes

Use of 137Cs
to estimate rates and patterns
of soil redistribution
on agricultural land
in Central-South Chile:
models and validation
Paulina Schuller
Rosa Eugenia Trumper
A. Castillo
Introduction
Soil conservation and sustainability is of maximum rclevance to
guarantee the increasing demand for food and raw materials in the
world. Therefore, soil dégradation and détérioration occurring
during the past need to be evaluated. For this purpose, reliable tech¬
niques for quantifying soil érosion and sédimentation hâve to be
improved. Developments made during the last décades in the use of
the anthropogenic radionuclide 137Cs as a tracer for determining
rates and patterns of soil redistribution were considered by many
authors as an important advance that overcomes many limitations of
the conventional monitoring techniques (Loughran, 1989; Ritchie
andMcHenry, 1990; Quine, 1997; Walling, 1998).
The purpose of this research was to evaluate the applicability ofthe
conventional 137Cs technique and a simplified and faster method for
386' Environmental Changes and Radioactive Tracers
estimating spatial distribution of medium-term soil érosion and
sédimentation rates on agricultural land in Central-South Chile. The
potential for using 137Cs as a tracer in soil redistribution investiga¬
tions had not been explored in Chilean soils.
Material and methods
Four study sites were selected in the Coastal Mountain Range in
Central-South Chile (38°40'S, 72°30'W), where sustainable deve¬
lopment of agricultural production need to be assessed due to éro¬
sion problems affecting this area. The mean annual rainfall at the
study area is 1160 mm.y1 and the typical soil type a Palehumult,
Metrenco Séries (IREN, 1970) containing about 65% clay and 7%
organic matter (Ellies and Contreras, 1997). The study fields were
under contrasting land use and management: crop land and semi-
permanent grassland, both under subsistence and commercial
management. The main characteristics ofthe sites are summarised
in Table 1 .
Site code
Use
Management
Surface (m2)
Slope (%) U; M; L'
Plough depth (m)
Density (kg m-3)
Sampling grid (mxm)
A
Crop land
Subsistence
22000
6; 19; 7
0.12±0.3
1250
16x20
B
Crop land
Commercial
4000
13:16:3
0.17±0.5
1000
7x10
C
Grassland
Subsistence
5000
9; 13; 7
0.12±0.3
1180
10x10
D
Grassland
Commercial
2000
36; 48; 17
0.15±0.4
1060
6x6
Determined at the upper (U), middle (M), and low (L) sector of the site.
I Table 1
Characteristics of the study sites.
P. Schuller ef al. Use of '37Cs to estimate soil redistribution on agricultural land T 387
To estimate the 137Cs référence inventory, four appropriate sites that
had been under herbaceous cover for at least four décades were cho-
sen. For measuring the 137Cs inventories, soil cores of 0.072 m in
diameter were collected with an auger down to at least the pénétra¬
tion depth between June and August 1998. At the référence site
samples were taken on a 6 m by 6 m grid. The grid spacing at the
study sites are shown in Table 1 .
To save time on the gamma analysis, the feasibility of measuring
137Cs inventories of composite soil samples for estimating soil redis¬
tribution was also tested at ail four sites. For this purpose, in
October and November 1999 soil samples were collected at the
same grid spacing as previously, but ail cores from transects along
the same contour line were bulked proportionally. The viability of
this approach relies on the similar topography in parallel downslope
transects, and on the almost uniform soil properties of each analy¬
sed site. Additionally, it is based on the assumption that soil redis¬
tribution occurred approximately to the same extend in parallel
downslope transects of the field.
Soil redistribution rates were quantified using the refined mass
balance model for cultivated soils incorporating soil movement by
tillage described by Walling and He (1999), adapted to the site spé¬
cifie conditions of the studied fields. The time-course of the 137Cs
deposit within the study area was estimated on the basis of the
annual deposit of 90Sr and 137Cs reported at the nearest site, located
at 41°26'S, 73°07'W (Health and Safety Laboratory, 1977; Juzdan,
1988; Larsen, 1985; Larsen and Juzdan, 1986; Monetti and Larsen,
1991; UNSCEAR, 1982), and considering the activity ratio
'"Cs/^Sr in déposition constant at 1.6 (UNSCEAR, 1982).
In order to examine the validity of the results obtained by the 137Cs
method, expérimental érosion plots of 10 m2 were installed in the
upper part of the fields A and C for obtaining référence values of the
annual sédiment flux at the corresponding area. In addition, the
results obtained by the 137Cs method were compared with redistri¬
bution rates obtained by pedological observations, comparing the
depths ofthe horizon ofthe area affected by soil redistribution with
that of a référence area with scarce intervention.
388 T Environmental Changes and Radioactive Tracers
S Results and discussion
The measured référence inventory was 525+12 Bq.nr2 (a= 94
Bq.nv2, n=61, référence date: January 1998). The plough depth
determined at the studied fields (Table 1) varied according to the
tillage technique employed, ranging from 0.12±0.03 m depth at
sites ploughed by animal traction to 0.17±0.05 m in sites ploughed
by mechanical traction. The 137Cs pénétration depth ranged accor¬
ding to the position along the slope transects, from 0.12 m at eroded
points to 0.34 m at points of sédiment accumulation. Other values
for the parameters required were taken from the literature (Walling
and He, 1999) or estimated by using information on local soil pro¬
perties and annual distribution and intensity ofthe rainfall.
Figure 1 represents the soil érosion (négative values) and déposition
rates (positive values) estimated at the four analysed areas as a func¬
tion ofthe field topography using the conventional grid framework
sampling and évaluation of 137Cs inventory at each point. The gra¬
phies were obtained using Surfer 7, and kriging geostatistics.
The results shown in Figure 1 allow a comparison of the modelled
estimâtes of the net soil redistribution rates and their spatial distri¬
bution at the four studied sites.
At the crop land site under subsistence management (A) high éro¬
sion rates were observed at the upper border, due to repeated plou-
ghing normal to the downslope gradient and due to the obstruction
of sédiment flux from the adjacent area into this field by a dense
shrub fence. The other sector of high érosion rates is located in the
area of maximum slope, which also promotes higher amounts of
soil érosion. The sédimentation area is positioned at the sector of
minimal slope located at hillfoot in water run-on sites. Intensive
annual cultivation processes and fréquent transit of animal plough
Systems could influence the high érosion rates obtained in this field
(Schuller et al, 1999).
Similar spatial distribution of soil érosion rates was observed on the
crop land under commercial management (B): The highest érosion
rates were estimated at the top ofthe field and at the sector of maxi¬
mum slope. The highest sédimentation rates were obtained at a
-4 -2 o 2
1Figure 1
Soil redistribution rates estimated by 137C S inventory evaluation at grid points: crop land under subsistence (A)
and commercial management (8), grassland under subsistance (C) and commercial management (0) .
390 T Environmental Changes and Radioactive Tracers
dépression at the hillfoot. The érosion rates quantified in field B are
lower than in field A. The soil loss estimated in field A is influen¬
ced by the site isolation from upper and latéral surrounding fields,
which hinder sédiment input into the study area. The field B is not
isolated from surrounding crop fields, and therefore part of the soil
loss could be compensated by sédiment input moving from adjacent
areas.
A form of expressing tolerable érosion is that the soil loss should
not exceed their natural production (Hudson, 1971). Due to the dif¬
ficulty in measuring this last parameter it is considered that the soil
use is sustainable, when the annual loss does not exceed a thou-
sandth of their total mass (Peralta, 1976). The values ofthe érosion
rates measured in the crop land sites are mostly below this tolerable
limit estimated at about 1 kg.nr2.yr', but in sectors with large slope
it exceeds this value by far.
At the semi-permanent grassland site under subsistence manage¬
ment (C) the areas showing the highest érosion rates are located in
the middle sector of the field, where the slope is steepest. At this
field, high sédimentation rates caused by sédiment flow into an
adjacent stream and by sédiment accumulation during the swelling
periods of the stream were observed at the hillfoot.
At the non-permanent grassland site under commercial manage¬
ment (D) the prédominant process caused by soil movement was
sédimentation. This area could be affected by déposition of sédi¬
ments moving from an upper flat adjacent cultivated area, which
was ploughed perpendicular to the hillslope ofthe grassland throu¬
ghout several décades. The highest sédimentation areas were loca¬
ted in water flow concentration sites: at footslope and at the
midslope where the inclination is maximal. The midslope sector is
positioned at a concavity of latéral hillslopes, and could be therefore
affected by sédiment coming from latéral pronounced slopes. Both
grassland sites are not isolated from the surrounding fields, and are
probably affected by soil movement from and towards the adjacent
areas.
As shown in Figure I, the 137Cs method allows to discriminate bet¬
ween the long-term soil redistribution processes according to the
use (crop or grassland) and management (commercial or subsis¬
tence) ofthe soil.
P. Schuller et al. Use of t37Cs to estimate soil redistribution on agricultural land T 391
The soil érosion rates estimated by the 137Cs method were first com¬
pared with sédiment flux measured at érosion plots during three
years. The mean annual sédiment loss determined with an expéri¬
mental plot located at the upper sector of site A was 0.28 + 0.08
kg.m"2.yr'. The calculated érosion rate at the sampling points adja¬
cent to the plot area varied from 0.09 to 0.6 kg.m^.yr1 (mean value
0.25 kg.m^.yr1). The sédiment flux determined with the expéri¬
mental plot simulating the management conditions of grassland C
was 0.10 ± 0.03 kg.m"2.yr' and the calculated érosion rates at sam¬
pling points of similar slope in that grassland fluctuated between
0.04 and 0.19 kg.m'2.yr' (mean value 0.11 kg.m^.yr1). The annual
sédiment loss measured with érosion plots is in good agreement
with the estimated érosion rates at adjacent positions in the fields.
Nevertheless, the measured flux represents net soil export from the
plots, because of their isolation from the soil redistribution pro¬
cesses operating in the surrounding area. Moreover, sédiment flux
is time dépendent and therefore valid for the period for which it is
calculated.
The soil redistribution rates quantified by the l37Cs technique repre¬
sent a time-integrated, medium-term, average for the last 40-45
years, and are therefore less influenced by extrême events.
Additional redistribution rates obtained by pedological observa¬
tions, which also consider the accumulative effect of past soil redis¬
tribution processes, are represented in Figure 2. The rates estimated
by the pedological observations reflect a similar pattern of spatial
distribution, in relation to the soil redistribution rates quantified by
the 137Cs method. Nevertheless, the values of érosion and sédimen¬
tation rates obtained by this method are expressed on relative scales,
because ofthe difficulty in determining the period of cultivation of
each field and the depth of the référence horizons. Therefore, addi¬
tional medium-term estimations of érosion and sédimentation rates
are required for validating the values estimated by the I37Cs method.
In the future, it is also necessary to study the applicability of the
method under other climatic conditions and soil types occurring in
Chile in which érosion is not so évident, in order to prevent it.
The érosion and sédimentation rates caused by tillage, water and the
net rates obtained along the slope transect at each study site using
the simplified 137Cs inventory évaluation are represented in Figure 3.
-1 o 1
--r--r·+--r--r·-r·
2
1 Figure 2
Soil redistribution rates estimated by pedolog ical obser vations: crop land under subsistence (A) and commerc ial management (B),
grassland under subsistance (C) and commerc ial management (0) .
1 ··.··Rt 1
··.··Rw r---
-~
~r-, A.
<,h-/
1
c
(/)
ID
a
:0
fq
:I:
C
r-
r-
m
:Il
!.t
III
:-
20
18
16
146 10
Relative heigth (m)
8 12
Relative helgth (m)
4
2
o
.1 .L- l..-__-.J. -'- -'-__--'
10
25
8
20
4 6
Relative heigth (m)
10 15
Relative heigth (m)
2
5
.-1-~1 ··.··Rt
··.··Rw
...~\ .'~ ..... ~."11! :-Rn 1
"'-
~
.' .~4 !1 ~ ï1.v:\....•~ '.-
r:
·8
o
(/)
CD
ë ·2
C
.2
'5 -4
.0
.-=
<;; ·6
'6
CD
a:
(/)
CD
"§ 2
C
o
S
:g 0
<;;
'6
CD
a: .2
o
A:: 4
~Ë 2
ci>
::s 0
1Figure 3
Soil redistribution ratescausedby tillage (At), by water (Aw), and net rates (An) along the slope transectestimaledby mcs
invenloryevalualion al contourlines:crop land under subsistence (A) and commercial management (8),
grassland undersubsistence (C) and commercial management (0).
394 T Environmental Changes and Radioactive Tracers
Comparing the mean redistribution rates estimated at similar alti¬
tude along the slope transects obtained by a 137Cs inventory évalua¬
tion of individual samples collected in a grid pattern with the ones
obtained by an inventory évaluation in composed samples taken at
contour lines, the corrélations between the redistribution rates cau¬
sed by tillage, by water and net rates are strongly significant at the
0.01 level in each site, with exception of redistribution caused by
water (significant at level 0.05) in site A. The simplified method is
suitable for giving assessment on soil loss and sédiment accumula¬
tion in areas exhibiting simple topography, showing almost similar
slopes at constant-altitude transects. It reduces considerably the
measuring time to estimate the soil redistribution.
The simplified method allows a fast estimation of soil redistribution
rates, providing the possibility to estimate soil redistribution in lar¬
ger areas in a shorter time. Nevertheless, the sampling and 137Cs
inventory quantification strategy must be selected according to the
resolution of the required information, and extension and com¬
plexity of the landscape relief.
Aknowledgements
This study was supported by the Fondo Nacional de Investigacion Cientifica
y Tecnolôgica, Chile (FONDECYT 1970662),
the International Atomic Energy Agency (IAEA CHI-8899),
and the Direction de Investigacion y Desarrollo,
Universidad Austral de Chile. The authors wish
to thank thèse institutions for their financial support.
P. Schuller ef al. Use of '37Cs to estimate soil redistribution on agricultural land '395
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Environmental changes and radioactive traces

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