Soil erosion from shifting cultivation and other smallholder
land use in Sarawak, Malaysia
Andreas de Neergaarda,*, Jakob Magida, Ole Mertzb
aDepartment of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
bDepartment of Geography and Geology, Faculty of Science, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
Received 23 August 2007; received in revised form 17 December 2007; accepted 18 December 2007
Available online 13 February 2008
The sustainability of shifting cultivation systems and their impact on soil quality continues to be debated, and although a growing body of
literature shows a limited impact on, e.g. soil carbon stocks, shifting cultivation still has a reputation as detrimental to the environment. We
Weusedtwosample sites withinthehumid tropicallowlandzoneinSarawak,Malaysia.Bothareashadsteepslopes between258and508,and
were characterised by a mosaic land use of native forest, secondary re-growth, upland rice fields and pepper gardens. Soil samples were
collected to 90 cm depth from all three land use types, and analysed for various chemical parameters, including texture, total organic matter
and137Cs content.137Cs is a radioactive isotope derived from nuclear fallout, and was used to estimate the retention of topsoil in the profiles.
Soil chemical parameters in upland rice fields, such as extractable cations, pH and conductivity, indicated limited soil transportation
downslope, and depletion of cations from upslope samples are most likely caused by leaching and losses via ashes after clearing and burning.
The position on slope had no significant effect on soil texture, carbon or P content, indicating very limited physical movement of soil
downslope. A soil carbon inventory to 90 cm depth on the three land uses only showed a higher carbon concentration in the top 5 cm offorest
anduplandrice plots.Whencorrected for soil density,therewas no effectof land useon the carbon inventory.Moreover,the carbon content in
the top 30 cm contributed <50% of the total carbon inventory, hence even significant effects of land use on carbon content in the upper soil
layers, are unlikely to change the carbon inventory dramatically.137Cs content in the soil profile indicated largest retention of original topsoil
in the native forest plots, and a loss of 18 and 35% of topsoil from upland rice and pepper gardens, respectively, over the past 40 years. When
comparing to 30 cm depth, soil loss was 30% from both upland rice and pepper fields. Low137Cs activity in deeper soil layers rendered a total
profile inventory impossible. It is concluded that shifting cultivation of upland rice in the current system is not leading to degradation of soil
# 2008 Elsevier B.V. All rights reserved.
137Cs; Erosion; Upland rice; Black pepper; Soil carbon; Slash-and-burn; Swidden farming
Soil erosion and loss of soil carbon is repeatedly being
mentioned as a global threat to environment and food supply
(Pimentel et al., 1995; Pimentel, 2006), but the available
knowledge on erosion in smallholder farming systems is
relatively limited. Shifting cultivation has for decades been
accused of causing soil erosion, and already more than 30
years ago Lal (1974) noted that the topic was frequently
discussed in the literature although little empirical evidence
was available. The problem does, like other conventional
understandings ofshifting cultivation(Mertz, 2002),prevail,
one of the main environmental and productivity problems in
shifting cultivation (Brady, 1996; Harwood, 1996; Devendra
and Thomas, 2002; Borggaard et al., 2003; Rasul et al.,
2004)—a view often criticised for being far too simplistic
* Corresponding author. Tel.: +45 35333499; fax: +45 35333460.
E-mail address: email@example.com (A. de Neergaard).
0167-8809/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
(Kleinman et al., 1995; de Jong, 1997; Schmidt-Vogt, 1998;
Fox, 2000). Some empirical studies are now available, but
results diverge and the methods used are very different –
ranging from direct measurements to the use of various
proxies – and Sidle et al. (2006) argue that most studies do
not distinguish between soil loss processes, e.g. land slides
vs. run-off. The mentioned studies often indicate that
shifting cultivation may be sustainable at low intensity, but
rarely specify this level in any detail (e.g. Juo and Manu,
1996). In a recent study, Mertz et al. (2008) found very poor
correlation between fallow length (and other intensity
indicators) and crop productivity and soil fertility in shifting
cultivation systems on Borneo, illustrating that the interac-
tions are not straightforward.
Lal (1987) summarized work at the International Institute
of Tropical Agriculture showing very low soil erosion of a
traditional farming practice resembling shifting cultivation
and studies in Thailand have shown sediments production
from shifting cultivation to be limited compared to other
land uses (Forsyth, 1994; Ziegler et al., 2004). In
Bangladesh, on the other hand, soil loss during run-off in
shifting cultivation was estimated to remove 27% of the
nutrients in the top 10 cm of the soil (Gafur et al., 2000),
though much of the soil material lost from the fields is
deposited within the watershed. Evidence from Guatemala
also indicates high erosion on shifting cultivation fields
(milpas) (Beach, 1998). Wairiu and Lal (2003) used soil
organic carbon (SOC) depletion as a proxy for soil erosion
and thus proposed strong erosion of shifting cultivation
compared to natural forest in the Solomon Islands. SOC
depletion following shifting cultivation has been observedin
other sites (van Noordwijk et al., 1997; Roder et al., 1997),
but was not observed in Sarawak where either no change in
SOC was observed (Bruun et al., 2006) or SOC levels
quickly recovered during fallow (Kendawang et al., 2004).
In Sarawak, recent studies have indicated the presence of
anthropogenic soil erosion as far back as 6000 years (Hunt
and Rushworth, 2005), but only one study have been carried
out on soil erosion of current land use practices. Using
experimental plots in two sites in Sarawak, Tanaka et al.
(2004) found that few nutrients were lost through run-off on
clayey soils and on sandy soils most nutrients were lost
through leaching, and in general impacts of shifting
cultivation on soil properties after more than 10 years of
fallow and 1 year of cultivation were found to be limited
(Tanaka et al., 2005; Kendawang et al., 2005).
The more intensive cultivation of black pepper (Piper
nigrum L.) often associated with shifting cultivation in
Sarawak has been even less studied in terms of run-off and
soil loss. In India, pure stands of black pepper resulted in
more soil erosion than cardamom or mixed stands of the two
(Moench, 1991) and in Sarawak, studies in the 1970s
indicated thatblackpepper cultivationcausedhighererosion
than other upland farming practices (Hatch, 1981, 1982).
This is particularly important when black pepper is farmed
on clean-weeded, relatively steep slopes with poorly
constructed terraces, which are common in Sarawak
Soil texture, soil carbon, nutrients and137Cs content –
emanating from fallout after nuclear bomb testing between
1952 and 1963 – has been widely used as indicators of soil
erosion (reviewed in e.g. Zapata, 2003).137Cs is strongly
fixed in mineral complexes in clay soils, and hence
effectively labels the top soil at the time of deposition.
The mobility in soil from leaching and plant uptake is very
limited. For137Cs, peak deposition occurred in 1963, thus
137Cs is a suitable indicator for erosion studies on a 30–50
year timescale. The lower fallout on the southern hemi-
sphere and on equator compared to the northern hemisphere
makes the method more difficult to use there, however the
absence of later pollution with137Cs from Chernobyl makes
calculations straightforward (Schuller et al., 2003).
Obviously, there are both gaps in knowledge and
considerable disagreement about the effects of smallholder
farming systems on soil erosion, and the aim of this paper is
to broaden the knowledge base by comparing soil erosion
and land degradation over the past 40 years in natural forest,
shifting cultivation of upland rice and black pepper in two
areas of Sarawak, Malaysia. The paper investigates the
impact of medium intensity shifting cultivation and more
intensive pepper cultivation on soil carbon inventories, soil
fertility and topsoil losses as estimated by137Cs inventory.
2. Materials and methods
2.1. Study area
Samples were taken from the Niah Sub-District, Miri
Division and Padawan District, Serian Division in Sarawak,
average annual temperatures of 27 8C and little annual and
diurnal variation (Halenda, 1989). In Niah, annual precipita-
tion is 2700 mm; in Padawan about 4000 mm (Kuching
station). In both areas the driest month is August and the
wettest month is January. The soils sampled in both areas
resemble tropudults in the USDA Soil taxonomy (Tie et al.,
1989) (dystric nitosols according to FAO classification).
In May 2002 samples were collected in Niah from fields
and forest belonging to the Iban communities of Rumah
11384505500E) and Rumah Ulat (63 households, location:
0380304600N and 11384404000E). Details of slope and relief of
samples fields are given in Table 1. In May 2003, samples
0181001800N and 11081300500E) and Parang (34 households,
location: 0181005200N and 11081205200E). The Padawan area
has steeper slopes than Niah, as described in Table 1.
The vegetation is characterised by tropical rainforest in
various stages of re-growth. The land use practices resemble
those described for other areas of Sarawak (Cramb, 1993;
Mertz and Christensen, 1997; Wadley and Mertz, 2005):
fields for upland rice are cleared in June and usually burnt in
August, weather permitting. Upland rice (Oryza sativa L.) is
planted within 1 week of burning and harvested in February–
March. Fields are usually only cultivated 1 year, although
vegetables including chilli (Capsicum spp.), cassava (Man-
ihot esculenta Crantz), bananas (Musa spp.) and other
into the fallow period. Average fallow length inthe Niah area
2008), hence the fields selected for sampling are more
intensively cultivated than the average in the area (Table 1).
nigrum L.)is cultivated on separate, permanent fields that are
maintained for up to 22 years, but averaging 8 years. High
infection levels of pestsand diseases are common reasons for
abandoning an area and establishing new fields. Undisturbed
forest plots were identified by local farmers and included
sacred forest, protected areas and fields that had not been
cultivated for more than 60 years.
2.2. Sampling for nutrient content and soil quality
In Niah, soil samples from 16 upland rice fields were
collected with a soil core auger (Eijkelkamp, i.d. 34 mm) to
30 cm depth. 25 soil cores taken within a 10 m ? 10 m
square were combined in one sample. Samples were
collected from top of slope, middle of slope and bottom
of slope, three at each level, totalling nine independent
samples from each plot (each consisting of 25 cores). Soil
was air dried, crushed and mixed and sub-samples were
taken for analysis at the laboratory of Semongok Agricul-
tural Research Centre, Kuching, Sarawak.
2.3. Sampling for137Cs and soil carbon
In Niah, soil samples were collected from three upland
rice fields, two pepper gardens and three undisturbed forest
sites. Soil was collected to 90 cm depth with a soil core
auger (Eijkelkamp, i.d. 18 mm), and divided in 0–30 cm,
30–60 cm and 60–90 cm intervals. 6 cores taken within a
10 m ? 10 m square were combined in one sample. Three
samples (each consisting of 6 cores) were taken upslope,
three midslope and three downslope, totalling nine
independent samples for each plot. The samples were air
dried and transported to Copenhagen, Denmark for analysis.
In Padawan, soil samples were collected from three
upland rice fields, three pepper gardens and three
undisturbed forest sites. Soil was collected volume specific
with soil sample rings (Eijkelkamp, 5 cm internal diameter;
100 cm3) from the vertical face of exposed soil profiles.
Samples were collectedat 0–5 cm, 15–20 cm, 40–45 cmand
70–75 cm depth, representing the following intervals:
0–5 cm, 5–30 cm, 30–60 cm and 60–90 cm.
Samples were collected upslope, midslope and down-
slope. At each level, duplicate soil ring cores from three
adjacent soil profiles were collected and combined to one
sample (consisting of 6 rings). Three samples were collected
from each level (upslope, midslope and downslope),
totalling 9 independent samples. The samples were air
dried and transported to Copenhagen, Denmark for analysis.
2.4. Sample analyses
Soil samples were analysed at the Chemistry Laboratory
at the Agricultural Research Centre, Semongok in Sarawak.
Particle size distribution was determined by the Hydrometer
Characteristics of sampled fields
Cover Slope (8) Orientation of
Years of cultivationAverage fallow
8 times since 1950
7 times since 1950
8 times since 1952
9 times since 1954
7 times since 1964
11 times since 1930
Method (FAO, 1970), contents of soil organic carbon by
the Walkley–Black Method (Nelson and Sommers, 1996),
total nitrogen by the Dumas combustion method (Bremner,
1996), available phosphorus by the Bray and Kurtz (1945)
method, Cation Exchange Capacity (CEC) by the
Ammonium Acetate Method buffered at pH 7 (Sumner
and Miller, 1996), and contents of exchangeable basic
cations by leaching the soil with 1 M ammonium acetate
buffered to pH 7 (Thomas, 1982). Subsequently the
contents of cations in the leachate were measured by
means of AAS. pH was measured at a 1:2.5 soil:water ratio
at a pH meter with an accuracy better than 0.05 unit (Chin,
Soil bulk density was determined by drying the soil ring
samples (100 cm3) at 105 8C and weighing them. In
Denmark, samples were finely ground and analysed for
total C and N content using an mass spectrometer (Europa
Scientific, 20–20) coupled to an ANCA-SL sample
preparation module (Europa Scientific, Crewe, UK). Soil
C mass was calculated using bulk density and soil carbon
137Cs content was measured at the Danish National
Institute of Radiation Protection, Copenhagen using a
sample of approximately 200 g sieved soil in cylindrical
containers 5 cm high, 8 cm diameter. The soil content of
137Cs was measured around 662 keV on a cooled Germa-
nium spectrometer coupled to a multi-channel analyser
using GammaView software. Due to the low activity of the
samples, counting time was 24 h, yielding a lower detection
limit of 0.3 Bq kg?1.
2.5. Statistical analysis
The experiment compared three land use types (shifting
cultivation rice, forest and pepper), three positions on slope
(top, middle and bottom), and three (Niah) or four
(Padawan) soil depths. The data were statistically analysed
by ANOVA. Differences between land uses, slope position
and soil depths were considered significant when LSD
values had P values lower than 0.05.
3.1. Niah data
Soil pH, extractable Ca and P from upland rice fields in
Niah are depicted in Fig. 1. Soil pH and extractable Ca were
significantly affected by position on slope (P < 0.001 and
P < 0.05, respectively). Highest values were found at the
bottom of the slope, lowest values at the top. Level fields
showed intermediate values. Extractable soil P showed the
same trend, but differences between sample positions on the
slope were not significant.
Soil extractable Mg and K from the same fields
demonstrated same trend as Ca and pH (Fig. 2). Values
were significantly lower at top of slope than bottom of slope
(P < 0.001 for both Mg and K).
Soil solute conductivity in shifting cultivation rice fields
showed same trend asthe nutrients in Figs. 1 and2, namely a
significant effect of sample position on the slope and a
higher level at the bottom of the slopes (P < 0.001) (Fig. 3).
All other measured parameters than those depicted in
Figs. 1–3 (total soil C and N, extractable Na, CEC and soil
texture), did not show any significant correlation with
position on slope.
137Cs activity (Bq kg?1soil) in three depths of the soils
from upland rice fields, pepper gardens and undisturbed
forest are shown in Fig. 4. The activity was not significantly
affected by land use in the top soil layer, but significantly
two cultivated areas.
3.2. Padawan data
Soil carbon concentration (in %) in upland rice fields,
pepper gardens and undisturbed forest at four soil depth
intervals are shown in Fig. 5. Carbon content was
significantly higher in forest and rice than in pepper in
Fig. 1. Average pH and extractable P and Ca in topsoil as function of
position on slope from 16 upland rice fields in Niah, Sarawak. Average
values from 6 level fields are indicated separately. Error bars indicate S.E.
(n = 16). pH and Ca are significantly affected by position on slope
(P < 0.001 and P < 0.05, respectively).
Fig. 2. Extractable Mg and K in topsoil as function of position on slope
from 16 upland rice fields in Niah, Sarawak. Average values from 6 level
fields are indicated separately. Error bars indicate S.E. (n = 16). Both are
significantly affected by position on slope (P < 0.001).
the top5 cm of soil (P < 0.001). For deeper soillayers,there
was no effect of land use on C content.
Total carbon inventory in kg C m?2to 90 cm depth in
upland rice, pepper gardens and undisturbed forest are
shown in Fig. 6. Carbon inventory was calculated on the
basis of carbon content and soil density at the different soil
depths. Soil carbon stocks were not significantly affected by
land use in any of the soil depths, nor in the profile as a
whole. There was also no significant effect of position on
slope on the soil carbon stocks.
137Cs concentrationinthesoil(Bq kg?1soil)atfourdepth
intervals in shifting cultivation rice, pepper gardens and
undisturbed forest is shown in Fig. 7.137Cs concentration in
topsoil was significantly affected by land use (P < 0.001);
undisturbed forest contained four times as much137Cs as the
pepper gardens and twice as much as the upland rice fields.
There were no significant differences in137Cs in the subsoil
layers, forest subsoil below 30 cm contained less137Cs than
the detection limit. Total137Cs inventory in Bq m?2to 90 cm
were close to or below the detection limit (average values
0.15–0.4 Bq kg?1, detection limit 0.3 Bq kg?1), hence the
Even with just a few samples above the detection limit, the
to the other land use types. Consequently, in the discussion,
only the top soil layer will be evaluated. There was no
the137Cs content of the soil.
As described in the introduction, the effects of shifting
cultivation and other smallholder farming practices on soil
erosion are not well understood. Empirical data sets are in
disagreement – both in data shown and interpretation – and
often text books and other literature uses anecdotal rather
than strong empirical data for describing shifting cultivation
as environmentally detrimental (Watson, 1989; Beets, 1990;
Rasul and Thapa, 2003). This study shows that in the case of
shifting cultivation practices in Niah and Padawan Districts
of Sarawak, Malaysia, there is indeed a limited amount of
soil erosion from shifting cultivation, but the magnitude of
this erosion must be analysed in the context of alternative
Fig.3. Topsoilconductivityasfunctionof positiononslopefrom 16upland
ricefieldsin Niah, Sarawak. Averagevalues from6 levelfieldsare indicated
separately. Error bars indicate S.E. (n = 16) (P < 0.001).
gardens and native forest plots in Niah, Sarawak. Error bars indicate S.E.
(n = 3, each consisting of 9 independent samples).
137Cs activity in three soil layers from upland rice fields, pepper
Fig. 5. Soil organic carbon concentration in four soil layers from upland
rice fields, pepper gardens and native forest in Padawan, Sarawak. Carbon
concentration is significantly lower in the top 5 cm of pepper gardens
compared to the other land uses (P < 0.001). Error bars show S.E. (n = 3,
each consisting of 9 independent samples).
Fig. 6. Total organic carbon in profile, calculated from carbon concentra-
tion and bulk density of soil, from upland rice fields, pepper gardens and
native forest in Padawan, Sarawak. Land uses are not significantly different.
Error bars show S.E. (n = 3, each consisting of 9 independent samples).
4.1. Soil chemical indicators
In the present study, the soil content of base-forming
cations, K, Ca, Mg and Na, showed significantly higher
values on the downslope samples compared to upslope
samples. Similarly, conductivity was significantly greater in
the downslope samples. As soil conductivity is an integral of
all dissolved ions, and Ca and Mg make up a significant part
of the total cations, it is not surprising that conductivity
exhibits the same trend as these ions.
These results indicate a larger loss of cations from the
upslope soil, but are not necessarily an indication of soil
erosion. The upslope soils are better drained than the
downslope areas, which are often close to a river or wetland.
Downward water movement will eventually leach out
cations from the soil, and this process will be most
pronounced in the upslope areas. Another possible
mechanism is the loss of ash by wind or water transport
immediately after burning the forest. Ashes contain the non-
volatile minerals from the burntplant material, including the
base-forming cations. It is likely that there can be a
and stabilisation of the ash layer by emerging rice plants
(Bruunetal., 2006).Thiswouldalsoexplainwhy itwasonly
the non-volatile cations found in the ashes (Ca, K, Mg), and
parameters affected by these (conductivity) increased at the
foot of the slope, while other measured soil parameters that
would mainly change as a direct effect of soil movement
(soil carbon, texture and CEC) were not affected.
cations with percolating water or from ashes. As the base-
forming cations leach out, the relative dominance of H+ions
of the exchangeable cations and cations in solution
increases, leading to a decrease in pH. Extractable soil P
showed the same trend as the cations and pH, but the
difference between slope positions was not significant.
Phosphorus is much less soluble in soil solution than the
cations, and only leaches from soils in significant amounts
when the sorption capacity of the soils is exceeded, which is
far from the case in the study area. The less pronounced
effect on soil P would therefore indicate that downhill
transport of topsoil is less significant in explaining the
differences between upslope and downslope samples, as this
mechanism should effect the cations and phosphorus in the
None of the other measured soil quality indicators (clay
content, cation exchange capacity, organic carbon and
nitrogen) were significantly different between slope posi-
tions. Loss of any of these parameters (and partly P as well),
erosion, either downward by micro-erosion through soil
cracks and pores or downslope during run-off. The fact than
none of these parameters are depleted in the upslope
samples, indicates that erosion from these sites and
subsequent deposition at the foot of the hill was not a
significant process in this area. In a similar study researchers
found no steep gradients in soil pH and phosphorus across
slopesandconcluded thatcropsandresidue formed anatural
barrier against soil erosion during cultivation (Rodenburg
et al., 2003).
4.2. Soil carbon
Analysis of carbon in the soil profile from the Padawan
site highlighted the importanceofsampling the entire profile
and correcting for soil density. In the top soil layer, pepper
gardens contained significantly lower concentrations (C g?1
soil) of carbon, than the rice and forest plots. However, due
to significantly higher soil density in the pepper gardens
(data not shown), the total amount of carbon (g m?2) was
equal in all three land uses. In the soil layers below 5 cm
depth, both density and carbon concentration were similar in
all three land uses. The contribution to the total carbon
inventory from the top 5 cm of the profile (where both
density and carbon concentration were affected by land use)
is marginal, constituting less than 10% for all three land
uses. Hence, even if total carbon content of the top of the
profile would be affected by land use, this is not likely to
gardens and native forest plots in Padawan, Sarawak.
subsoil of upland rice fields was below detection limit (0.3 Bq kg?1soil).
Error bars indicate S.E. (n = 3, each consisting of 9 independent samples).
137Cs activity in four soil layers from upland rice fields, pepper
137Cs content in
Fig. 8. Total137Cs in soil profile, calculated from137Cs activity and bulk
density of soil, from upland rice fields, pepper gardens and native forest
below detection limit (0.3 Bq kg?1soil). Error bars indicate S.E. (n = 3,
each consisting of 9 independent samples).
affect the carbon storage of the profile as a whole. However,
top soil properties may be significantly affected by the
variation in carbon content and bulk density, affecting water
infiltration rates, nutrient dynamics and root interception.
The results highlight the importance of volume specific
sampling within the entire soil profile when evaluating land
use effects on soil quality.
Soils were sampled to 90 cm depth. It was not suspected
that land use would affect soil C content below this depth.
supports this assumption; hence sampling to greater depth is
unlikely to have changed the result.
Yemefack et al. (2006) reported from a study of shifting
cultivation in Cameroon that soil C was much less affected
than other indicators as pH, Ca and P when sampled to
20 cm. They explained this by a ‘‘dilution’’ effect of the Ah
horizon and top soil layers by deeper (<10 cm) soil layers.
In the Niah site,137Cs activity in the topsoil layer was not
affected by land use/cover, but in the 30–60 cm depth
interval, the activity was significantly higher in the
undisturbed forest. The soils were sampled using a 90 cm
soil core auger (18 mm inner diameter), and then dividing
the soil tube in the relevant depth intervals. The high content
of137Cs in the deeper soil layers, together with observations
during sampling, led to suspicion that contamination from
thetopsoiltothe deeper layers occurred during sampling. As
137Cs is very immobile in soils with active clays, it is
difficult to imagine which processes would lead to an
accumulation in the subsoil of an undisturbed forest.
However, micro-erosion of clay particles through macro-
pores in the soil could be such a pathway. In order to avoid
the possibility of contamination of subsoils during sample
collection, the sampling strategy was changed at the
Padawan site, also allowing for calculation of the total Cs
and C inventory by taking volume specific samples.
At the Padawan site, the separate sampling of the 0–5 cm
layer, yielded much higher activities (>8 Bq kg?1soil) than
the sampling of the 0–30 cm depth interval at Niah. This
movement of soil has not altered this distribution. The forest
sites contained about four times the activity of the pepper
gardens, and about twice the activity of the rice fields.
When corrected for bulk density, the differences are less
pronounced. If excluding the data for the 5–30 cm layer(due
to uncertainty of estimates in rice and pepper fields caused
by the low activity of137Cs in the soils), the Cs inventory in
rice and pepper fields is 82 and 65% of the forest soil,
respectively, indicating soil losses of 18 and 35%, but only
from a very shallow soil layer. If including the 5–30 cm
layer, the Cs inventory is just around 70% for both land uses.
The combination of steep slopes (15–508 in this study),
clearing of land and exposing the soil, combined with heavy
rainfall events and an annual precipitation of 2500–
4000 mm, certainly can be conductive for significant soil
erosion. Studies have shown a relatively direct correlation
between slope gradient and erosion (Zhang et al., 2003). In
other tropical farming systems, with steep slopes and high
rainfall intensity, traditional farming systems have devel-
oped well functioning erosion control measures (Malley
et al., 2004; He et al., 2007). The two communities in
Sarawak do not at all consider soil erosion an issue, and
mainly associate the term with rare incidents of landslides,
indicating that soil erosion is not obvious at field level. The
shifting cultivation system practiced by Iban and Bidayuh
communities in Sarawak is characterised by a quite short
period of exposed soil. After cutting the trees, the soil is still
covered by undergrowth, besides the branches and falling
leaves. After burning, litter often remains in the fields due to
incomplete combustion, and vegetables and upland rice are
planted within a few days using a planting stick—hence the
soil disturbance is minimal. The system could rightfully be
compared to conservation tillage systems, which are
recommended for their erosion controlling abilities. The
clearing period also coincides with the driest months of the
year, in order to facilitate burning of the vegetation; hence
the impact of rainwater is minimal. After planting, the rice
andvegetables(and weeds) form aclosed canopy within3–6
weeks. The mechanical weeding that takes place has inother
studies been shown to cause only minor soil erosion, when
compared to the water erosion in the system (Turkelboom
et al., 1999; Ziegler et al., 2007). The main reason for this is
the very shallow ‘‘tillage’’ depth of 1–2 cm. During harvest
only the rice panicle is removed, leaving the straw, residual
crops, weeds and undergrowth to develop into fallow
vegetation with complete soil coverage already from the
time of harvest. Similar studies have highlighted the
importance of un-burnt material, crops and crop residue
in controlling surface erosion (Rodenburg et al., 2003).
Consequently, the soil is only exposed for a period of 4–8
weeks during the driest months of each cropping cycle and
with fallow lengths of 5–10 years, this only result in an
average annual soil exposure of about 1 week. Hence, it is
not surprising that we find the cumulated soil loss over the
past 40 years in areas affected by shifting cultivation to be
very limited. In a short review, Juo and Manu (1996)
presented the view that as long as shifting cultivation fields
were small and surrounded by forest, the cultivation period
<2 years, and the fallow period sufficiently long (not
specified), shifting cultivation may adequately conserve soil
nutrient stocks. However, they also expressed the view, that
this situation was very rare, and increasingly so. This study
suggests that these ideal conditions may not be so rare.
When evaluating the environmental effects of shifting
cultivation, it must be compared with alternative farming
practices—not only with forestry or natural forest. Being a
farming system that relies on field crop production, shifting
cultivation cannot be expected to be more environmentally
more and better environmental services than other more
intensive farming systems. The forest and crop litter,
combined with the minimal soil disturbance in the shifting
cultivation systems, forms an almost continuous covering
layer that protects the soil from erosion. Although we
this is more likely attributed to ash dispersal after burning,
depleted of137Cs compared tothenatural forest, the total soil
loss after numerous cultivation cycles was calculated to be
fertility andquality loss as a result ofshifting cultivationisof
minor importance, and besides the temporary loss of above
ground biomass, these systems are not net emitters of carbon
compared to natural forest.
The research was funded by the Danish SLUSE
university network for Sustainable Land Use and Natural
Resource Management. The authors are indebted to Klaus
Ennow from Danish National Institute for Radiation
Protection for handling the Cs analysis, and Jona Anak
Kerani for field assistance. We thank the villagers from
Rumah Muyang and Kampung Assom for their hospitality
and kind cooperation.
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