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Soil erosion from shifting cultivation and other smallholder land use in Sarawak, Malaysia
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Agriculture, Ecosystems and Environment 125 (2008) 182–190 www.elsevier.com/locate/agee
Soil erosion from shifting cultivation and other smallholder land use in Sarawak, Malaysia Andreas de Neergaard a,*, Jakob Magid a, Ole Mertz b a
Department of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark b Department 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
Abstract 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 wished to compare soil erosion from three land use types in a shifting cultivation system, namely upland rice, pepper gardens and native forest. We used two sample sites within the humid tropical lowland zone in Sarawak, Malaysia. Both areas had steep slopes between 258 and 508, 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 and 137Cs 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 of forest and upland rice plots. When corrected for soil density, there was no effect of land use on 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. Low 137Cs 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 chemical and physical quality. The soil carbon inventory is not affected by land use in this analysis, due to the contribution from the deeper soil layers. # 2008 Elsevier B.V. All rights reserved. Keywords:
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Cs; Erosion; Upland rice; Black pepper; Soil carbon; Slash-and-burn; Swidden farming
1. Introduction 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 * Corresponding author. Tel.: +45 35333499; fax: +45 35333460. E-mail address: [email protected] (A. de Neergaard). 0167-8809/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2007.12.013
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 of shifting cultivation (Mertz, 2002), prevail, and many studies still refer to soil erosion and fertility loss as 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
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(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 interactions 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 observed in 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 that black pepper cultivation caused higher erosion 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
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constructed terraces, which are common in Sarawak smallholder systems. Soil texture, soil carbon, nutrients and 137Cs 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. For 137Cs, peak deposition occurred in 1963, thus 137 Cs is a suitable indicator for erosion studies on a 30–50 year timescale. The lower fallout on the southern hemisphere and on equator compared to the northern hemisphere makes the method more difficult to use there, however the absence of later pollution with 137Cs 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 by 137Cs 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, Malaysia, both located within the lowland humid tropics with average annual temperatures of 27 8C and little annual and diurnal variation (Halenda, 1989). In Niah, annual precipitation 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 Muyang (42 households, location: 038450 2400 N and 1138450 5500 E) and Rumah Ulat (63 households, location: 038030 4600 N and 1138440 4000 E). Details of slope and relief of samples fields are given in Table 1. In May 2003, samples were collected in Padawan from fields and forest belonging to the Bidayuh communities of Assom (30 households, location: 018100 1800 N and 1108130 0500 E) and Parang (34 households, location: 018100 5200 N and 1108120 5200 E). 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;
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Table 1 Characteristics of sampled fields Cover
Slope (8)
Orientation of slope (8)
Years of cultivation
Average fallow length (years)
Padawan site Rice 1 Rice 2 Rice 3 Pepper 1 Pepper 2 Pepper 3 Forest 1 Forest 2 Forest 3
30–35 40 25 30 35 35 30 Variable, 20–50 40
250 40 20 40 330 80 300 0 240
8 times since 1950 7 times since 1950 8 times since 1952 8 years 15 years 22 years
6.6 7.6 6.4
Niah site Rice 1 Rice 2 Rice 3 Pepper 1 Pepper 2 Pepper 3 Forest 1 Forest 2 Forest 3
25 25 25 20–25 20–25 – 15–20 15–20 25
60 110 50 180 50 – 60 340 60
9 times since 1954 7 times since 1964 11 times since 1930 7 years 9 years
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 (Manihot esculenta Crantz), bananas (Musa spp.) and other perennial crops may be cultivated and harvested several years into the fallow period. Average fallow length in the Niah area is about 11 years and in Padawan about 13 years (Mertz et al., 2008), hence the fields selected for sampling are more intensively cultivated than the average in the area (Table 1). Field sizes vary greatly, but average about 1 ha. Pepper (Piper nigrum L.) is cultivated on separate, permanent fields that are maintained for up to 22 years, but averaging 8 years. High infection levels of pests and 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 Agricultural Research Centre, Kuching, Sarawak.
Years of fallow
>60 >60 >60 5.4 5.6 6.5
>50 60 >60
2.3. Sampling for
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Cs 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 collected at 0–5 cm, 15–20 cm, 40–45 cm and 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 downslope. 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
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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, 2000). 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 concentration. 137 Cs 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 137 Cs was measured around 662 keV on a cooled Germanium 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 kg1. 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.
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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).
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 as the nutrients in Figs. 1 and 2, 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. 137 Cs activity (Bq kg1 soil) 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 higher in the 30–60 cm layer in undisturbed forest than in the 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
3. Results 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
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).
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Fig. 3. Topsoil conductivity 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) (P < 0.001).
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).
the top 5 cm of soil (P < 0.001). For deeper soil layers, there was no effect of land use on C content. Total carbon inventory in kg C m2 to 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. 137 Cs concentration in the soil (Bq kg1 soil) at four depth 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 much 137Cs as the pepper gardens and twice as much as the upland rice fields. There were no significant differences in 137Cs in the subsoil layers, forest subsoil below 30 cm contained less 137Cs than the detection limit. Total 137Cs inventory in Bq m2 to 90 cm depth is shown in Fig. 8. Most subsoil samples in all land uses were close to or below the detection limit (average values 0.15–0.4 Bq kg1, detection limit 0.3 Bq kg1), hence the uncertainty of the estimates of subsoil 137Cs inventory is high. Even with just a few samples above the detection limit, the
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Fig. 4. 137Cs activity in three soil layers from upland rice fields, pepper gardens and native forest plots in Niah, Sarawak. Error bars indicate S.E. (n = 3, each consisting of 9 independent samples).
Cs inventory in forest subsoil would have been comparable to the other land use types. Consequently, in the discussion, only the top soil layer will be evaluated. There was no significant effect of position on slope (top, middle, bottom) on the 137Cs content of the soil.
4. Discussion 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 systems.
Fig. 6. Total organic carbon in profile, calculated from carbon concentration 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).
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Fig. 7. 137Cs activity in four soil layers from upland rice fields, pepper gardens and native forest plots in Padawan, Sarawak. 137Cs content in subsoil of upland rice fields was below detection limit (0.3 Bq kg1 soil). Error bars indicate 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 nonvolatile minerals from the burnt plant material, including the base-forming cations. It is likely that there can be a downward movement of ashes in the period between burning
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and stabilisation of the ash layer by emerging rice plants (Bruun et al., 2006). This would also explain why it was only 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. The lower pH upslope can also be explained by the loss of cations with percolating water or from ashes. As the baseforming 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 same way. None of the other measured soil quality indicators (clay content, cation exchange capacity, organic carbon and nitrogen) were significantly different between slope positions. Loss of any of these parameters (and partly P as well), is usually associated with loss of topsoil by soil movement— 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 slopes and concluded that crops and residue formed a natural barrier against soil erosion during cultivation (Rodenburg et al., 2003). 4.2. Soil carbon
Fig. 8. Total 137Cs in soil profile, calculated from 137Cs activity and bulk density of soil, from upland rice fields, pepper gardens and native forest plots in Padawan, Sarawak. 137Cs content in subsoil of upland rice fields was below detection limit (0.3 Bq kg1 soil). Error bars indicate S.E. (n = 3, each consisting of 9 independent samples).
Analysis of carbon in the soil profile from the Padawan site highlighted the importance of sampling the entire profile and correcting for soil density. In the top soil layer, pepper gardens contained significantly lower concentrations (C g1 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 m2) 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
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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. The finding that only the top 5 cm were significantly affected supports this assumption; hence sampling to greater depth is unlikely to have changed the result. Other researchers have reported similar findings. 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. 4.3. Soil
137
Cs
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 of 137Cs in the deeper soil layers, together with observations during sampling, led to suspicion that contamination from the topsoil to the deeper layers occurred during sampling. As 137 Cs 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 macropores 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 kg1 soil) than the sampling of the 0–30 cm depth interval at Niah. This illustrates how Cs is confined to the top soil layers, if physical 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 of 137Cs 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 developed 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 and vegetables (and weeds) form a closed canopy within 3–6 weeks. The mechanical weeding that takes place has in other 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.
5. Conclusions When evaluating the environmental effects of shifting cultivation, it must be compared with alternative farming
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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 friendly than natural forest, but it would in most cases provide 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 observed a gradient in selected soil nutrients down the slopes, this is more likely attributed to ash dispersal after burning, rather than soil erosion. The carbon inventory was not affected by land use, and although the topsoil of the cultivated land was depleted of 137Cs compared to the natural forest, the total soil loss after numerous cultivation cycles was calculated to be <30%. We conclude that at the given cultivation intensity, soil fertility and quality loss as a result of shifting cultivation is of minor importance, and besides the temporary loss of above ground biomass, these systems are not net emitters of carbon compared to natural forest.
Acknowledgements 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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Agriculture, Ecosystems and Environment 125 (2008) 182–190 www.elsevier.com/locate/agee
Soil erosion from shifting cultivation and other smallholder land use in Sarawak, Malaysia Andreas de Neergaard a,*, Jakob Magid a, Ole Mertz b a
Department of Agricultural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark b Department 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
Abstract 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 wished to compare soil erosion from three land use types in a shifting cultivation system, namely upland rice, pepper gardens and native forest. We used two sample sites within the humid tropical lowland zone in Sarawak, Malaysia. Both areas had steep slopes between 258 and 508, 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 and 137Cs 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 of forest and upland rice plots. When corrected for soil density, there was no effect of land use on 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. Low 137Cs 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 chemical and physical quality. The soil carbon inventory is not affected by land use in this analysis, due to the contribution from the deeper soil layers. # 2008 Elsevier B.V. All rights reserved. Keywords:
137
Cs; Erosion; Upland rice; Black pepper; Soil carbon; Slash-and-burn; Swidden farming
1. Introduction 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 * Corresponding author. Tel.: +45 35333499; fax: +45 35333460. E-mail address: [email protected] (A. de Neergaard). 0167-8809/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2007.12.013
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 of shifting cultivation (Mertz, 2002), prevail, and many studies still refer to soil erosion and fertility loss as 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
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(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 interactions 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 observed in 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 that black pepper cultivation caused higher erosion 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
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constructed terraces, which are common in Sarawak smallholder systems. Soil texture, soil carbon, nutrients and 137Cs 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. For 137Cs, peak deposition occurred in 1963, thus 137 Cs is a suitable indicator for erosion studies on a 30–50 year timescale. The lower fallout on the southern hemisphere and on equator compared to the northern hemisphere makes the method more difficult to use there, however the absence of later pollution with 137Cs 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 by 137Cs 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, Malaysia, both located within the lowland humid tropics with average annual temperatures of 27 8C and little annual and diurnal variation (Halenda, 1989). In Niah, annual precipitation 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 Muyang (42 households, location: 038450 2400 N and 1138450 5500 E) and Rumah Ulat (63 households, location: 038030 4600 N and 1138440 4000 E). Details of slope and relief of samples fields are given in Table 1. In May 2003, samples were collected in Padawan from fields and forest belonging to the Bidayuh communities of Assom (30 households, location: 018100 1800 N and 1108130 0500 E) and Parang (34 households, location: 018100 5200 N and 1108120 5200 E). 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;
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Table 1 Characteristics of sampled fields Cover
Slope (8)
Orientation of slope (8)
Years of cultivation
Average fallow length (years)
Padawan site Rice 1 Rice 2 Rice 3 Pepper 1 Pepper 2 Pepper 3 Forest 1 Forest 2 Forest 3
30–35 40 25 30 35 35 30 Variable, 20–50 40
250 40 20 40 330 80 300 0 240
8 times since 1950 7 times since 1950 8 times since 1952 8 years 15 years 22 years
6.6 7.6 6.4
Niah site Rice 1 Rice 2 Rice 3 Pepper 1 Pepper 2 Pepper 3 Forest 1 Forest 2 Forest 3
25 25 25 20–25 20–25 – 15–20 15–20 25
60 110 50 180 50 – 60 340 60
9 times since 1954 7 times since 1964 11 times since 1930 7 years 9 years
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 (Manihot esculenta Crantz), bananas (Musa spp.) and other perennial crops may be cultivated and harvested several years into the fallow period. Average fallow length in the Niah area is about 11 years and in Padawan about 13 years (Mertz et al., 2008), hence the fields selected for sampling are more intensively cultivated than the average in the area (Table 1). Field sizes vary greatly, but average about 1 ha. Pepper (Piper nigrum L.) is cultivated on separate, permanent fields that are maintained for up to 22 years, but averaging 8 years. High infection levels of pests and 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 Agricultural Research Centre, Kuching, Sarawak.
Years of fallow
>60 >60 >60 5.4 5.6 6.5
>50 60 >60
2.3. Sampling for
137
Cs 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 collected at 0–5 cm, 15–20 cm, 40–45 cm and 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 downslope. 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
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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, 2000). 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 concentration. 137 Cs 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 137 Cs was measured around 662 keV on a cooled Germanium 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 kg1. 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.
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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).
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 as the nutrients in Figs. 1 and 2, 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. 137 Cs activity (Bq kg1 soil) 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 higher in the 30–60 cm layer in undisturbed forest than in the 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
3. Results 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
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).
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Fig. 3. Topsoil conductivity 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) (P < 0.001).
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).
the top 5 cm of soil (P < 0.001). For deeper soil layers, there was no effect of land use on C content. Total carbon inventory in kg C m2 to 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. 137 Cs concentration in the soil (Bq kg1 soil) at four depth 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 much 137Cs as the pepper gardens and twice as much as the upland rice fields. There were no significant differences in 137Cs in the subsoil layers, forest subsoil below 30 cm contained less 137Cs than the detection limit. Total 137Cs inventory in Bq m2 to 90 cm depth is shown in Fig. 8. Most subsoil samples in all land uses were close to or below the detection limit (average values 0.15–0.4 Bq kg1, detection limit 0.3 Bq kg1), hence the uncertainty of the estimates of subsoil 137Cs inventory is high. Even with just a few samples above the detection limit, the
137
Fig. 4. 137Cs activity in three soil layers from upland rice fields, pepper gardens and native forest plots in Niah, Sarawak. Error bars indicate S.E. (n = 3, each consisting of 9 independent samples).
Cs inventory in forest subsoil would have been comparable to the other land use types. Consequently, in the discussion, only the top soil layer will be evaluated. There was no significant effect of position on slope (top, middle, bottom) on the 137Cs content of the soil.
4. Discussion 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 systems.
Fig. 6. Total organic carbon in profile, calculated from carbon concentration 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).
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Fig. 7. 137Cs activity in four soil layers from upland rice fields, pepper gardens and native forest plots in Padawan, Sarawak. 137Cs content in subsoil of upland rice fields was below detection limit (0.3 Bq kg1 soil). Error bars indicate 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 nonvolatile minerals from the burnt plant material, including the base-forming cations. It is likely that there can be a downward movement of ashes in the period between burning
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and stabilisation of the ash layer by emerging rice plants (Bruun et al., 2006). This would also explain why it was only 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. The lower pH upslope can also be explained by the loss of cations with percolating water or from ashes. As the baseforming 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 same way. None of the other measured soil quality indicators (clay content, cation exchange capacity, organic carbon and nitrogen) were significantly different between slope positions. Loss of any of these parameters (and partly P as well), is usually associated with loss of topsoil by soil movement— 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 slopes and concluded that crops and residue formed a natural barrier against soil erosion during cultivation (Rodenburg et al., 2003). 4.2. Soil carbon
Fig. 8. Total 137Cs in soil profile, calculated from 137Cs activity and bulk density of soil, from upland rice fields, pepper gardens and native forest plots in Padawan, Sarawak. 137Cs content in subsoil of upland rice fields was below detection limit (0.3 Bq kg1 soil). Error bars indicate S.E. (n = 3, each consisting of 9 independent samples).
Analysis of carbon in the soil profile from the Padawan site highlighted the importance of sampling the entire profile and correcting for soil density. In the top soil layer, pepper gardens contained significantly lower concentrations (C g1 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 m2) 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
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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. The finding that only the top 5 cm were significantly affected supports this assumption; hence sampling to greater depth is unlikely to have changed the result. Other researchers have reported similar findings. 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. 4.3. Soil
137
Cs
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 of 137Cs in the deeper soil layers, together with observations during sampling, led to suspicion that contamination from the topsoil to the deeper layers occurred during sampling. As 137 Cs 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 macropores 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 kg1 soil) than the sampling of the 0–30 cm depth interval at Niah. This illustrates how Cs is confined to the top soil layers, if physical 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 of 137Cs 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 developed 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 and vegetables (and weeds) form a closed canopy within 3–6 weeks. The mechanical weeding that takes place has in other 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.
5. Conclusions When evaluating the environmental effects of shifting cultivation, it must be compared with alternative farming
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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 friendly than natural forest, but it would in most cases provide 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 observed a gradient in selected soil nutrients down the slopes, this is more likely attributed to ash dispersal after burning, rather than soil erosion. The carbon inventory was not affected by land use, and although the topsoil of the cultivated land was depleted of 137Cs compared to the natural forest, the total soil loss after numerous cultivation cycles was calculated to be <30%. We conclude that at the given cultivation intensity, soil fertility and quality loss as a result of shifting cultivation is of minor importance, and besides the temporary loss of above ground biomass, these systems are not net emitters of carbon compared to natural forest.
Acknowledgements 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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