Influence of Burning Practice in Shifting Cultivation under Different ...

S. Tanaka: Influence of burning in shifting cultivation on nutrient dynamics403Influence of Burning Practice in Shifting Cultivation underDifferent Climates on Nutrient DynamicsSota TANAKA*Graduate School of Kuroshio Science, Kochi University, B200 Monobe, Nankoku 783-8502, JapanKeywords: Japan, nutrient dynamics, shifting cultivation, Sarawak, ThailandAbstractThis paper presents a comparison between the traditional shifting cultivation systems that have been reportedin previous studies. The aim is to examine the influence of different burning practices on soil nutrient dynamics undertemperate (Japan), tropical monsoon (northern Thailand) and tropical rain forest (the Malaysian state of Sarawak, Borneo)climates. The system in Japan includes cereal cropping continuously for three to four years. The amount of fuel (plantmaterial to be burned) is estimated to be 50 t ha -1 at most. Ignition is started from an upper slope, and the land is burnedslowly and carefully downward. The system used by the Karen people of Thailand consists of a single year cropping of ricefollowed by 10–15 years of fallow. Burning is carried out at the end of the dry season and a fire is ignited at a lower slope. Thesystem in Sarawak is similar to that employed in Thailand. In each of these two regions, the amount of fuel is around 100 tha -1 after 15 years of fallow. The increases in soil temperature from burning are in the order Thailand > Japan > Sarawak,where the differences can be attributed to soil moisture content and the direction of fire ignition. The effect of soil burning isthus more obvious in Thailand and Japan than in Sarawak. After three to four months of burning, the soil mineral N contentin many regions decreases to the initial pre-burning level. A comparison between the level of nutrients in ashes and soilssuggests that the addition of ash plays an important role in elevating nutrient levels in Japan and Sarawak. However, this maynot be the case in Thailand. The increased nutrients return to their initial levels within one year in Sarawak and Thailandand after three or four years in Japan, which could be due to differences in the nutrient-holding abilities of the soils as wellas to climatic influences. These differences in soil nutrient dynamics are considered to be reflected by the different croppingsystems used in the regions.1. IntroductionShifting cultivation systems are agricultural methodsthat require low input. Such methods are still commonlyconducted in tropical regions; however, in the past,they were also widely practiced in temperate regions.Traditional shifting cultivation systems practiced byindigenous farmers are generally believed to be welladaptedto the climate and environment conditions ofparticular regions, and therefore to be well-balancedand ecologically sound. Intensified systems, with anextended cropping period and a shortened fallow period,can cause land degradation, such as nutrient depletion anderosion. Irrespective of whether a system is traditional orintensified, the role of burning practices is considered tobe positive in terms of land preparation, weed and pestcontrols, and increased soil fertility. Although numerousstudies have been conducted on burning practices fromthe viewpoint of soil science, most of them have focusedon changes in soil nutrient levels caused by burning andcrop production within one field or one region. In contrast,several comprehensive investigations have reviewed*Corresponding author: Sota Tanaka, E-mail: sotatnk@kochi-u.ac.jp, Tel: +81-88-864-5183, Fax: +81-88-864-5183Received 22 August 2011; accepted 22 December 2011

404S. Tanaka / Pedologist (2012) 403-414previous studies of tropical and subtropical regions, andhave evaluated the influence of burning practices on soils(Giardina et al., 2000). Because the main purpose of suchresearch is to construct a model of the nutrient dynamicsin shifting cultivation ecosystems, data are compiled foranalysis from a variety of regions.However, assuming that a shifting cultivation systemis indeed well-adapted to the climatic and environmentalconditions of a given region, the significance and extentof the influence of burning practices on soil nutrientdynamics should change under different shifting cultivationsystems and climates. To investigate this hypothesis, thepresent paper examines and compares the influence ofburning practices under temperate, tropical monsoon andtropical rain forest climates, based mainly on reporteddata from Japan, northern Thailand and Sarawak inBorneo, respectively. However, in Japan, quantitative datarelated to nutrient dynamics under shifting cultivation arescarce because, except for a few isolated cases, shiftingcultivation stopped being practiced in 1950s or 1960s.2. Characteristics of shifting cultivation systemsThe characteristics of traditional shifting cultivationsystems in several Asian countries are compared in Table 1.In Japan, burning in a traditional shifting cultivationsystem had three timings for burning, spring, summer, andautumn (Tachibana, 1995). A fire was ignited at an upperslope, which then slowly moved downward. Unburned orcharred fuels were subsequently piled up together andreburned. Many different phrases were used to expressthis burning method (Sasaki, 1972), for example, “honemade yaku” (burn to the core (hone = bone)) and “sokomade yaku” (burn down to the bottom).Among the groups practicing shifting cultivationin northern Thailand and Myanmar, the Karen and Lua’people practiced shifting cultivation with short croppingand long fallow periods (Kunstadter, 1978). The time forslashing and cutting was traditionally January to Februaryand that for burning was March to April (the end of dryseason). In Borneo, many tribes conducted shiftingcultivation, but the practices were similar to each other,with slashing and cutting done in June and July and burningdone in August and September. In terms of the locationof fire ignition, the area between Yunnan and Thailandseems to have formed a boundary; to the south of thisboundary, the initial burning position was generally on alower slope. The author has observed that the Iban peoplein Sarawak always started burning from the lower part ofa slope, even for young fallow forests of less than 5 yearsTable 1. Characteristics of traditional shifting cultivation systems throughout Asia.Region Tribe Ignition from CroppingperiodCropReferenceJapan upper 3–4 y cereals Sasaki (1972)upper 3–4 y cereals, oriental paper bush + paper mulberryupper 1–2 y turnip + forest plantationYunnan upper 1–5 y rice, cereals Yin (2000)Thailand Lua’ upper 1 y rice Kunstadter (1978)Karen lower 1 y ricePhilippines Hanunóo upper (secondary forest)lower (primary forest)1 y rice Conklin (1957)Borneo Iban lower 1 y rice Freeman (1970)Kantu’ windward 1 y rice Dove (1981)Kenyah lower 1 y rice Chin (1985)Cereals in Japan are typically:(1st and 2nd years) Japanese barnyard millet (Echinochloa esculenta, hie), foxtail millet (Setaria italica, awa), and buckwheat (Fagopyrumesculentum, soba)(3rd and 4th years) soybean (Glycine max, daidzu), and adzuki (Vigna angularis, adzuki)

S. Tanaka: Influence of burning in shifting cultivation on nutrient dynamics405old. Reburning was common in all regions. Fire ignitedfrom lower slopes moved rapidly upward in comparisonwith ignition from an upper slope.The length of cropping periods was also regiondependent. In the temperate regions, the cropping periodextended up to four or five years, whereas in the tropicalmonsoon and tropical rain forest regions, a single yearof cropping was common. In Japan, cereals were usuallyplanted. Among them, Japanese barnyard millet, foxtailmillet and buckwheat were planted in the first and secondyears and beans (soy and azuki) were planted in the thirdand fourth years. In northern Thailand and Borneo, theprimary crop was upland rice.3. Amount of aboveground biomass to be burnedSugawara and Sindo (1985) experimentally estimatedthat the actual amount of aboveground biomass slashedand cut to be burned (fuel) was 26.7 t ha -1 for vegetationin Iwate Prefecture in Japan that had lain fallow for 10years and was dominated by Quercus serrata (Table 2).In contrast, Takahasi (1947) reported fuels amounts of18.5 t ha -1 and 16.6 t ha -1 in his experiment in YamagataPrefecture. Although the length of the fallow periodswere not described in his paper, these are considered tobe around 7–8 years or longer judging from the context.For slashing and burning agriculture accompanied bytimber harvesting or forest plantation (Yakihata-zorin),fuels were mainly composed of the branches and leavesleft after timber harvesting. Fuel amounts were reportedto be 40.7 t ha -1 for a deciduous broad-leaved forest mixedwith Pinus densiflora in Shimane Prefecture (Su et al.,1995), and to be 15.2 t ha -1 for a 78-year-old Cryptomeriajaponica stand (Ohtsuka et al., 2006). On the other hand,much of the literature on folklore studies and culturaland social anthropology (e.g., Sasaki, 1972; Tachibana,1995) has described that for spring burning, the timingof slashing and cutting fuel was in the previous summeror autumn, because of the long duration required to drythe fuel. Although fallow forests that were old or hadlarge amounts of aboveground biomass were used inspring burning, a significant portion of the biomass wasremoved to use as timber and fire wood. In areas proneto heavy snowfall, large-diameter trees were occasionallycut down on ground covered deeply with snow since theycould easily be transported by using a sledge. In this case,because trees were typically cut at the snow-surface level,large stumps were left standing. In addition, some trunkswere kept aside, and after burning the remaining fuel,these trunks were laid down along the contour lines toprevent erosion and to create footholds on steep slopes. Incontrast, summer and autumn burnings were conductedafter drying the fuel for only 1–2 weeks, suggesting thatyoung fallow forests with small amounts of the biomasswere used for this. Judging from these facts, relativelysmall amounts of fuel seem to have been burned inJapanese shifting cultivation systems, perhaps around 50t ha -1 at most for a spring burning and 10–20 t ha -1 at mostfor summer and autumn burnings.Figure 1 shows the amounts of aboveground biomassin fallow forests that are under shifting cultivation systemsby the Karen and Lua’ people in northern Thailand andMyanmar, and by several tribes in Borneo. In contrast, theactual fuel amounts for burning have seldom been reported.For the same number of fallow years, the amounts ofaboveground biomass did not differ substantially betweenforests in Thailand and Myanmar and forests in Borneo,although they were generally higher in the former if threeBorneo plots with large biomass are excluded. It mightbe surmised that vegetation recovery in Thailand andBiomass(t ha -1 )Table 2. Aboveground biomass to be burned (fuels) in Japan.Forest conditionReference26.7 10 y fallow forest Sugawara and Shindo (1985)18.5 7–8 y fallow forest Takahashi (1947)6.6 7–8 y fallow forest Takahashi (1947)40.7 Secondary forest after timber harvesting Su et al. (1995)15.2 78-year-old C. japonica plantation after timber harvesting Ohtsuka et al. (2006)

406S. Tanaka / Pedologist (2012) 403-414Fig. 1. Number of fallow years and the aboveground biomass of forests under shifting cultivation systemsMyanmar is slow compared with Borneo, due to therebeing less precipitation and severe dry seasons in theformer. However, Fukushima et al. (2007) reported that theKaren people’s method of cutting trunks at 1 m above theground when felling trees led to faster regrowth and thatthe growth of bamboo species in this region was vigorous,resulting in rapid recovery of aboveground biomass.Lua’ and Karen people are also reported to have cut offbranches and left the trunk alive for large-diameter trees(Kunstadter, 1978). Traditionally, shifting cultivators inBorneo would cut down trees at a point above the buttressroots in primary forests or old fallow forests. However,nowadays, shifting cultivation systems are rarely used forsuch large forests and cultivators usually clear-cut fallowforests with a chainsaw. In addition to the differencebetween slashing and cutting methods, higher soil nutrientcontent in Thailand (as discussed below) might alsocontribute to the fast biomass recovery. Thus, assumingthat the amount of aboveground biomass is equivalent andthat tree species dominate fallow vegetation, the amountof fuel for burning is expected to be considerably higher inBorneo than in Thailand and Myanmar—at least under theKaren and Lua’ people’s systems.4. Soil temperature increase during burningFigure 2 plots the maximum soil temperaturesrecorded by a number of investigations during burningin the three regions. Data (13) and (14) of Thailand andall Sarawak data were measured in shifting cultivationexperiments by the current author (Tanaka et al., 2001;Kendawang et al., 2004; 2005). Within each region, the soiltemperature tended to be higher with increasing amountsof fuels. However, a comparison of temperatures and fuelamounts among regions shows that temperature increaseswere considerably higher in Japan and Thailand than inSarawak. The differences between soil temperatureincreases might be explained based on the position onthe slopes for ignition and the soil moisture conditions asfollows.For shifting cultivation in Japan, the period for dryingfuels was from several months to a half year for springburning and one to two weeks for summer and autumnburnings. The amount of fuels seemed to be smallirrespective of the burning season. However, becauseburning was initiated from an upper slope, the soil surfacecould be heated for a long duration, resulting in sufficienttime for heat transfer into subsurface soils. Conversely,with soils usually moist, even in spring (a relativelydrier season), and with water having a large specific heatcapacity and heat of vaporization, the soil temperaturecould increase above 100 °C only for surface soils at adepth of about 1–2 cm. In Thailand, by burning time atthe end of the severe dry season (three months aftercutting), the fuels seem to be sufficiently dry. Althoughin the author’s experiment, fuels were ignited at a lowerslope, the positions for ignition were not reported fordata (9)–(12). In the author’s experiment, the fuels werealmost completely burned. Figure 2 indicates that soiltemperatures at 2–3 cm depth could increase to about100–200 °C because of the soil’s low moisture contentat the end of the dry season. In Sarawak, the period fordrying fuels was commonly about 2–3 months, and so inexperiments, large tree trunks were cut into pieces of 1 mlength in order to hasten drying. Therefore, the fuels wereconsidered to be well-dried and the burning comparable

S. Tanaka: Influence of burning in shifting cultivation on nutrient dynamics40798.5 t/ha45 t/ha34 t/ha18.5 t/ha16.6 t/ha15 t/haN.D.Large stemsBranches & leavesReburnReburn100 t/ha20 t/haN.D.300 t/ha100 t/ha300 t/ha100 t/ha300 t/ha100 t/haFig. 2. Soil temperature increase during burningto the shifting cultivation system of the local people.However, soil temperature increases were small, up toabout 100–150 °C at 3 cm depth even when burning 300t ha -1 of fuels. This small increase is attributed to the fuelignition on the lower part of the slope as well as the moistsoils in the area. Irrespective of the region, increases insoil temperature below a depth of 5 cm were negligible.5. Nutrient transfer from fuels to ashAsh addition by burning is widely accepted toalleviate soil acidity and fertilize soils to elevate nutrientcontent. However, only a portion of nutrients accumulatedin aboveground biomass are transferred into ash, and onlya proportion of those nutrients in the ash are infiltratedinto the soils.Nutrients in the aboveground biomass are releasedinto the atmosphere through volatilization or the dispersalof minute particles during burning, where the extent ofthese losses is dependent on the type of nutrient and theburning conditions (Raison et al., 1985). Volatilizationtemperatures of typical soil nutrients are 277 °C for P, 774°C for K, 1,107 °C for Mg and 1,484 °C for Ca, althoughthese temperatures are different depending on theelements’ form. Several organic forms of N decompose ataround 200 °C and are released as NH 3 or NO x into theatmosphere, whereas nitric acid is volatilized at 82.6 °C.Because fuel combustion temperatures are from severalhundred to more than one thousand degrees Celsius,the majority of N and a significant portion of P, K and Mgare lost due to volatilization. In contrast, temperaturesseldom exceed the level required for Ca volatilization.The remaining nutrients during burning are transferredinto the resulting ash and charcoal. However, a percentageof ash will also be lost due to updrafts during burning.Few studies of shifting cultivation systems in Japanand Southeast Asia have been concerned with nutrienttransfer from aboveground biomass to ash and from ash tosoils. Therefore, nutrient data compiled by Giardina et al.(2000) in Central and South America are listed in Table 3with some modification. The percentage of each nutrienttransferred from vegetation into ash were estimated to

408S. Tanaka / Pedologist (2012) 403-414AbovegroundBiomass(t ha -1 )Table 3. Nutrients in aboveground biomass and ash (Giardina et al., 2000).Biomass nutrient (kg ha -1 ) Ash nutrient (kg ha -1 )N Ca Mg K P N Ca Mg K PMexico 110 944 1535 ND 346 27 28 696 ND 90 11Brazil 34 342 311 35 75 7 6 92 9 20 2Brazil 95 834 783 95 270 24 4 177 16 54 5Brazil 292 1390 90 ND 545 62 47 477 ND 211 18Brazil 361 2420 955 ND 500 62 40 486 ND 316 35ND: no data available.be 3% for N, 49% for P, 50% for Ca and 57% for K. Afterburning has been completed, a proportion of nutrientstransferred to the ash will then be lost due to wind andrunoff water. Although scant data of such losses areavailable, in the case of Mexico in Table 3, the amountsof N and P that infiltrated into soils were 7 and 5 kg ha -1 ,respectively (Giardina et al., 2000).6. Effect of ash addition on soil propertiesAsh addition to soils elevates soil nutrient levelssuch as exchangeable base cations and available P. Levelsof N are also increase to an extent if vast amounts of fuelare burned. As discussed above, nutrient input to soilsthrough ash addition is affected by various factors bothduring and after burning. Therefore, a detailed discussionabout absolute amounts of nutrients added to soils, orabsolute increases in soil nutrient levels due to ashinputs, has little meaning in comparison of the ash effectsamong regions, as in the present paper. In this section,the significance of ash addition effects on soil fertility isdiscussed in terms of the duration of these effects.Table 4 lists surface soil properties of areas undershifting cultivation systems before burning and in fallowTable 4. Surface soil properties of areas under shifting cultivation systems before burning or under fallow.Depth pH T-N CEC Exchangeable cations Available ReferenceCa Mg K P(cm) (g kg -1 ) (cmol ckg -1 ) (mg kg -1 )JapanIwate* 0–5 4.98 28.6 0.47 0.45 0.11 ND Sugawara and Shindo (1982)Iwate* 0–5 5.10 28.7 0.42 0.49 0.10 ND Sugawara and Shindo (1983)Iwate* 0–5 5.00 29.3 0.37 0.44 0.11 ND Sugawara and Shindo (1985)Niigata* 0–5 5.60 5.0 ND 17.4 5.45 1.90 7.07 Ohtsuka et al. (2006)ThailandKhon Kaen* 0–5 6.3 3.0 20.3 16.1 5.9 0.4 6.67 Tulaphitak et al. (1983)Mae Hong Son** (Sedimentary) 0–10 5.90 2.7 17.2 3.15 2.09 0.76 5.02 Funakawa et al. (1997)Mae Hong Son** (Granite) 0–10 5.85 3.3 17.0 1.85 1.75 0.67 5.45 Funakawa et al. (1997)Sarawak, MalaysiaBalai Ringin* 0–5 4.01 3.4 19.4 0.19 0.26 0.2 5.3 Tanaka et al. (2004)Sabal* 0–5 4.77 2.2 4.88 0.04 0.34 0.08 3.6 Tanaka et al. (2004)Niah* 0–5 4.62 2.2 14.9 1.25 1.01 0.43 13.8 Tanaka et al. (2005)Mujong** 0–10 4.95 2.6 20.4 0.83 0.79 0.28 7.5 Tanaka et al. (2007)ND: no data available; *before burning; **average values of fallow forests of various ages.T-N: total nitrogen; CEC: cation exchange capacity.

S. Tanaka: Influence of burning in shifting cultivation on nutrient dynamics409forests. In spite of the limited available data and theexceptions in those data, the pH and exchangeable cationlevels were in general highest in Thailand, followed byJapan and then Sarawak. The levels of available P werelow in all regions. The level of CEC was highest in Japan,suggesting a larger content than in the other regionsof soil organic matter and 2:1 type clay minerals. Fromthese properties, the inherent soil fertility level in termsof soil pH and nutrients is considered to be higher inThailand than in Japan and Sarawak. Table 5 comparesthe soil nutrient stocks at several of sites listed in Table4, calculated from the soil properties at depths of 0–5 cmor 0–10 cm. Relating the soil nutrient stocks in Table 5with the ash nutrients in Table 3, the burning of only 50 tha -1 of fuels in Japan (spring burning) and Sarawak (abouthalf of the fuel amounts burned in Mexico (110 t ha -1 )or Brazil (95 t ha -1 )) would be expected to substantiallyelevate soil nutrient levels. For the summer and autumnburnings in Japan, ash effects are considered somewhatunlikely because the amounts of the fuel are thought to be10–20 t ha -1 as discussed in Section 3. In contrast, becausethe levels of soil nutrients are originally relatively high inThailand, ash effects might not be appreciable, althoughthe alkaline components could play an important role inalleviating the acidity levels in the subsoils (Funakawa etal., 1997).Table 6 lists the duration after which elevated pH andnutrients levels due to ash addition return to their initialpre-burning levels. In Japan, according to a series of studiesby Sugawara and Sindo (1982; 1983; 1985), this period wastypically after three to four years (third or fourth harvest).However, in many cases, the levels still remained high atthe last measurement. In Ohtsuka et al.’s study (2006),the levels of pH, exchangeable Ca and available P werestill elevated at the end of their measurement period(three months after burning). In the cases of ThailandTable 5. Nutrient stocks of surface soils before burning.Depth T-N Exchangeable (kg ha -1 ) Available Reference(cm) (kg ha -1 ) Ca Mg K P (kg ha -1 )Iwate, Japan 0–5 ND 47.0 27.3 21.5 ND Sugawara and Shindo (1982)Mae Hong Son, Thailand 0–10 2700 630 240 297 5.0 Funakawa et al. (1997)Balai Ringin, Sarawak 0–10 3200 25.5 21.9 63.6 3.9 Tanaka et al. (2004)Mujong, Sarawak 0–10 2750 166 91 109 7.5 Tanaka et al. (2007)The bulk density in calculation was assumed to be 1.0 g mL -1 .Table 6. Duration after which elevated pH and nutrients levels due to ash addition returned to initial pre-burning levels.Measurement intervals pH Exchangeable Available ReferenceCa Mg K PMonths after burningJapanIwate 1, 7, 19, 31, 43 19 31 43 19 ND Sugawara and Shindo (1982)Iwate 1, 4, 16, 28, 40 40 NR NR NR ND Sugawara and Shindo (1983)Iwate 1, 4, 16, 28, 40 28 NR 16 NR ND Sugawara and Shindo (1985)Niigata 0, 0.25, 1, 2, 3 NR NR 1 2 NR Ohtsuka et al. (2006)ThailandKhon Kaen 0, 3, 8, 11, 16, 18, 22 11 ND ND 8 NR Tulaphitak et al. (1983)Sarawak, MalaysiaBalai Ringin 0, 3, 7, 12 12 NR NR 12 7 Tanaka et al. (2004)Sabal 0, 3, 7, 12 3 NR 7 7 7 Tanaka et al. (2004)Niah 0, 1, 3, 7, 12 NR NR 3–7 7 3–7 Tanaka et al. (2005)NR: not returned to; ND: no data available

410S. Tanaka / Pedologist (2012) 403-414(Tulaphitak et al., 1983) and Sarawak (Tanaka et al.,2004, 2005), elevated pH, exchangeable K and Mg, andavailable P returned to the initial levels by harvestingtime, or within one year. Despite evidence being scarcedue to the lack of information, several potential reasonsfor the differences seen in the duration of ash effects aretentatively proposed as follows. The first reason is thedifference between clay mineralogical composition andthe charge characteristics of the soils under differentclimates. The second reason is that leaching of ashnutrients might be more severe in Thailand and Sarawakthan in Japan. Although the annual precipitation is similarin Thailand and Japan, cropping in Thailand occurred inthe rainy season. The third reason is that the differencemight be partially attributed to the geographical locationof Japan, which is adjacent to the Chinese continent andsurrounded by ocean—the passageway of many typhoons.The annual amounts of nutrients passed to soils byprecipitation in Kochi Prefecture have been reported tobe 5.3 kg ha -1 for Ca, 3.1 kg ha -1 for Mg and 2.6 kg ha -1for K (Yamada et al., 2004), suggesting that in comparisonwith the amounts of nutrients contained in ash and soils,an appreciable amount of nutrients can be added throughprecipitation. At a location facing or near the ocean, seasalts can also be added during the passing of a typhoon.In addition, nutrient addition through dry deposition canbe expected. Inoue et al. (1998) reported that yellow sandphenomenon derived from the Chinese continent couldtransport nutrients, such as Ca, to Japan.7. Soil burning effectThe soil burning effect is a phenomenon in which theNH 4 -N level of soils is increased during burning and theN mineralization is stimulated after burning. Note thatthe soil burning effect in situ implicitly includes somecontribution from ash of mineral N and the stimulation ofN mineralization as a result of increased soil temperaturedue to removal of forest cover. Although the majority ofprevious studies on the soil burning effect have beenconcerned only with N, increases in other nutrients byheating, such as available P and exchangeable K, can bealso included (Andriesse and Koopmans, 1984). Accordingto Sakamoto et al. (1993) and Simamoto et al. (1995), theincrease in NH 4 -N can be attributed to a deaminationreaction caused by heating. Those authors also indicatedthat the amounts of both NH 4 -N and easily decomposableorganic N begin to increase at a relatively low temperatureof 50 °C. Furthermore, they suggested that, whereas Nfractions at around 50 °C are derived mainly from themicrobial debris resulting from soil heating, the proportionof N originating from soil organic matter increases withtemperature above 100 °C. According to a series of studiesconducted by Mitsui et al. during World War II (Moritsukaand Matsuoka, 2008), the maximum levels of NH 4 -N andN mineralization were recorded at 200 °C, and the levelsFig. 3. Mineral nitrogen levels in slash and burn experiments in Otoyo town, Kochi Prefecture, Japan, innorthern Thailand and in Sarawak

S. Tanaka: Influence of burning in shifting cultivation on nutrient dynamics411decreased at temperatures of more than 300 °C. NO 3 -N disappeared at 200 °C. Although these studies wereconducted under laboratory conditions, the temperatureswere mostly within the range reported during burning inshifting cultivation systems (Table 2).Figure 3 reveals changes in mineral N levels of0–5-cm soils during slash and burn experiments by thepresent author in Otoyo Town, Kochi Prefecture, Japan(unpublished data), in northern Thailand (Tanaka etal. 2001) and in Sarawak (Kendawang et al., 2005). Soilsamples were collected from reburned spots in KochiPrefecture, and from plots after 100 t ha -1 fuel burnings inThailand and Sarawak. The soil moisture content beforeburning and the maximum temperature during burningwere 35% and 90 °C (temperature measured at 3 cm depth)in Kochi Prefecture, 17% and 297 °C (2.5 cm) in Thailand,and 38% and 70 °C (3 cm) in Sarawak, respectively. Beforeburning, the levels of mineral N and the proportions ofNO 3 -N in the mineral N were higher in Kochi Prefectureand Sarawak than in Thailand, suggesting that the formerhad higher soil microbial activities due to preferableconditions and high soil moisture contents. The amountof NH 4 -N was increased by the soil burning effect in allregions. The NH 4 -N level was highest in Kochi Prefecture,and is attributed to the higher soil organic matter in thisregion and the longer heating duration due to burningfrom an upper slope and reburning. However, comparingthe pre-burning levels, the increase in NH 4 -N was moreremarkable in Thailand, followed by Japan and Sarawak,reflecting the differences in soil moisture content andin the soil temperature increase. Nitrate N levels weredecreased by burning in all cases, especially for Sarawak,which partially offset the benefit of NH 4 -N increases.Nitrification activities recovered at a relatively earlystage of cropping. Note that in all regions, the soil burningeffect diminished during a single crop cycle, suggestingexhaustion of the available N pool.According to Japanese farmers’ perceptions, reburningresulted in high crop productivity (Sasaki, 1972; Nomoto,1984). Based on the author’s observations, the colorof surface soil at around a depth of 1 cm occasionallychanges to red or brown at such reburned spots, as wellas at well-burned points of a single burning, indicatingthe combustion of soil organic matter and the resultingloss of N. Although, to the author’s best knowledge, aninvestigation of this phenomenon has not been conducted,such perception might be explained thusly: a significantamount of N supplied from thick ash deposition at suchpoints may offset the loss of soil N by burning. In addition,heat can be conducted to deeper soil layers duringreburning and the soil burning effect may occur theretoo.Although the soil burning effect is indispensablefor crop production under a shifting cultivation systemwithout fertilizer application, the nature of the effect canbe thought of as squeezing the soils’ available N pools tosupply mineral N to crops at the expense of NO 3 -N.8. ConclusionBy taking into account the variations between regionalash effects and soil burning effects, as discussed above,the differences between shifting cultivation systemsunder temperate (Japan), tropical monsoon (Thailand) andtropical rain forest (Sarawak) climatic conditions can beinterpreted as follows. In Japan, ash effects that elevatethe levels of exchangeable cations and available P can lastfor a long duration (three to four years after burning). Ifthe ability of soils to supply N, which decreases duringcropping, can be compensated for, cropping can continue.Such compensation is the reason for cropping beans inthe third and fourth years in Japan (Table 1). In Thailand,although the soil burning effect is important for cropproduction, ash effects might be relatively unimportantbecause of the inherently high level of soil nutrients thatalready exist. In contrast, in Sarawak, ash effects areimportant to alleviate soil acidity and supply basic cationsand P during crop production, but the soil burning effectmight be relatively insignificant. Farmers in Thailandand Sarawak must abandon their fields after one cropbecause of the decrease in available N in Thailand, and thedecreases in all nutrients in Sarawak.Although only a rough comparison could be performedin this study because of the lack of available data, furtherinvestigation can contribute to our understanding of therole of fallow periods under low input shifting cultivationsystems, as well as to clarifying the mechanisms of landdegradation under intensified systems. Presently in

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