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Regolith and soils in Bhutan, Eastern Himalayas

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Regolith and soils in Bhutan, Eastern Himalayas
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  Regolith and soils in Bhutan, Eastern Himalayas I. C. B AILLIE a,b , K ADO  T SHERING a , T SHERING  D ORJI a , H. B. T AMANG a , T SHETEN  D ORJI a , C HENCHO N ORBU a , A. A. H UTCHEON a & R. B A ¨ UMLER c a National Soil Services Centre, Ministry of Agriculture, PO Box 119, Thimphu, Bhutan,  b National Soil Resources Institute, Cranfield University, Silsoe, Bedfordshire MK45 4DT, UK, and   c Department of Soil Science, Technical University of Munich, 85350 Freising-Weihenstephan, Germany Summary Bhutan lies at altitudes of 100–7500m on the steep, long and complex southern slopes of the EasternHimalayas. Soil surveys show that, despite steep gradients, there are many moderately or deeplyweathered soils. Many slopes are mantled with polycyclic, layered drift materials, so soil horizons oweas much to regolith heterogeneity as to pedogenesis. In the limited arable areas soil profiles are furthercomplicated by rice cultivation and the construction, maintenance and irrigation of flat terraces on steepslopes. Some natural pedogenic horizonation is apparent, and there is an altitudinal zonation of soiltypes. Although the climate is warm and seasonally wet, most soils on the subtropical southern foothillsare not particularly weathered and leached. The foothills are seismically active, and many soils areformed in unstable landslide debris. Elsewhere the regoliths are more stable. The main soils up to about3000m in the inner valleys are moderately weathered and leached, and have bright subsoil colours andthin dark topsoils. Above these there is a zone of bright orange-coloured non-volcanic andosolic soils.Further upslope there are acid soils with thick surface litter, stagnogleyic topsoils, and drab brownsubsoils with organic cutans. These grade to weak podzols, which extend from about 3500m up to thetreeline, around 4000m. Above this, alpine turf soils, with deep, dark, and friable topsoils and yellowishfriable subsoils, are intermixed with unweathered glacial deposits. The interactions between pedogenesisand the deposition of the varied and layered drift materials complicate mapping and classification of thesoils. Introduction Many accounts of Himalayan soils emphasize their shallow-ness, stoniness and limited development. Such generalizationsare more valid for dry areas in the west (Gupta & Tripathi,1996) than for the humid Central Himalayas where extensivesurveys show that many soils are moderately deep, weathered,leached, and show marked pedogenic development (Carson etal  ., 1986; Maharjan, 1991; Ba ¨umler & Zech, 1994a). Thereare no published soil surveys of comparable scope for the non-Bhutan sections of the Eastern Himalayas, although therehave been limited studies of hill soils in Sikkim and ArunachalPradesh (e.g. Das  etal  ., 1996; Walia & Chamauli, 1996).The kingdom of Bhutan lies on the southern slopes of theEastern Himalayas. Only about 8 % of the country is cultivatedbecause high altitudes, harsh climate and steep slopes limitcropping, and this is unlikely to exceed 10 %  without risk of severe land degradation. Despite the scarcity of arable land,over 80 % of Bhutan’s workforce is engaged in agriculture andrelated activities, so careful soil management and conservationare essential for sustainable development. The Royal Govern-ment therefore established a National Soil Services Centre,with assistance from Denmark, the Netherlands and the Euro-pean Union. The Centre includes a soil survey unit, and thispaper draws on data from its first 4years of fieldwork. We aimto identify aspects of the formation and distribution of soils,and to assess the relative contributions of regolith heterogen-eity and pedogenesis to soil patterns. Methods The study area Bhutan is the only independent survivor of the formerBuddhist monarchies and theocracies in the Himalayas, asLadakh, Mustang, Tibet and Sikkim are now subsumed intolarger states. Bhutan ended its seclusion in the second half of the 20th century, but its location, topography and Correspondence: I.C. Baillie. E-mail: Ian.Baillie@lineone.netReceived 9 April 2002; revised version accepted 18 March 2003 European Journal of Soil Science , March 2004,  55,  9–27 doi: 10.1046/j.1365-2389.2003.00579.x #  2003 Blackwell Publishing Ltd  9  infrastructure mean that it is still fairly isolated. It lies betweenthe Indian states of Sikkim and Arunachal Pradesh andspans latitudes 26  47 0 N to 28  26 0 N and longitudes 88  52 0 E to92  03 0 E. It is similar to Switzerland in size (about 46000km 2 )and shape (Figure 1).Bhutan’s geological structure results from the intense tec-tonic activity that resulted from the collision of the Indian andAsian continental plates and the uplift of the interveningHimalayas. It is mostly underlain by thick sheets of highlymetamorphosed gneisses, quartzites, schists and marbles,which were emplaced in a series of southward and eastwardthrusts since the middle Tertiary. The sheets show inversemetamorphism, with more weakly metamorphosed schists out-cropping through windows in the overlying gneisses. Themetamorphic rocks are intruded by granitic and ultramaficbodies, ranging from veins to large batholiths. The intenselymetamorphosed rocks are the eastwards continuation of theHimalayan Central Crystalline Complex. In the CentralHimalayas the complex is flanked to the south by wide out-crops of the less metamorphosed rocks of the Lesser Himalayanformations. In Bhutan the gneisses stretch southwards almost tothe alluvial piedmont plain of India, and cover more than 70 % of the country. The Lesser Himalayan formations are com-pressed into a narrow belt along the southern border, exceptin the southeast (Bhargava, 1995). The active Main BoundaryFault runs through southern Bhutan, which experiences fre-quent earth tremors. There is less seismic activity in the centreand north of the country.Since losing most of its piedmont territories to British Indiain the 19th century, Bhutan has consisted almost wholly of high mountains, and is one of the most rugged countries in theworld. Altitudes range from 100 to 7500m above sea level,with more than two-thirds of the country above 2000m, andalmost a quarter above 4000m. Slopes are steep, long andcomplex, and virtually the whole country qualifies for thehigh and very high mountain relief types and most ruggedrelief roughness class of Meybeck  etal  . (2001).The landscape is shaped by the ongoing uplift of theHimalayas, and by the relative lithological homogeneity. Themain features are the High Himalayas, which is a range of veryhigh mountains aligned more or less east–west along thenorthern border, and the spur ranges of high mountains anddeep valleys that run southwards from it (Figure 1). The north– south ranges are very uneven and rugged, but have stretcheswith summits approximately concordant at 3500–4500m. Attheir southern ends, they plunge steeply down to the jumbledand rugged foothills that rise abruptly out of the piedmontplains (Figure 2).The main rivers flow steeply more or less southwards to theBrahmaputra. Unlike the Central Himalayas, there are few 50   km28 ° N92 ° E91 ° E90 ° E89 ° EINDIABAYOFBENGAL Figure1  Sketch map of Bhutan, showing main rivers, peaks of the High Himalayas ( n ), north–south ranges (grey), and inner valleys (black). 10  I. C. Baillie  et al . # 2003 Blackwell Publishing Ltd,  European Journal of Soil Science ,  55,  9–27  significant east–west subsequent valleys due to lithologicalcontrasts within the Lesser Himalayan beds. The steep riversare fast flowing, turbulent, and carry much sediment to theBrahmaputra plains.All the rivers are working to similar base levels, set by theBrahmaputra, but the histories of their dissection and theirlongitudinal profiles vary considerably. The two main rivers inthe east are antecedent. They rise on the Tibetan Plateau andcut gorges through the High Himalayas. Their valleys in Bhutanare deep and steep, with narrow thalwegs and little alluvium.The main rivers in the centre and the west of the country riseon the southern slopes of the High Himalayas and havestepped profiles with multiple knickpoints. The valleys of some of them widen out upstream of major knickpoints intheir middle reaches to form relatively gentle basin-likesections, which are isolated by rugged terrain up- and down-stream (Figures 1 and 2). These are the inner valleys, and theyhave straight or concave lower slopes, and thalwegs up to 1kmwide, which are floored with alluvium of various types andages. Their side slopes are moderately graded by Bhutanesestandards, but still fit the roughest relief category of Meybeck etal  . (2001). Despite this and their limited extent, the innervalleys are the historical and cultural heartland of Bhutan, andmany people live and farm here. Population and cultivationare also relatively dense in parts of the southern foothills,where development is more recent.Temperature decreases with altitude at lapse rates of 5–6  Cper vertical kilometre. Mean air temperatures range from over20  C to below zero. There are no data on soil temperatures,but regimes probably range from thermic to cryic (Soil SurveyStaff, 1999). The winters are dry and bright, and are littleaffected by the westerly disturbances that bring some winterrain to the Western Himalayas (Mani, 1981), and most of therainfall occurs in the summer monsoon. The Eastern Hima-layas are near the head of the Bay of Bengal (Figure 1), whichis the monsoon’s landfall, and precipitation is generally greaterthan in the Central and Western Himalayas (Miehe  etal  .,2001). Although partly screened from the monsoon by theMeghalaya plateau to the south, the orographic rainfall inthe southern foothills of Bhutan is heavy and intense, withannual means exceeding 4m in places and occasional storms of more than 500mmday  1 .There is a sharp decrease in precipitation northwards, andmany stations record annual means of less than 1000mm.Many of the rain gauges are located in settled areas on thefloors and lower slopes of the inner valleys, and this gives anexaggerated impression of general dryness. Cloud cover andrainfall in the valley floors are suppressed by strong andprolonged up-valley and up-slope winds. Stream flows, forestphysiognomy (Ohsawa, 1987), and limited data for rainfall onthe mountains indicate that the middle and upper slopes areconsiderably moister than the valley floors, receiving well over 400020006000    A   l   t   i   t  u   d  e   (  m   a   b  o  v  e  s  e  a   l  e  v  e   l   ) SoutherngorgeInnervalleyMontanetorrentPiedmontSouthernfoothillsNorth–south rangeHighHimalayas13456782Latitude27 ° N 28 ° N0 Figure2  Generalized north–south transect of Bhutan, showing profile of main interfluve crest (solid line), river profile (dashed line), and altitudinaldistribution of soil zones: (1) raw subtropical soils on unstable slopes; (2) cultivated drift soils on floors of inner valley; (3) soils poor in organicmatter under blue pine forest; (4) non-volcanic Andosols; (5) drab brown soils under mixed conifer forest; (6) weak podzols under silver fir forest;(7) alpine turf soils; (8) periglacial debris. Regolith and soils in Bhutan  11 #  2003 Blackwell Publishing Ltd,  European Journal of Soil Science ,  55,  9–27  a metre of rain per year. Although there are hardly any recordsfor the High Himalaya range it appears to be generally dry.We have no soil moisture data, but most soils appear tohave udic regimes, judging from the moist subsoils seen inprofile pits at all times of the year. Also the onset of growthin the natural vegetation and weeds in spring precedes thepre-monsoon rains and appears to be controlled mainly bytemperature. A few non-irrigated arable crops can be grownin early spring on residual soil moisture. However, some soilson the floors and lower slopes of the inner valleys appear tobecome dry enough in winter to have ustic moisture regimes,and have been correlated accordingly (Soil Survey Staff, 1999).Over two-thirds of the country is forested, and the proportionis increasing. The vegetation varies widely with temperature andrainfall, ranging from subtropical mesic semi-evergreen rainforest in the southern foothills, through temperate deciduousbroadleaf and subalpine coniferous forests at intermediatealtitudes, to alpine grassland and scrub above the treeline atabout 4000m. Bhutan is biologically very diverse, and its con-servation importance and record are internationally recognized(Wikramanayake  etal  ., 2002).Single-cropped rice on flood-irrigated basin terraces is themain cropping system on the predominantly small ( < 2ha)farms. Most terraces are on fairly gently sloping alluvialterraces and fans, but some are constructed on slopes of over30degrees. Figure 3(a) shows that the construction of terraceson such steep slopes involves considerable effort and yieldsonly narrow and fragmented strips of cropland (Vincent,1995). Other cropping systems include rainfed maize, potatoesand buckwheat; orchards of apples and citrus, and groves of cardamom; and long fallow rotations that use fire to liberateaccumulated nutrients from the organic matter of eitherheaped topsoil (Roder  etal  ., 1993) or woody biomass. Bhutan-ese farmers use few mineral fertilizers (FAO, 1990) or otheragrochemicals, but organic fertilizers are applied regularly tomost arable land. Data collection Most of our initial soil surveys are of parts of the inner valleys,and we have only limited experience of the southern foothillsand northern mountains. The data summarized here areselected from about 450 profiles, supported by about 2100auger inspections. The main horizons of the profiles wereanalysed as follows (Soil and Plant Analytical Laboratory,1993): pH by pH meter in 1:2.5 suspensions of soil in distilledwater; organic carbon by Walkley–Black oxidation; total N bymicro-Kjeldahl digestion and colorimetry; available P by Brayand Kurtz 0.5 M  HCl and 1 M  NH 4 F extraction, and color-imetry; exchangeable Ca 2 þ , Mg 2 þ , K þ and Na þ by leachingwith excess 1 M  NH 4 OAc and flame photometry (K, Na) oratomic absorption spectrophotometry (Ca, Mg); cationexchange capacity (CEC) by leaching the ammonium withexcess 1 M  KCl and colorimetry; Al and H (for samples withpH in water below 5.0) by 1 M  acidified KCl extraction, andsequential titrations with 0.05 M  NaOH and 0.05 M  HCl;particle-size fractions by pipette sampling of soil:water suspen-sions after H 2 O 2  oxidation and dispersion with sodium hexa-metaphosphate.The analytical characteristics of the soil profiles are listed inTable 1. Results and discussion Four main trends in soil features and distribution patterns arehighlighted: (i) the depth of chemical weathering at highaltitudes; (ii) the extent, depth and complexity of surfacedrift materials; (iii) the marked effects of rice terrace construc-tion and cultivation; and (iv) the altitudinal zonation of pedogenesis. Depth of weathering Road cuttings reveal extensive deep weathering in gneiss ataltitudes up to 4000m. The saprolite is soft to at least 8m, butthe quartz veins survive undisturbed .  This suggests that chem-ical weathering is as important as physical processes. However,light grey colours predominate, and there is little rubification.Motegi (1998) has suggested that the roughly concordantsummits on the north–south ranges are remnants of upliftedand dissected erosion surfaces. The weathering may havestarted at low altitudes and in warmer climates, and thewhole landscape has since been elevated. The Himalayas arestill rising rapidly, and Gansser (1983) recognizes uplifts of upto 5000m, so this idea is possible. Low altitude weathering andsubsequent uplift, however, may not be essential, as consider-able  in situ  chemical weathering occurs at high altitudes intemperate mountains elsewhere (e.g. Derry & France-Lanord,1997). Moreover, if the weathering started at lower altitudes, itshould have been accompanied by rubification, as in gneisscurrently undergoing chemical weathering at altitudes belowabout 3000m.Most biotite disappears early, but muscovite persists, and itsfine glinting flakes are common in the deeply weathered gneissand in soils throughout Bhutan. Such flakes contribute to thelarge contents of silt and fine sand in many topsoils. Largequantities of silt generated by the weathering of gneisses andsimilar rocks are found elsewhere in the Himalayas, includingsites at low altitudes (Gardner, 1994; Srivastava, 1998), and donot always indicate predominantly physical weathering. Thesilts are susceptible to mobilization and transport by wind, andare an important source of aeolian material. Polygenetic regoliths The survival of deep saprolites indicates that erosion is not souniformly intense as to strip all slopes of loose materials.However, most of these are actively moving downhill, so that 12  I. C. Baillie  et al . # 2003 Blackwell Publishing Ltd,  European Journal of Soil Science ,  55,  9–27  (a) (b)(d)(c) Figure3  (a) Narrow rice terraces cut into a steep slope, close to Profile 5. (b) Profile 3, showing lithological discontinuity in a rice soil on fan depositsof greyish gneissic sandy clay loam over reddish calcsilicate clay. (c) Pronounced organic cutans in aeolian over alluvial deposits at 2700m.(d) Subalpine podzol with moderately developed B s  but no E horizon, under silver fir at 3800m. Regolith and soils in Bhutan  13 #  2003 Blackwell Publishing Ltd,  European Journal of Soil Science ,  55,  9–27
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