Soil Formation In Phobjikha Valley, Central Bhutan With Special Regard To The Redistribution Of Loessic Sediments

摘要

Soil development and landscape evolution were studied in the basin-shaped Phobjikha Valley at 2900-3200 m a.s.l, to the west of the Black Mountain Range, West Central Bhutan. The local environmental setting with strong along-valley winds, frequent freeze-thaw cycles, extensive dry periods and sparse vegetation cover seems to encourage the generation and short-distance transport of silt-sized particles. The effects of this process are evidenced in the smooth valley morphology and in the nature of the examined pedons. Their involvement in continuing redistribution of local sediments is reflected by a homogeneous silty-clayey and stone-free texture, varying profile depths, buried topsoils and weakly developed recent A horizons. In protected locations, in situ weathering of metamorphic parent materials results in alu-andic features with bulk densities < 0.9 g cm~(-3), (Al~o + 1/2Fe_o) > 2%, and phosphate retention > 95%. Dominance by Al-hydroxy interlayered clay minerals and large amounts of well-crystallised iron oxides indicate an advanced stage of weathering. In areas of preferred eolian deposition, argic and ferralic features emerge, with clay contents of up to 60% and surface areas of > 50m~2g~(-1). Under forest, umbric horizons can develop. CEC_(eff) is below 10 cmol_c kg~(-1) at all sites. Cluster and factor analyses of soil chemical and physical parameters confirm the redistribution of local sediments as a dominant factor behind the measured variables. No clear indication of glacial activities in the area was found, whereas the massive silty sediments in the lower parts of most profiles, the presence of debris slopes, and the asymmetric cross sections of the side valleys suggest periglacial conditions. Buried topsoils dated at about 2000 conventional ~(14)C years BP indicate a weakening or absence of sediment influx under wetter conditions towards the end of the Holo-cene climate optimum. Charcoal on top of paleosols suggests that human activities of deforestation, grazing and arable agriculture may have contributed to the reactivation of local sediment redistribution until today.