Red and orange iron-stained sands of relatively recent age occur widely in both coastal dunefields and continental deserts in the tropics and subtropics. Red desert dune sands have been described from elsewhere ( Figure 1). Not all tropical and subtropical dunes are red, but in general aeolian red sands are more common in these areas than in temperate and high latitudes.The question of the nature and significance of red sediments in general has concerned geologists for many years, since reddening is often an important process associated with diagenesis, and because ancient red sediments occur widely in the geological record (Van Houten, 1973). The debate concerning red sediments has centred around two main questions; first, the nature of the climatic conditions which best favour reddening, and hence the palaeoclimatic inferences which can be drawn from fossil red beds; and, second, the role of time in the reddening process.Several authors have discussed these issues with reference to recent sediments in particular areas (e.g. Norris, 1969; Folk, 1976;Walker, 1979), but little attention has been given to patterns of reddening at the wider global scale.The purpose of this paper is to review the present state of knowledge regarding the nature, origin and palaeoclimatic significance of red dune sands within the context of late Quaternary environmental change. The paper is therefore concerned with reddening of sands under subaerial conditions rather than at high temperature and pressures. Processes of reddening during diagenesis have recently been reviewed by Walker (1976) and Turner (1980) in relation to the general problem of continental red beds. The discussion stems in part from recent investigations carried out by the authors on red dune sands in southern India and northeast Australia. These investigations will be described in more detail elsewhere.
IThe nature of red dune sands .
Between the elevations of 1000 and 2000 m in the mid-hills of Nepal, over 12 million people subsist on land-holdings of less than 0Á5 ha. These farmers have limited access to commercial inputs such as fertilisers and are reliant on organic manures for soil fertility maintenance. Participatory research was conducted with farmers on bari land (upper slope rain-fed crop terraces) in the hill community of Landruk (bench terraces 0-58 slope, 3000-3500 mm annual rainfall, which aimed to develop soil and water management interventions that controlled erosion without resulting in high leaching, and so were effective in minimising total nutrient losses. Interventions tested were the control of water movement through diversion of run-on and planting fodder grasses on terrace risers on bench terraces. The interventions were effective in reducing soil loss from the bari land in comparison with existing farmer practices, but no effect was observed on nutrient losses in solution form through runoff and leaching. Losses of NO 3 -N in leachate ranged from 17Á3 to 99Á7 kg ha À1 yr
À1, but only 0Á7 to 5Á6 kg ha À1 yr À1 in runoff. The overall nutrient balance suggests that the system is not sustainable. Fertility is heavily dependent on livestock inputs and if the current trends of declining livestock numbers due to labour constraints continue, further losses in productivity can be expected. However, farmers are interested in interventions that tie ecosystem services with productivity enhancement and farmers' priorities should be used as entry points for promoting interventions that are system compatible and harness niche opportunities.
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