An experiment conducted from 1988 to 1997 to determine the effects of the timing of application and nutrient supply (particularly of phosphorus) is reported. The sources of applied nutrients that were compared were farmyard manure, pre-rice green-manuring with Sesbania aculeata and fertilizer application in a rice-wheat rotation on a typic ustifluvent. The application of seven tonnes farmyard manure per hectare to both the rice and the wheat crops over eight years increased organic carbon levels from 1.4 to 1.6% but had no yield effect on either crop. Phosphorus application through farmyard manure was not adequate for rice, whilst an application of 34 kg P ha −1 to the rotation gave an economic yield increase only in rice and then only in the first four years of the experiment. From the third year, green manuring was able to replace the effects of the recommended nitrogen, phosphorus and potassium fertilizer applications in increasing rice yield. Green manuring had no effect on the wheat yield but the recommended fertilizer application increased yield. Green manuring increased soil total nitrogen and available potassium levels and reduced base saturation. After adjusting rice yields for variation in transplanting date between years there was no statistical evidence of a yield trend in either crop over the period of the experiment. Farmers' practice of applying seven tonnes farmyard manure per hectare appears adequate to produce stable rice paddy yields of 4-6 t ha −1 a −1 . I N T RO D U C T I O NTraditionally, wetland soils in the Wangdue-Phodrang valley of Bhutan have been used to grow one summer crop of rice (Oryza sativa) per year. Increasingly, winter crops (wheat [Triticum aestivum var. aestivum] or mustard [Brassica campestris var. toria]) or, less commonly, a spring rice crop are also being grown on these soils. To meet the manuring requirements of this cropping intensification, farmers need to spread their traditional but increasingly limited farmyard manure (FYM) resources more thinly (i.e. between two crops rather than one) and are supplementing the use of FYM with the use of inorganic fertilizers, particularly urea. Farmers do not use green manures.In a recent survey (DRDS, 2001b) in the valley, the farmers' estimate of mean yield was 3.50 (s.e. = 0.22) t ha −1 for paddy and 0.75 (s.e. = 0.18) t ha −1 for wheat. These yields are close to the recent national average estimates of 3.55 t ha −1 for rice and 0.93 t ha −1 for wheat (RGoB, 2000). Urea was applied by 52% of farmers to rice §Corresponding author, G. B. Chettri:

Subsistence Bhutanese farmers spread across different agro-ecological zones maintain large species and varietal diversity of different crops in their farm. However, no studies have been undertaken yet to assess why farmers conserve and maintain large agro-biodiversity, the extent of agro-ecological richness, species richness, estimated loss of traditional varieties and threats to the loss of on-farm agro-biodiversity. Information on the number of varieties cultivated by the farmers for six important staple crops were collected from nine districts and twenty sub-districts spread across six different agro-ecological zones of the country to understand farmers reasons for maintaining on-farm crop diversity, estimate OPEN ACCESSAgriculture 2015, 5 3 agro-ecological richness, species richness and the overall loss of traditional varieties, to know the famers' level of awareness on climate change and the different threats to crop diversity. The results from this study indicated that an overwhelming 93% of the respondents manage and use agro-biodiversity for household food security and livelihood. The average agro-ecological richness ranged from 1.17 to 2.26 while the average species richness ranged from 0.50 to 2.66. The average agro-ecological richness indicates a large agro-ecological heterogeneity in terms of the different species of staple crops cultivated. The average species richness on the other hand shows that agro-ecological heterogeneity determines the type and extent of the cultivation of the six different staple cereals under consideration. The overall loss of traditional varieties in a time period of 20 years stands at 28.57%. On climate change, 94% of the farmers recognize that local climate is changing while 86% responded that they are aware of the potential impacts of climate change on their livelihoods. Climate change and associated factors was considered the most imminent threat to the management and loss of on-farm agro-biodiversity. The results from this study indicate that on-farm agro-biodiversity conservation, development and utilization programs have to be more specific to the different agro-ecological zones considering the agro-ecological heterogeneity. Attention has to be given to individual crops that have low average species richness and high percentage of loss of traditional varieties. The impact of climate change could offset the traditional seed system which primarily supports the persistence of on-farm agro-biodiversity in several ways.

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An analysis of food crop germplasm flows into and out of Bhutan was carried out to determine the extent of reliance of Bhutanese agriculture on introduced germplasm. Methods used included literature review, key informant interviews, field visits and crop pedigree analysis. Bhutan has been introducing foreign germplasm since the 1960s. By December 2015, about 300 varieties of 46 food crops including several non-traditional crops were introduced. Germplasm sources include CGIAR centres such as IRRI, CIMMYT, ICARDA, and AVRDC and countries such as Bangladesh, India, Japan, Korea, Nepal, and Thailand. Pedigree analysis of rice varieties indicated that 74% of the released varieties originated in other countries. Using imported germplasm, Bhutan has formally released over 180 varieties of cereals, fruits and vegetables. Initially, the germplasm flow was largely unregulated, but the country has been developing formal exchange mechanisms with the creation of the National Biodiversity Centre (NBC) in 1995. Findings point to a strong reliance on external germplasm for the country's major food crops. International germplasm exchanges will remain important to deal with new environmental and climatic conditions and given the limited national plant breeding capacity. National agricultural policy should give higher priority to collaborative development of new crops in the Himalayan region and beyond.

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