Claassen, 2005) and arbuscular mycorrhiza (Harrier and Watson, 2004). However, the use of microbes in P delivery to plants is complex. A better understanding of root growth is the optimal balance among plant, soil and microorgan­isms (Vance et al., 2003).
      More field research is required to optimize the selection and production of crop varieties/species that enrich the diet with such elements as Ca, Zn, Cu, and Fe. Given the usu­ally substantial residual effects of most of fertilizer nutrients (except N), they should be considered as investments in the future rather than annual costs. Replenishment of nutrients such as P, K, Ca, Mg, Zn through the use of agricultural by-products and biosolids and substitution and recycling of phosphorus (P) sources has been recommended (Kashma-nianetal., 2000).
       Soil conservation practices can reduce soil losses by wind and water erosion. Strategies for controlling sediment loss include (1) planting windbreaks and special crops to alter wind flow; (2) retaining plant residue after harvesting; (3) creating aggregates that resist entrainment, (4) increas­ing surface roughness; (5) improving farm equipment and (6) stabilizing soil surfaces using water or commercial prod­ucts (Nordstrom and Hotta, 2004).
       Improved management practices to prevent sediment loss may be effective (Nordstrom and Hotta, 2004). Many management techniques do not require sophisticated tech­nology or great costs to implement, but they may require farmer willingness to change practices. Barriers to adoption of conservation measures include start-up or transition costs associated with new methods or equipment, inadequate ed­ucation, reliance on past traditions, or a history of failed field experiments (Uri, 1999). Reluctance to implement soil conservation policies and practices can be overcome when severe erosion events associated with periods of drought re­mind society of the advantages of compatible methods of farming (Todhunter and Cihacek, 1999).
      Shifting cultivation leads to deforestation and degrada­tion, (Zhang et al., 2002). Most technical options to pre­vent agricultural expansion and abandonment are similar to those for preventing deforestation. They are also based on the promotion of off-farm employment (Mulley and Un-ruh, 2004), or the production of high-added value products combined with air transport. In order to increase farmers' natural capital and thereby increase long term flows of farm outputs, modifying the management of soil, water and veg­etation resources, based on agroecology, conservation agri­culture, agroforestry and sustainable rangeland and forest management, as well as wildlife biology and ecology has been supported (Buck et al., 2004).
       Cultivation of new lands in some biomes would nei­ther compensate nor justify the loss of irreplaceable eco­logical services. Other biomes are less sensitive and would not be similarly affected. The functional complementation of biomes is an effective land use option to explore on a broad scale (Viglizzo and Frank, 2006). For example, ag­ricultural  expansion  in  South  America   (Argentina,  Bo­livia, Brazil, Colombia) was based on the replacement of natural forests by cattle ranching and soybean cropping (Cardille and Foley, 2003; Vosti et al., 2003; Etter et al., 2006). There are potential benefits to conservation man­agement that arise from agricultural land abandonment or

extensification. In China conversion of cultivated land has not always decreased national food security, since many converted lands had low productivity (Deng et al., 2006). Abandonment of agricultural land does increase the vulner­ability of farmers. Positive outcomes in one sector can have adverse effects elsewhere (Rounsevell et al., 2006). Mod­ern biomass energy will gain a share in the future energy market and abandoned agricultural land is expected to be the largest contributor for energy crops; the geographical potential of abandoned land for 2050 ranges from about 130 to 410 EJ yr1 and for 2100, from 240 to 850 EJ yr-1. At a regional level, significant potentials are found in the for­mer USSR, East Asia and South America (Hoogwijk et al., 2005).

       Large scale, high input options. Large scale approaches to soil management are available and based on the replenish­ment of soil nutrients, site specific nutrient management and zero tillage. These approaches include: adoption of crop models to synchronize N supply with crop demand (Fage-ria and Baligar, 2005; Francis, 2005); adoption of precision agriculture and variable rate technologies for inputs such as nutrients, pesticides and seeds (Adrian et al., 2005); and improvement of N fertility for non-legumes by legume fixa­tion, fertilizers, manures and composts.
        Nitrogen use efficiency is currently less than 50% world­wide, thus increasing N efficiency may reduce the use of N fertilizers (Sommer et al., 2004; Fageria and Baligar, 2005; Ladha et al., 2005). Deep rooting crops could potentially serve to redistribute N for crops in areas with nitrate pol­luted groundwater (Berntsen et al., 2006).
        Crop models assess tradeoffs among yield, resource-use efficiency and environmental outcomes (Timsina and Humphreys, 2006), but their effective adoption requires local calibration and validation, improved farmer knowl­edge, cost-effective and user friendly techniques (Ladha et al., 2005). The adoption of precision and variable rate tech­nologies by farmers is significantly affected by their percep­tion of usefulness and net benefit (Adrian et al., 2005). To be of more benefit to farmers, crop models need to more effectively couple the spatial variability of crop yields and soil properties obtained by remote sensing and variable rate machinery needs improvement. Motivations for widespread uptake adoption of these technologies may come from envi­ronmental legislation and public concern over agrochemical use (Zhang et al., 2002).
        Efficient use of N fertilizer requires that the amount and timing of the fertilizer application be synchronized with the needs of the crop (Ladha et al., 2005). The availability of the soil to supply N to the crop is closely linked with soil organic matter; maintenance of soil organic matter is a key factor in maintaining N fertility (Robson et al., 2002). Legumes are grown in rotations both for the contribution to the residual N and for the value of the crop itself (i.e., forage or food). To encourage the adoption of modern agricultural technologies governments and others will need to ensure farmers have access to technical advice, economic incentives and public education programs.
         Whereas N efficiency and uptake is key for some re­gions, in others soil erosion control practices, such as con­tour cropping and terracing in soils of better quality (Popp