Much agricultural production in Africa, Asia and South America is currently as­sociated with a depletion of nutrients or ‘nutrient mining’ (de Koning et al. 1997; Syers et al. 1997; Sanchez 1999; Tilman et al. 2002). Increasing the use of crop residues for biofuel production may in practice exacerbate the latter problem (Troeh et al. 1999; Sauerbeck 2001). On the other hand, including woody perennials in agricultural strategies may well benefit both biomass-for-energy production and the long-term sustainability of food production, because there is evidence that woody perennials may recycle leached nutrients to near surface layers (Mele et al. 2003) and because several woody perennials are conducive to N-fixation (Sanchez 1999).

Long-term studies on the sustainability of crop production in industrialized coun­tries (Vance 2000) are not easy to interpret as far as nutrients are concerned, because, as pointed out above, there are large unintended inputs of nutrients. However, such unintended inputs are usually well below those necessary for high-productivity crop­ping. Against this background, when harvest residues such as cereal and rape straw are used as biofuel, reuse of ash on arable land has been advocated (Sander and Andren 1997).

Sustainable enhancement of biomass production can be achieved if there are ways to increase nutrient availability indefinitely (Vance 2000; Bhattacharya et al.

2003) . For several nutrients, this is, technically speaking, not a major problem be­cause the elements concerned are relatively abundant. Mg, K and Ca are in this category. However, P especially is geochemically scarce, and N nutrients are often generated by using geochemically scarce fossil fuels. This becomes even more of an issue, because increased biofuel production is expected to be partly based on in­creased yields of crops per hectare, which is linked to intensification of agriculture, including increased N and P inputs (Tilman et al. 2001; Searchinger et al. 2008).

Net natural inputs to farms of P, associated with weathering and deposition (Hedin et al. 2003), allow for very low primary productivity on farms (Newman 1997). Raising the availability of P if compared with pre-industrial times has mainly been achieved by relying on phosphate ore deposits, and there is no known al­ternative to that natural resource for doing so. So, sustainable use necessitates an extremely slow depletion of this stock. Ore deposits deplete rapidly when P used is not retrieved with high efficiency and fed back into biomass production. Non­retrieval associated with agriculture originates in harvesting, erosion (see Sect. 3.2) and leaching. Leaching is increased in agricultural land, if compared with soils un­der native forest (Williams and Melack 1997), and can be very high when soils are saturated with P (Liu et al. 2008). Preventing saturation leads to the need to restrict P additions to agricultural land.

Losses of P may also be linked with activities following harvesting. Much of the phosphate wastes associated with consumption and industrial processing of har­vested biomass currently end up in wastewater. The recycling of such phosphate back into the economy is currently poorly developed (Sims and Riddell-Black 1998; Kvarnstrom and Nilsson 1999). One may also fail to retrieve P when there is burn­ing of biomass. When biomass is burned, P ends up largely in ashes. When there is co-firing with, for instance, coal, such ashes are often considered unfit for agricul­tural use (Woodbury et al. 1999; Nugterenet al. 2001; Adriano et al. 2002; Reijnders 2005), whereas forced extraction of nutrients such as P from such ashes is not prac­ticed. Even in the case of burning pure chicken manure, the composition of ashes may be such that nutrients are not fed back to agriculture (Reijnders and Huijbregts, 2005). Feed additives with high concentrations of trace elements such as Cu and Zn are responsible for this problem. On the other hand, in the case of biomass present in sewage sludge, a process has been developed for the fractionation and recovery of phosphate (Lundin et al. 2004), and this type of approach can, in principle, be applied to all wastes that contain substantial amounts of phosphate.

It has been estimated that the resulting net loss of P from the world’s cropland is about 10.5 x 106 Mg per year, nearly one half of the amount of P that is extracted yearly as phosphate ore (Liu et al. 2008). Such large losses of P associated with pro­duction and use of biomass cannot be maintained indefinitely without jeopardizing adequate P levels in soils. If one will no longer be able to add substantial amounts of P to soils, primary productivity will ultimately plummet, negatively affecting both food and biofuel production (Newman 1997). So, indefinitely increased availabil­ity of P in soils is critically dependent on high-efficiency recycling of P involved in biomass production, while keeping soil concentrations of hazardous compounds below critical levels (Kvarnstrom and Nilsson 1999). A major effort is needed to apply this principle to biomass-for-energy.

The increased availability of nitrogen compounds to be used as fertilizer, if compared with the situation before the industrial revolution, is mainly based on the Haber synthesis, which converts fossil methane into ammonia (Galloway et al. 2008). As it stands, there is a large-scale leakage of added nitrogen compounds from biomass production systems such as plantations and annual crops. In well-managed intensive agriculture, the recovery of nitrogen in products is around 50% or less (Tinker 1997; Tilman et al. 2002). Moreover, N compounds present in biomass will largely get lost on burning. Basing the Haber synthesis on fossil fuels cannot be maintained indefinitely, as fossil carbon is virtually non-renewable. In this case, one may circumvent inputs of virtual non-renewables. For instance, hydrogen necessary for converting nitrogen present in air into N fertilizers can also be generated by hydrolysis powered by solar energy, a way of production that may be maintained in­definitely. There may also be scope for improved biogenic nitrogen fixation, which converts N2 into plant nutrients and may partially replace fertilizer amendments.