Studies regarding the prospects for future modern biomass production tend to rely fully or overwhelmingly on land as the place where biomass is grown for this pur­pose. Studies with high estimates regarding the technical potential of biomass sup­ply often have most of that potential met by energy crops that have high yearly yields per hectare (Hall and Rosillo-Calle 1998; Berndes et al. 2003; Hoogwijk et al. 2003; de Vries et al. 2007). Hoogwijk et al. (2003), for example, assumed a dry weight productivity of biomass-for-energy plantations on surplus agricultural land of up to 20Mgyear-1ha-1. Such yields may be achieved. Actual experience shows that breeding efforts may increase biomass yields (dry weight) in the range of 6.7-11.3Mgha-1year-1 to yields greater than 16Mg(Volk etal. 2003). And there is much research aiming at further yield increases, for example, by lengthening the growing season without risking frost damage, limiting remobilisation of nutrients following senescence and improving drought resistance (Karp and Shield 2008). However, in general, highly productive species and varieties tend to be relatively inefficient in their resource use (Wood 1998), which is not in line with sustainable resource use (Pimentel et al. 2002; Reijnders 2006). Indeed, sustainable productiv­ity is limited due to restrictions on water and nutrient use and the need to maintain adequate soil carbon levels.

In a first approximation to the levels of biomass production that may be produced in a sustainable way on land, it would seem useful to focus on natural net primary production (NNPP), which varies geographically (Havstad et al. 2007; Campbell et al. 2008). Kheshgi et al. (2000) estimated average natural NNPP on land at 4 Mg (= 106 g) of dry biomass per year. Campbell et al. (2008), who studied abandoned agricultural soils, estimate that potential production rates on such soils average 4.3 Mg dry biomass ha-1 year-1. As pointed out earlier in this chapter, it may well be that recycling nutrients in the case of biofuel use is less efficient than in natural systems and that a part of carbon fixed in NNPP may be necessary to maintain soil organic carbon in a steady state. Thus, it is likely than on average, a lower amount of biomass can be harvested sustainably than 4-4.3 Mgha-1year-1. Pimentel et al. (2002) have suggested that in tropical and temperate areas, on average, approxi­mately 3 Mg ha-1 of woody biomass can be harvested in a sustainable way per year.

Again, there are geographical differences in sustainably harvestable biomass due to climate and water and nutrient availability (e. g. Nabuurs and Lioubimov 2000; Gough et al. 2008).

To get an idea of what a sustainable yield of feedstock may mean for energy sup­ply, it would seem interesting to focus on agricultural land that has been abandoned (including currently fallow land). Field et al. (2008) and Campbell et al. (2008) es­timate that the total area of such land is about 385-472 x 106 ha. We further assume that, after restoration of nutrients and soil organic matter, on these lands, a yearly sustainable yield of about 3 Mg (Pimentel et al. 2002) biomass with a lower heating value of 20MJkg-1 (Field et al. 2008) may be achieved. This would correspond with about 23-28 EJ (= 1018 J) year-1. As pointed out in Chap. 1, use of primary energy for the transport sector is currently about 100 EJ.

Another option that may be considered in the context of sustainable supply re­gards biofuels produced from what are currently ‘wastes’, such as organic urban wastes, biomass from forest remediation and residues from forestry and agricul­ture which are not used as animal feed. The worldwide amount of such wastes is currently estimated at between 50 and 100 EJ (Swedish Environmental Advisory Council 2007; Lysen and van Egmond 2008). Unfortunately, it is not clear how much thereof is necessary for maintaining the future productivity of arable lands and forests in line with the sustainability requirements for soil organic matter dis­cussed in Sect. 3.2. However, even when only 10-20% thereof could be diverted to transport biofuel production, this would represent a substantial contribution to the transport fuel supply.