A data base was constructed of the % N and plant d. wts (W) in t ha" 1 of C3 and C4 crops that had been grown with sufficient nitrogen to permit maximum growth rate. The % N of all crops declined sharply with increase in Wbut this decline differed between C3 and C4 crops. When Wwas greater than I t ha" 1 , 86% of the variance in In % N was removed by the model % N = aW' b with b =-05 for all crops, and a = 5-7 % for C3 crops and 4-1 % for C4 crops. The same model gave a good description of data on C3 and C4 crops entirely independent of that used for developing the model. According to this relationship the fractional decline in % N with increase in plant mass was the same for both types of crops, but C4 crops contained about 72% of the nitrogen in C3 crops at equivalent d. wts. As approx. 32% more dry matter was produced per unit of intercepted radiation for C4 and C3 crops, the N uptake (or weight of plant protein produced) per unit of intercepted radiation was approximately the same for both types of crops. A small improvement in the degree of fit to % N = aW~" was obtained by allowing both a and b to vary with the crop. Values of b obtained in this way for tall fescue, lucerne and winter wheat, but not for potato and sorghum, were consistent with Hardwick's 'skin core' hypothesis (Annals of Botany, 1989,60,439-46). The entire data set was, however, consistent with Caloin and Yu's model (Annals of Botany, 1984, 54, 69-76) in which there is a conceptual N pool for photosynthesis and another N pool for the other processes.

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Crop solar radiation balance can be established as absorbed or intercepted radiation. Absorbed radiation is the balance between received radiations (incident and soil reflected radiations) and "lost" radiations (crop reflected and transmitted radiafions). Soil reflection is neglected in Gallagher and Biscoe (1978) formulae. Intercepted radiation is the difference between incident and transmitted radiation. The fractions of absorbed and intercepted photosynthetically active radiation are close but the fractions of absorbed or intercepted radiation are different between PAR and total radiation.The main methods to obtain radiation balance components are given; the main difficulty is to measure transmitted radiation. This component can be obtained from a randomly located set of several stationnary sensors or a moving sensor across the crop, or a linear sensor (some manufactured radiation sensors are given). Some values of interception (or absorption) coefficient K from the relationships between the fraction of intercepted (or absorbed) radiation and leaf area are given.total solar radiation -PAR -solar radiation balance -efficiency of absorbed radiation -efficiency of intercepted radiation

Biomass from dedicated crops is expected to contribute significantly to the replacement of fossil resources. However, sustainable bioenergy cropping systems must provide high biomass production and low environmental impacts. This study aimed at quantifying biomass production, nutrient removal, expected ethanol production, and greenhouse gas (GHG) balance of six bioenergy crops: Miscanthus 9 giganteus, switchgrass, fescue, alfalfa, triticale, and fiber sorghum. Biomass production and N, P, K balances (input-output) were measured during 4 years in a long-term experiment, which included two nitrogen fertilization treatments. These results were used to calculate a posteriori 'optimized' fertilization practices, which would ensure a sustainable production with a nil balance of nutrients. A modified version of the cost/benefit approach proposed by Crutzen et al. (2008), comparing the GHG emissions resulting from N-P-K fertilization of bioenergy crops and the GHG emissions saved by replacing fossil fuel, was applied to these 'optimized' situations. Biomass production varied among crops between 10.0 (fescue) and 26.9 t DM ha À1 yr À1 (miscanthus harvested early) and the expected ethanol production between 1.3 (alfalfa) and 6.1 t ha À1 yr À1 (miscanthus harvested early). The cost/benefit ratio ranged from 0.10 (miscanthus harvested late) to 0.71 (fescue); it was closely correlated with the N/C ratio of the harvested biomass, except for alfalfa. The amount of saved CO 2 emissions varied from 1.0 (fescue) to 8.6 t CO 2 eq ha À1 yr À1(miscanthus harvested early or late). Due to its high biomass production, miscanthus was able to combine a high production of ethanol and a large saving of CO 2 emissions. Miscanthus and switchgrass harvested late gave the best compromise between low N-P-K requirements, high GHG saving per unit of biomass, and high productivity per hectare.

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