Carbon allocation and NUE in annual versus perennial crops

A key challenge for the production of next generation bioenergy feedstocks is increasing yields while maintaining sustainability. As mentioned previously, the existing agricultural concept of NUE relates N uptake to yield (Moll et al., 1982), generally in terms of grain production, and thus has severe limitations in comparing annual to perennial crops. In ecological studies, NUE is associated with whole-plant physiology, the assimilation of N, and other nutrients that are necessary for carbon fixation into sugars and carbon allocation into tissues forming stems, leaves and roots. For bioenergy crops, an assessment of the growth habit and life cycle of the crop is necessary in order to compare NUE of seed or oil crops to lignocellulosic energy. In addition, it is clear that NUE should be calculated from harvestable rather than total biomass (Weih et al., 2011). In general, NUE for bioenergy crops is not well studied or characterized, and most studies do not address integration of processes. Whereas annuals depend more on acquired nutrients for growth (Chapin et al. , 1990), perennial crops have an advantage with traits such as rapid spring regrowth from existing carbon reserves and generally higher NUE (Jorgensen & Schelde, 2001). Lignocellulosic crops such as poplar, willow, Eucalyptus, and Miscanthus have higher NUE than traditional annual cereal crops in part due to differences in harvest time or multiple year rotations which allow higher rates of translocation of N to storage organs like stems and roots (Jorgensen & Schelde, 2001). Ecological studies suggest that NUE is the product of mean retention time (MRT), defined as the length of time a unit of N is present in a population, which is representative of N carryover from annual to perennial plant parts (Berendse & Aerts, 1987; Aerts & Chapin, 2000; Weih et al., 2011). Thus, perennials may compensate for lower N acquisition capabilities by having higher N retention due to a lower total biomass turnover rate (Aerts & Chapin, 2000). A high NUE does not necessarily indicate that the system as a whole is more efficient (Jorgensen & Schelde, 2001). One of the criticisms leveled at bioenergy crops is an increased use of N fertilizers derived from fossil fuels and associated greenhouse gas (GHG) emissions (Scharlemann & Laurance, 2008; Erisman et al., 2010). Most of the major industrialized areas of the world, including the United States, European Union, and China have proposed increasing sustainable energy sources through the development of bioenergy crops. However, there have been few discussions over the environmental impacts of changes in the N cycle as a result of increasing biomass production. Thus, improvements in NUE of bioenergy crops will be crucial for mitigation of GHG associated with the production of biofuels (Erisman et al. , 2010). NUE of perennial biofuel crops can be improved through a combination of optimizing soil, fertilizer and water interactions, as well as through improvement in traits associated with the physiology of N uptake and assimilation. Development of higher yield bioenergy crops with increased NUE and decreased or neutral soil and atmospheric N losses is critical in order to create a sustainable source of energy for increasing world energy consumption (Erisman et al., 2010).

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