Evolved Strategies in Nitrogen Acquisition by Plants

Eleven features of nitrogen metabolism in the plant and in the ecosystem are delineated, each appearing to have negative survival value, especially negative effects on yield. These include apparent failures in controls over N2 fixation and over nitrate uptake; preventable energy losses in soil microbial metabolism and in root reduction in nitrate; preventable mass losses of nitrogen via denitrification and ammonia volatilization, and insufficient partitioning of photosynthetic energy to support N2 fixation in N-limited ecosystems. A quantitative examination of the energetics of metabolizing N2, NH3, and NO3 - reveals behind often high costs some energy inefficiencies required for sufficiently fast kinetics and some compromises with other metabolisms. Cost: benefit ratios can be defined for assimilation of the three forms of N, and each ratio may vary significantly according to other physiological demands, such as pH control at the rhizosphere or stomatal closure for efficient use of water. For the individual plant, I then proposed a number of metabolic principles based on the energetics inside the plant, suitably constrained, and also compromised with the metabolic strategy of soil bacteria. These principles rank-order the preferences for N sources. They also outline complementary roles for normal fast or feedback controls based on immediate environmental variations, integrative controls over relative growth rates of tissues, and nonnutritional controls that schedule maturation. The overall view is that plants in their N-nutrition follow a set of quantifiable strategies, or simple patterns in metabolic adaptations to varied conditions. The goal is to maximize yield over a range of conditions, but at times to sacrifice yield potential for clear requirements such as early competitiveness in canopy expansion. The strategies are heavily conditioned by energetics. Indeed, many are principally patterns of optimally allocating photosynthetic (energy) resources between alternatives such as N2 versus NO3 - assimilation. I note that agricultural breeding to alter adaptation ranges and to increase yield readily incurs conflicts of desired traits, such as that between ability to use high levels of fertilizer N and ability to mature. I propose that conflicts are easier to foresee between strategies (controlled blocks of biochemical activities, in genetically fixed patterns) than between individual biochemical activities. A second general use of the strategic view may be simpler prediction of overall plant responses to environmental changes (in atmospheric CO2 levels, soil aeration and acidity, etc.). When one considers competitive uptake of nitrogen by all the plants in an ecosystem, additional plant responses are seen as necessary. Also, a few inherent instabilities of strategies are seen in N2 fixation and in denitrification.