Why are Nitrogen Concentrations in Plant Tissues Lower under Elevated CO₂? A Critical Examination of the Hypotheses

Abstract

Plants grown under elevated atmospheric [CO 2 ] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO 2 ]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO 2 . This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO 2 , although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO 2 , plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.Key words: carbon dioxide; dilution; elevated CO 2 ; graphical vector analysis; nitrogen; plants; root uptake; tissue concentrations. Growth of plants at atmospheric concentrations of carbon dioxide (CO 2 ) greater than the current ambient can greatly affect plant tissue chemistry (Poorter et al. 1997;Loladze 2002). One of the most commonly seen effects is a decrease in the dry mass concentration of N (N m ). Cotrufo et al. (1998), synthesizing data from a broad range of studies, found mean decreases in N m of 14% in aboveground tissues and 9% in roots. This compares closely with the findings of other data syntheses, which have found elevated [CO 2 ] mediated decreases in N m Received 25 Feb. 2008 Accepted 11 Jun. 2008 Supported by the Cullen Fund of Southwestern University to D. R. Taub.* Author for correspondence. Yin (2002) found that the effect of elevated [CO 2 ] was greatest for woody deciduous species and that decreases in N m under elevated CO 2 were most pronounced at high light levels, high temperatures and large pot sizes. Yin (2002) and Taub et al. (2008) both found that the effect of [CO 2 ] on N m was reduced by N fertilization. Several studies have reported that the effect of elevated [CO 2 ] on N m is less for nitrogen-fixing species than for other types of plants (Cotrufo et al. 1998; Jablonski et al. 2002;Taub et al. 2008). Ainsworth and Long (2005) and Taub et al. (2008) found that the effect of elevated CO 2 on N m increased under ozone stress (although see Taub et al. 2008 for divergent results for soybean).Although it has been well established that elevated [CO 2 ] typically decreases N m , the mechanisms by which this occurs are n...

The Autoregulation Gene SUNN Mediates Changes in Root Organ Formation in Response to Nitrogen through Alteration of Shoot-to-Root Auxin Transport

“…As expected, increased C also reduced tissue [N], which could be due to a dilution effect by increased C assimilation and by a decrease in N uptake under high CO 2 concentrations (Taub and Wang, 2008).…”

Root growth and N dynamics in response to multi-year experimental warming, summer drought and elevated CO2 in a mixed heathland-grass ecosystem

“…Different studies have provided highly variable results with increasing, decreasing or unchanged uptake under elevated CO 2 (Rogers et al 1994;Newbery et al 1995;Jackson and Reynolds 1996;Bassirirad 2000;Gavito et al 2001;Bielenberg and Bassirirad 2005;Taub and Wang 2008). Dijkstra et al (2010) found that an increase in plant biomass with CO 2 enrichment was mostly a result of increased nitrogen use efficiency, whereas increased plant N uptake contributed to the increase in biomass with increased soil moisture.…”

“…Elevated CO 2 commonly results in lower tissue N concentration, for reasons that are not fully understood but include the dilution effect due to the accumulation of nonstructural carbohydrates (Bassirirad et al 1997;Taub and Wang 2008). The stimulated root growth results in more roots that can exploit soil nutrients better than root systems in plots maintained at ambient CO 2 concentrations.…”

Nutritionally Enhanced Staple Food Crops

“…The environmental trade-offs between yield and nutritional quality could result either from the variation in soil health or quality (nutrientdeficient soils or soils affected by salinity/alkalinity or acidity; such soil conditions lead to nutrient imbalance in the soil) and soil fertility related factors (i.e., NPK effect on seed composition) or due to drought and high temperature during the seed development. For example, reduced seed protein and minerals and altered lipid composition have been reported in barley, potato, rice, and soybean as a consequence of nitrogenous fertilizer application (Riedel 2010) or global climatic changes, especially due to high temperature and high CO 2 concentrations in the environments (H€ ogy and Fangmeier 2008;Taub and Wang 2008;Pleijel and Danielsson 2009;Sinha et al 2009;DaMatta et al 2010;Erbs et al 2010). The challenge for the agricultural research community is to minimize any possible negative trade-offs between yield and nutrient concentrations, to provide nutritious staple foods for growing populations (Davis 2009).…”