Effects of elevated carbon dioxide and nitrogen fertilization on nitrate reductase activity in sweetgum and loblolly pine trees in two temperate forests
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Nitrogen (N) availability is a major factor limiting plant production in many terrestrial ecosystems and is a key regulator of plant response to elevated CO2. Plant N status is a function of both soil N availability and plant N uptake and assimilation capacity. As a rate-limiting step in nitrate assimilation, the reduction of nitrate is an important component of plant physiological response to elevated CO2 and terrestrial carbon sequestration. We examine the effects of elevated CO2 and N availability on the activity of nitrate reductase, the enzyme catalyzing the reduction of nitrate to nitrite, in two temperate forests—a closed canopy sweetgum (Liquidambar styraciflua) plantation in Tennessee (Oak Ridge National Laboratory (ORNL)) and a loblolly pine (Pinus taeda) stand in North Carolina (Duke). Both CO2 and N enrichment had species specific impacts on nitrate reductase activity (NaR). Elevated CO2 and N fertilization decreased foliar NaR in P. taeda, but there were no treatment effects on L. styraciflua NaR at ORNL or Duke. NaR in 1-year P. taeda needles was significantly greater than in 0-year old needles across treatments. P. taeda NaR was negatively correlated with bio-available molybdenum concentrations in soils, suggesting that CO2 and N-mediated changes in soil nutrient status may be altering soil-plant N-dynamics. The variation in response among species may reflect different strategies for acquiring N and suggests that elevated CO2 may alter plant N dynamics through changes in NaR.
KeywordsFree-air carbon dioxide enrichment (FACE) Liquidambar styraciflua Micronutrients Molybdenum NO3− assimilation Pinus taeda
nitrate reductase activity
free-air carbon dioxide enrichment
soil organic matter