ReviewNutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO2) concentrations in the earth's atmosphere are projected to rise from current levels near 400 ppm to over 700 ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO2 (eCO2) and have the potential to mitigate the impact of anthropogenic CO2 emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO2, highlighting key discoveries made in the field, particularly from free-air CO2 enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.