Atmospheric nitrogen deposition at a conifer forest: response of free amino acids in Engelmann spruce needles

Recent increases of nitrogen (N) deposition to forest ecosystems have had a variety of effects on plant species including mineral imbalance, growth disturbance and the accumulation of foliar-free amino acids. The purpose of this study was to determine the existence and degree of correlation between variable atmospheric nitrogen deposition and the concentrations of foliar free amino acids in Engelmann spruce (Picea engelmannii). Needle samples were collected during July and August of 1996 from a site in the Colorado Rocky Mountains that has large diurnal variations in atmospheric nitrogen deposition and receives approximately 4–8 kg N ha−1 year−1 total deposition. Amino acid concentrations were quantified with foliar % N also determined. Atmospheric nitrogen concentrations of gaseous HNO3 and NH3, as well as particulate NH4+ and NO3−, were determined simultaneously with needle sampling, and were subsequently converted to atmospheric N flux estimates. Foliar concentrations of arginine (July intensive) and glutamic acid (August intensive) in adult trees showed strong positive correlations with variations in atmospheric N flux, dominated by nitric acid, after a 4 ±2 h time delay. The results of this study support previous work that suggest that arginine may serve as an indicator of atmospheric N deposition. More importantly, however, this study has shown that foliar amino acid responses to atmospheric N flux (especially arginine) may occur after approximately a 4±2 h time delay.