Foliar uptake of 15N from simulated cloud water by red spruce (Picearubens) seedlings

One hypothesis to explain dieback of red spruce (Picearubens Sarg.) trees in high-elevation red spruce stands is that atmospheric deposition may introduce nitrogen in excess of plant needs (N saturation), which may disrupt normal metabolism and foliage development in this harsh environment. Some authors suggest that direct foliar uptake of N might contribute to N saturation. To examine the importance of foliar uptake by red spruce, we exposed seedlings in a greenhouse to a mist that simulated cloud water and contained either ammonium or nitrate as 15N (99 at.% excess at 2 mg/L). After 50 h exposure to mist, seedlings were washed and then separated into four tissue types: new foliage, old needles, stems, and roots. Total and isotopic nitrogen contents were determined for each tissue type. The accumulation rate of 15N in each tissue type was very low. Extrapolating our data on the basis of a year suggests that N from cloud water supplies only a small fraction of the N required for new growth (less than 1.5% for the seedlings we used). We observed that both ammonium-15N and nitrate-15N accumulated in stem tissues and could not be removed after repeated washings, which suggests an alternative mechanism for long-term N retention by red spruce stands.