Isotopic composition of throughfall nitrates in suburban forests with different vegetations

Isotopic studies on throughfall (precipitation shed from foliages) nitrates are rare despite their importance as a major nitrogen source in forest ecosystems. This study was conducted to examine a seasonal and species-related variability in isotopic composition of throughfall nitrates and its implications on forest nitrate sources. Event based throughfall samples were collected for a year from three locations (Chestnut, Pinus, and Mixed forests) along with rainfall (uninterrupted precipitation to the ground) samples from an open canopy location, and analyzed for nitrogen and oxygen isotopic composition of dissolved nitrates. δ15NNO3 and δ18ONO3 values of rainfall nitrates were greater during the non-growing season (Nov.~Mar.) than the growing season (Apr.~Oct.) as expected from the atmospheric nitrate cycles and associated isotopic effects. Although the isotopic composition of throughfall nitrates followed a similar seasonal pattern with that of rainfalls, their δ15NNO3 and δ18ONO3 values were consistently higher (Sep.~Dec.) or lower (Jan.~Apr.) than rainfalls and showed an abrupt decrease in July, possibly related to the incorporation of nitrates from soil and foliar origin. Isotopic composition of throughfall nitrates also varied systematically with vegetation types, indicating isotopic effects associated with nitrate uptake and release at the forest canopy. The seasonal and species-related variability in the isotopic composition of throughfall nitrates indicated combined effects of atmospheric NOx cycles, dry and wet depositions, and species-specific isotopic effects associated with nitrate use.