Nutrient leaching from pine needles impacted by acidic cloudwater

Abstract

In coastral and mountainous environments, fog and cloud droplets are frequently deposited onto terrestrial surfaces; this deposition pathway may account for a large fraction of both moisture and chemical loading. Stratus cloudwater was collected for chemical analyses at Henninger Flats (870 m MSL), a site in the foothills of the San Gabriel Mountains, 25 km northeast of downtown Los Angeles. Concurrent samples of intercepted cloudwater were manually removed, drop-by-drop, from needles on various species of pine trees upon which it had deposited. Samples were analyzed for pH, major cations, and anions. Solute concentrations were substantially higher in samples removed from pine needles compared to the suspended cloudwater. For example, cloudwater concentrations for nitrate and sulfate were measured between 0.25 and 4.5 meq L⁻¹ while deposited samples were 0.4 to 90 meq L⁻¹. This solute enhancement was due to evaporation following droplet deposition and to nutrient leaching. Nutrient leaching was indicated by (a) a disproportionate increase in the concentrations of cations such as K and Mg (a factor of 2 to 15 enrichment relative to sulfate), and (b) reductions in the leachate acidity and ammonium relative to the incident droplets. A relationship was observed between the enhancement of Na, Ca, and nitrate in pine needle leachate. This suggests that reactions at the foliar surfaces are occurring which involve gaseous HNO₃ and accumulated soil dust and sea-salt particles.