Foliar Uptake Of Atmospheric Reactive Nitrogen Pollution

Abstract

Several studies have established that gaseous reactive N is capable of being incorporated directly into the leaves and assimilated into plant metabolism. However, no studies have directly quantified the magnitude of foliar N uptake at realistic atmospheric N concentrations or how this entry pathway may influence vegetative responses to increasing atmospheric N deposition. This body of work expands on previous research by explicitly quantifying the direct foliar uptake of atmospheric nitrogen dioxide (NO2) using foliar delta exp(15)N and the mechanisms driving the capacity for species to incorporate atmospheric reactive N directly through the leaves into biomass over multiple scales in laboratory, mesocosm, and natural systems. In chapter one, I use foliar sigma exp(15)N as a tool to quantify the magnitude of foliar NO2 uptake in tomato (Lycopersicon esculentum) and tobacco (Nicotiana tabacum). This chapter establishes that gaseous 15NO2-N may be directly incorporated into the leaves of vegetation and is assimilated as a nutritional source under N-limited conditions. In chapter two, I examine a suite of factors including morphological, stable isotope, and nitrate reductase activity in L. esculentum and N. tabacum to quantify the amount of N incorporated via foliar 15 NO2 uptake, the variation in foliar uptake capacity between species, and determine whether realistic concentrations of 15NO2 influence plant N metabolism. This chapter provides evidence that the magnitude of foliar NO2 uptake has the potential to influence plant metabolism and that variation exists among species in the capacity to incorporate NO2 via the foliar uptake pathway. In chapter three, I investigate the roles of different root N sources (NO3- and NH4+) on enzyme expression and foliar uptake capacity of 15NO2 in N. tabacum. This chapter contributes to the mechanistic understanding of foliar uptake capacity by integrating relationships among several potential drivers of the foliar uptake pathway. Chapter four examines the variation in natural abundance foliar sigma exp(15)N patterns and uses these measurements to estimate the importance of soil sigma exp(15)N, mycorrhizal associations, and foliar N uptake on foliar sigma exp(15)N patterns. This chapter supports the use of natural abundance foliar sigma exp(15)N to detect the potential for foliar N uptake in several tree species in temperate forests.