Abstract

The present and past links between biogeochemical cycles, climate, and human activities are priority research questions. Temperature and rainfall have been suggested to be important controls for the N cycle and are often reflected in the δ15N values of soils, plants, and animals. Several global data syntheses have demonstrated an inverse relationship between foliar δ15N and mean annual precipitation (MAP) for a wide variety of ecosystems. Moreover, the isotopic signal changes predictably across trophic levels, with a ∼3–4 ‰ enrichment at each level. Our principal objective is to measure δ15N changes across different spatial and temporal scales and to establish how these changes relate to long-term dynamics in the N cycle. Using elevation and latitudinal gradients across the Atacama Desert, we first establish a modern analogue for the relationship between plant/feces δ15N and MAP. We then use feces from radiocarbon dated fossil rodent middens to reconstruct past changes in δ15N during the last 15,000 cal BP. These records will help us to understand the impact of climate and place any anthropogenic forcing of the N cycle into a historical context. We gathered modern Abrocoma and Phyllotis feces (n=100) and plant leaves (n=78) from the Antofagasta region of northern Chile (∼20–26°S). Ancient feces were collected from rodent middens that span the last 15,000 years. The isotopic results (δ15N) were measured at the Isotope Lab at Cornell University and at the Laboratorio de Biogeoquímica e Isótopos Estables Aplicados at the Pontificia Universidad Católica. The results show that modern average plant δ15N values range from -1.96‰ to 8.84‰, showing a significant inverse correlation with elevation (R2=0.46, P=0.00(1). Modern feces, δ15N ranges from 0.25‰ (4000 masl) to 13.57‰ (3200 masl), but do not show a consistent trend with elevation. Fossil samples (plants and feces) from the last 1,000 years exhibit a major ∼3–4‰ decrease in δ15N values from older to recent samples which could be related to human activity. The feces from the last 15,000 cal BP reveal an important shift in δ15N around 3000 cal BP. Between 15,000 and 3000 cal BP, δ15N values oscillate around 10‰, but from 3000 cal BP to the present there is ∼5 ‰ decrease. Thus, two different drivers of the N cycle appear to be at work in the Atacama at different timescales: humans and climate change. For the most part, modern samples are responding to local climate factors, mainly precipitation and elevation. Human effects appear to be present for at least the last 1000 years, possibly due to local food production, and in the last 200 years, combustion of fossil fuels and the Haber-Bosch process of N2 fixation. We found no direct links between variations in local rainfall and δ15N at millennial timescales, although the 5‰ shift at ∼3000 cal BP appears to be driven by wetter conditions associated with the expansion of human occupations in the Salar de Atacama basin. Whether this expansion also drove changes in the local N cycle remains to be evaluated.