Effect of diet, anthropogenic activity, and climate on δ15N values of cave bat guano

Few studies have attributed delta N-15 values of guano to a factor other than diet. A delta N-15 record obtained from a 1.5-m core of bat guano deposit from Zidita Cave (western Romania) provides a record of anthropogenic and climatic influence on the regional nitrogen pool. Nitrogen content is nearly constant (%N > 9) for over 1 m of the core, indicating limited diagenesis. The delta N-15 and delta C-13 time series are compared and the delta N-15 is also interpreted in light of previously published pollen assemblage from the same core. Using these comparisons the influence of anthropogenic activity and water availability is interpreted. Although some delta N-15 variation can be attributed to major changes in anthropogenic activities, additional variation implies an alternative control. The correlation between delta N-15 and delta C-13 values is significant (p < 0.01), but not strong, suggesting that both variables are influenced by water availability, known to be a primary control of delta C-13 values within C-3 ecosystems. Drier periods indicated by higher delta C-13 values correspond with lower delta N-15 values and vice-versa for wetter conditions. The instrumental climate record (precipitation amount) for the past 50 years nearby the cave supports this relationship. From 1000 to 1800 CE, the delta N-15 values fluctuated between 11.5 and 14 parts per thousand, then decreased in two abrupt steps, at 1800 and 1947 CE. The most significant decrease occurred after 1947 CE when values fell from 12.5 to below 7 parts per thousand, suggesting the N-cycle transitioned towards a more conservative state. The correlations between delta N-15 and delta C-13, and the instrumental precipitation record, along with the apparent negligible effect of diet on long term delta N-15 variation indicate that the delta N-15 values of guano can be used as an integrator of the local N-cycle. (C) 2016 Elsevier B.V. All rights reserved.