Climate and ecosystem 15N natural abundance along a transect of Inner Mongolian grasslands: Contrasting regional patterns and global patterns

Abstract

Nitrogen isotopes provide integrated information about nitrogen cycling in terrestrial ecosystems. This study explores the regional patterns of ecosystem 15N abundance along a 1200 km transect in Inner Mongolian grasslands and their relationships with climate. Results indicate that climatic variables control approximately 50% of the variation in ecosystem 15N abundance along the transect. Ecosystem 15N abundance decreases as both mean annual precipitation (MAP) and mean annual temperature (MAT) increase. Regional patterns obtained from our study differ from reported global patterns. Ecosystem 15N abundance is negatively correlated with MAT along the eastern part of the transect, while a positive correlation between MAT and ecosystem 15N is apparent at the global scale. MAP exerts stronger controls on ecosystem 15N abundance along the western part of the transect than what is shown in a global regression model. Ecosystem 15N abundance in the western part of the transect is substantially higher (ca. 3‰) than the values projected by a global model. The Δ15Nsoil‐plant (‰) (difference in δ15N values between plant and soil) values in Inner Mongolian grasslands are not significantly correlated with either MAP or MAT; but Δ15Nsoil‐plant values are positively correlated with MAT and negatively correlated with MAP at the global scale. These conflicting trends strongly indicate that climatic controls on ecosystem 15N abundance are scale‐dependent. Regional N deposition gradients, grazing‐induced ammonia volatilization, and variation in plant‐soil types are among the possible causes of these inconsistencies.