Spatial relationship between δ¹⁵N and elevation in agricultural landscapes

Abstract

Understanding of the nitrogen (N) cycle and its spatial variability is important for managing ecosystems. Soil δ15N, as an important indicator of different soil nitrogen cycling processes, may provide critical information about the spatial variability in soil N cycling. The objective of this study was to examine the dominant landscape scale variability of δ15N, the location of the variability and its spatial relationship with elevation. Soil δ15N and elevation were measured along two transects (Davidson and Elstow, Saskatchewan, Canada). Each transect had 128 points with 3 m sampling intervals. Higher δ15N values typically occurred in topographic depressions as compared to knolls. The coefficient of determination revealed a significant linear relationship between δ15N and elevation (r2=0.27) at Davidson whereas no relationship (r2=0.00) was detected for the Elstow transect. However, wavelet spectra, cross wavelet, and squared wavelet coherency analysis revealed spatial relationships between δ15N and elevation at both sites. A strong coherency between δ15N and elevation at large scales (96 m or more) was detected for both transects. The Davidson transect showed an out of phase coherency at a topographically elevated area at the beginning and the end of the transect. The Elstow transect had a strong out of phase correlation (negative relationship) at the middle of the transect (corresponding to a depressions) indicating a location dependent relationship between δ15N and elevation. The relationship between δ15N and elevation reflects the effects of hydrology and soil water content over the landscape on N cycling processes.