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.