Summary of nitrogen isotope ratios (d15N) in cultivated and uncultivated soil profiles of the Judith River Watershed

Soil samples were collected from 75 cultivated soils excavated in August-September 2012 in fallow fields, and two uncultivated soils excavated in August 2013, to 145 cm using a mini excavator, at three fields on key landforms in the watershed that were under similar management for non-irrigated wheat production. Samples were collected volumetrically in 15 cm depth increments (1500 cm3) and weighed. Field moist samples were dried at ~50°C for 24-48 hours, sieved to obtain the fraction <2mm, and subsamples of the fine fraction milled overnight in duplicate prior to weighing for total nitrogen and d15N analysis by combustion-IRMS (Delta V, Thermo) at the USGS Southwest Regional Isotope lab in Denver. Duplicates were included every six samples, and where concentration or isotopic differences were >10%, proximal samples were re-analyzed. Results are omitted for samples that contained insufficient nitrogen or where analysis was otherwise unsuccessful. Samples from 21 locations on two landforms, including two native range sites, were analyzed. A total of 62 analyses from these sites were successful, and results are presented as averages for each depth increment across all sites.