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.

本研究于2012年8-9月采集75份耕作土壤样品，采自某流域内3处位于关键地貌单元、采用相似非灌溉小麦种植管理模式的农田休耕地；2013年8月另采集2份非耕作土壤样品，所有样品均通过小型挖掘机（mini excavator）采集至地下145厘米深度。样品以15厘米为深度间隔进行体积定量采样，单份采样体积为1500 cm³，随后完成称重。田间湿样于约50℃烘干24-48小时，过筛获取粒径<2mm的组分，将细组分的副样分为两份平行样进行过夜研磨，随后称重并采用燃烧-同位素比值质谱法（combustion-IRMS，型号为Delta V，赛默飞世尔（Thermo）），于美国地质调查局（USGS）丹佛西南区域同位素实验室开展总氮（total nitrogen）与d15N分析。质量控制环节设置每6份样品插入一组平行样，若浓度或同位素比值差值超过10%，则对邻近样品进行复测。总氮含量不足或分析失败的样品，其结果未被纳入统计。本研究对覆盖两种地貌单元的21个采样点（含2处原生植被样地）的样品进行分析，最终获得62组有效分析结果，结果以所有采样点各深度间隔的平均值形式呈现。