Soil nitrate data for 120 point locations within limestone cedar glades at Stones River National Battlefield near Murfreesboro, Tennessee

This dataset contains data collected within limestone cedar glades at Stones River National Battlefield (STRI) near Murfreesboro, Tennessee. This dataset contains measurements of soil nitrate (parts per million) at certain quadrat locations (points) within 12 selected cedar glades. These measurements were obtained according to the following protocol: (1) for each quadrat location (point), one soil sample was obtained under sterile conditions, using a trowel wiped with methanol and rinsed with distilled water, and was placed into an autoclaved jar with a tight-fitting lid and placed on ice, (2) soil samples were transported to lab facilities on ice and immediately refrigerated, (3) soil samples were transferred to clean aluminum trays and dried at 35 degrees Celcius for 48 hours to produce air-dry soil, (4) air-dry soils were then passed through a number 10 sieve to remove rocks and root fragments, (5) soil slurries were prepared by adding 10 grams of sieved, air dry soil to 25 milliliters distilled water and one powder pillow of Hach nitrate ionic strength adjustor (ISA) (Hach Company, Loveland, CO, USA) into 125-milliliter plastic bottles, (6) soil slurry bottles were agitated on a reciprocating shaker for 20 minutes. Soil nitrate levels were measured using a nitrate-sensitive electrode (Hach Platinum Series combination nitrate electrode 51920, Hach Company, Loveland, CO, USA) connected to a Hach SensIon 1-millivolt meter (Hach Company, Loveland, CO, USA). Measurements were taken by immersing the nitrate electrode into each soil slurry and measuring readings in millivolts. Three millivolt readings were taken per soil slurry, and the electrode was thoroughly washed with deionized water and blotted dry between each slurry immersion. For each soil slurry, the mean was calculated of the three readings in millivolts. Soil slurry readings (means of three measurements) in millivolts were transformed into nitrate estimation using a two-stage calibration procedure. A calibration curve was established immediately prior to taking soil slurry readings, and another calibration curve was established immediately afterward. Both calibration procedures were performed at the same temperature as soil slurry measurements. A mean calibration curve was created using the mean of the pre- and post- sample calibration curves and the mean calibration curve was used to transform slurry millivolt readings into soil slurry nitrate estimates in parts per million. Estimates of soil nitrate (parts per million) were calculated by multiplying soil slurry nitrate estimates derived from the mean calibration curve by 2.5, according to the instruction manual for the Hach Platinum Series combination nitrate electrode 51920. Detailed descriptions of experimental design, field data collection procedures, laboratory procedures, and data analysis are presented in Cartwright (2014).References:Cartwright, J. (2014). Soil ecology of a rock outcrop ecosystem: abiotic stresses, soil respiration, and microbial community profiles in limestone cedar glades. Ph.D. dissertation, Tennessee State University.Cofer, M., Walck, J., and Hidayati, S. (2008). Species richness and exotic species invasion in Middle Tennessee cedar glades in relation to abiotic and biotic factors. The Journal of the Torrey Botanical Society, 135(4), 540–553.

本数据集采集自美国田纳西州默弗里斯伯勒附近的斯通河国家战场（STRI）内的石灰岩雪松荒原地块。数据集包含12处选定雪松荒原地内的样方（quadrat）点位（测点）的土壤硝酸盐含量（以百万分比计）测定数据。

该数据集的测定按照以下流程完成：
1. 针对每个样方点位（测点），采用经甲醇擦拭、蒸馏水冲洗过的小铲在无菌条件下采集一份土壤样品，将其放入带密封盖的高压灭菌罐中，并置于冰上保存；
2. 土壤样品全程以冰上运输至实验室设施后，立即进行冷藏处理；
3. 将土壤样品转移至洁净铝托盘，于35摄氏度下烘干48小时，制备得到风干土壤；
4. 将风干土壤通过10号筛，以去除石块与根系碎屑；
5. 制备土壤悬液：向125毫升塑料瓶中加入10克过筛后的风干土壤、25毫升蒸馏水，以及1包哈希硝酸盐离子强度调节剂（ISA，Hach nitrate ionic strength adjustor）；
6. 将土壤悬液瓶放置于往复式振荡器上振荡20分钟。

土壤硝酸盐水平采用硝酸盐敏感电极（nitrate-sensitive electrode）进行测定，该设备为哈希铂金系列复合硝酸盐电极51920（Hach Platinum Series combination nitrate electrode 51920，哈希公司（Hach Company），美国科罗拉多州拉夫兰），连接至哈希SensIon型1毫伏计（Hach SensIon 1-millivolt meter，哈希公司（Hach Company），美国科罗拉多州拉夫兰）。测定时将硝酸盐电极浸入每份土壤悬液，读取毫伏数值。每份土壤悬液采集3次毫伏读数，每次浸取电极后均用去离子水彻底冲洗并吸干。针对每份土壤悬液，计算3次读数的平均值。

土壤悬液的毫伏读数（3次测量的平均值）需通过两步校准流程转换为硝酸盐估算值。在开展土壤悬液测量前即刻建立一条校准曲线，测量结束后即刻再建立一条校准曲线。两条校准曲线均在与土壤悬液测量相同的温度下完成。随后以样前、样后校准曲线的均值构建平均校准曲线，并通过该平均校准曲线将悬液的毫伏读数转换为土壤悬液硝酸盐估算值（单位为百万分比）。根据哈希铂金系列复合硝酸盐电极51920的使用手册，将通过平均校准曲线得到的土壤悬液硝酸盐估算值乘以2.5，即可得到土壤硝酸盐含量（百万分比）的估算值。

本数据集的实验设计、野外数据采集流程、实验室操作流程及数据分析的详细说明详见Cartwright（2014）。

参考文献：
1. Cartwright, J. (2014). 岩石露头生态系统的土壤生态学：石灰岩雪松荒原地的非生物胁迫、土壤呼吸及微生物群落特征. 田纳西州立大学博士学位论文.
2. Cofer, M., Walck, J., & Hidayati, S. (2008). 田纳西州中部雪松荒原地的物种丰富度与外来物种入侵及其与非生物和生物因子的关联. 托里植物学会杂志, 135(4), 540–553.