Carbon and nitrogen storage in the topsoils of Inceptisols and Mollisols under native sage scrub and non-native grasslands in southern California

In southern California, native California sage scrub is increasingly being converted to non-native grasslands, a process known as type conversion. Here we present data that was used to test if type conversion influences C and N storage in surface soil (A horizon). To better understand the factors influencing regional nutrient storage, we examined total C and total N concentration (%) and quantity (g/m2), key soil properties, and microbial abundances and assemblages in sage scrub and non-native grassland habitats at three sites. The three sites were along a coast to inland gradient spanning both Los Angeles and San Bernardino counties. The coastal site was in the Santa Monica Mountains while the most interior site was in the Crafton Hills Conservancy, 155 km east. An intermediate site was the Robert J. Bernard Biological Field Station, located 101 km east of the Santa Monica Mountains and 55 km west of Crafton Hills. To compare total C and N, soil properties, and microbial abundances, we collected soil from six sampling locations in both habitat types at each site in the spring of 2016 (March 25 through April 1). At each sampling location, we collected five different types of soil samples. First, we collected an intact soil clod from the mineral soil surface (immediately below the O horizon) to determine bulk density using a modified version of the clod method—a measure of soil density that excludes rock fragments > 2 mm diameter (description of the method is attached). Second, we gathered ~30 ml of loose soil from the top ~10 cm of the soil profile (all within the A horizon) for analyses of percent C and N using an Elementar vario MICRO cube elemental analyzer (Elementar Mt. Laurel, New Jersey). Third, we collected ~250 mL of soil from the A horizon at each sampling location to send to Earthfort Laboratories (Corvallis, Oregon) to determine total and active bacteria and fungi. Total bacteria and fungi (µg/g) were determined through direct enumeration using microscopy. Bacteria were identified using the fluorescein isothiocyante method. Total fungal biomass was determined by converting width and length measurements. Active bacteria and fungi (µg/g) were quantified using direct microscopy after staining samples with fluorescein diacetate, which binds and fluoresces to metabolically active bacteria and fungi. Fourth, a composite soil sample comprised of soil from all six sampling locations within each habitat at each site was sent to the UC Davis Analytical Laboratory to determine organic matter content, CEC, pH, and soil texture. The final type of sample was 20 mL of loose soil from each sampling location for 16S and ITS amplicon metagenomics analysis, which involves the direct sequencing of the microbiomes in an environmental sample. Molecular data can be found on GenBank.

南加州本土加州鼠尾草灌丛正日益被转化为非本土草原，这一过程被称为类型转换。本数据集提供了用于检验类型转换是否会影响表层土壤（A层，A horizon）碳（C）与氮（N）储量的数据。为更好地解析区域养分储量的影响因子，我们选取了三个样点，对其灌丛与非本土草原生境中的总碳、总氮浓度（%）与储量（g/m²）、关键土壤理化性质，以及微生物丰度与群落组成进行了测定。这三个样点沿海岸到内陆的梯度分布，涵盖洛杉矶县与圣贝纳迪诺县。沿海样点位于圣莫尼卡山脉，最内陆的样点位于克里夫顿山保护区（Crafton Hills Conservancy），距沿海样点以东155公里；中间样点为罗伯特·J·伯纳德生物野外站（Robert J. Bernard Biological Field Station），距离圣莫尼卡山脉以东101公里，距克里夫顿山保护区以西55公里。为对比总碳、总氮、土壤性质与微生物丰度，我们于2016年春季（3月25日至4月1日）在每个样点的两类生境中各选取6个采样点开展土壤样品采集。每个采样点共采集5类土壤样品：第一，从矿质土壤表层（O层正下方）采集完整土块，采用改良土块法测定容重——该方法排除直径>2mm的岩屑以计算土壤密度，方法详情见附件。第二，从土壤剖面顶部约10cm（全部位于A层）采集约30ml松散土壤，使用Elementar vario MICRO cube元素分析仪（美国新泽西州劳雷尔山Elementar公司）测定碳、氮百分含量。第三，从每个采样点的A层采集约250mL土壤，送至Earthfort实验室（美国俄勒冈州科瓦利斯）测定总细菌与真菌量以及活性细菌与真菌量。总细菌与真菌量（µg/g）通过显微镜直接计数法测定：细菌采用异硫氰酸荧光素（fluorescein isothiocyante）法鉴定，总真菌生物量则通过测量菌丝宽度与长度后换算得到。活性细菌与真菌量（µg/g）则采用荧光素二乙酸酯（fluorescein diacetate）染色后直接镜检法定量——该染料可与代谢活跃的细菌和真菌结合并发出荧光。第四，将每个样点每类生境的6个采样点土壤混合制成混合样品，送至加州大学戴维斯分校分析实验室测定有机质含量、阳离子交换量（CEC）、pH值与土壤质地。第五，从每个采样点采集约20mL松散土壤，用于16S与ITS扩增子宏基因组学分析——该技术可直接对环境样品中的微生物组进行测序。相关分子数据可在GenBank数据库中获取。