Soil Nutrient Distributions in Grassland and Shrublands of the Sevilleta NWR

Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of 'islands of fertility' focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata, creosotebush. It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems.过去一个世纪以来，美国西南部的植被已从多年生草本草原转变为木本灌丛。过往针对“肥力岛”发育的研究，大多仅聚焦于植物生长最具限制性的必需元素——土壤氮（N）。本研究旨在解答两个核心问题：其一，在沙漠灌木三齿拉瑞阿（Larrea tridentata，俗称creosotebush）冠下，磷（P）与钾（K）这两种其他植物必需元素，是否呈现出与氮类似的浓度梯度；其二，氮、磷、钾的空间分布，与必需但非限制性营养元素（钙Ca、镁Mg、硫S）以及非必需元素（钠Na、氯Cl、氟F）的空间分布是否存在差异。研究人员在相邻的草原与灌丛样地中，分别采集植被冠下与植被间隙的表层土壤，并对一系列土壤营养元素开展了检测分析。土壤营养元素的分布在草原样地中呈现均匀模式，与丛生禾草的空间均质性相契合；而在灌丛样地中则呈现斑块状分布，与单株灌木的空间异质性相匹配。二者的核心差异在于：草原样地中所有元素均呈均匀分布；而在灌丛样地中，植物必需元素氮、磷、钾在灌木冠层下发生富集，而非限制性与非必需元素则要么富集于灌丛间隙，要么在冠下与间隙中分布浓度相当。本研究结果阐明了植被从草原向灌丛的转变，如何推动荒漠生态系统的结构与功能发生长期且广泛的改变。