DISSERTATION RESEARCH: Woody plant encroachment into grasslands: Impacts on landscape-scale patterns of soil carbon, nitrogen, and phosphorus storage and dynamics

很少下雨或下雪的土地约占地球陆地面积的40%。这些地区的大部分地区正在经历以草为主的植物被越来越多的树木和灌木所取代的过程。这种变化通常会增加植物和土壤中的碳含量，并影响对植物生长至关重要的其他元素（如氮和磷）的利用；碳积累也可能影响气候。氮和磷是通常限制植物生长速度的营养物质，其中一种的供应会影响其他元素。尽管碳、氮和磷在植物和土壤中的重要性，但很少有研究在树木和灌木数量增加的干燥景观中同时考虑这三种元素。该项目将测量树木和灌木在很大程度上取代了草的地区土壤中碳、氮和磷的储存模式。研究结果将提供关于为什么树木和灌木正在取代草以及这些变化对这些环境意味着什么的知识。这项研究获得的信息将用于改进描述气候和营养循环相互作用的数学模型，从而为这些旱地的保护和管理提供信息。有关这项研究的教育材料将与德克萨斯农工大学农业生物推广项目合作，为公众准备。长期以来，草地、稀树草原和沙漠生态学家一直对树木增殖后土壤养分池大小的变化感兴趣。然而，以往关于木本植物入侵后土壤养分动态的研究大多局限于小空间尺度，仅限于土壤剖面的最上部，主要关注C和/或N。本研究将量化整个土壤剖面的土壤有机C、全氮和全磷，以首次评估木本植物入侵草地后景观尺度的C:N:P土壤化学计量学。具体目标为：(1)考察植被覆盖变化是否改变了景观尺度下土壤碳、氮、磷储量的三维空间格局；(2)在固氮剂侵蚀的土壤中，土壤P与碳、氮是否呈等距分布，以及这些等距分布是否随土壤深度的变化而变化。在亚热带热带稀树草原景观中，固定氮的木本植物在过去的一个世纪里侵入了草原，在1.2米深的空间特定土壤岩心中，养分储存将被量化。研究结果将为全球干旱和半干旱生态系统中木材侵蚀对碳、氮和磷循环相互作用的影响提供新的认识，并增强我们在生物地球化学-气候模型中表征这些相互作用的能力。

Lands that receive small amounts of rain or snow occupy approximately 40% of the Earth's land surface. Large portions of these areas are experiencing a replacement of plants that are mostly grasses with increased numbers of trees and shrubs. This change generally increases the amount of carbon found in both plants and soils, and impacts how other elements that are important for plant growth, such as nitrogen and phosphorus, are used; carbon accumulation may also impact climate. Nitrogen and phosphorus are nutrients that often limit the rates at which plants grow, and the supply of one or the other can affect other elements. Despite the importance of carbon, nitrogen and phosphorus in plants and soils, very few studies have been conducted that consider all three elements together in dry landscapes, where the numbers of trees and shrubs are increasing. This project will measure patterns of storage of carbon, nitrogen, and phosphorus within the soils in areas where trees and shrubs have largely replaced grasses. The results will provide knowledge on why trees and shrubs are replacing grasses and what these changes mean for those environments. The information gained by this research will be used to improve mathematical models that describe the interactions of climate and nutrient cycles that in turn can inform the conservation and management of these dry lands. Educational materials about the research will be prepared for the general public in cooperation with the Texas A&M AgriLife Extension program.Modification of soil nutrient pool sizes following woody proliferation has long been of interest to grassland, savanna, and desert ecologists. However, most previous studies examining nutrient dynamics following woody encroachment have been confined to small spatial scales, limited to the uppermost portions of the soil profile, and focused primarily on C and/or N. This research will quantify soil organic C, total N, and total P throughout the entire soil profile in order to make the first assessment of landscape-scale C:N:P soil stoichiometry following woody plant encroachment into grassland. Specific objectives are: (1) Examine whether vegetation cover change alters the 3-dimensional spatial patterns of soil C, N, and P storage at the landscape scale; and (2) Test whether soil P scales isometrically with respect to C and N, and whether these isometric patterns change with soil depth in N-fixer encroached systems. Nutrient stores will be quantified in spatially-specific soil cores taken to a depth of 1.2 meters in a subtropical savanna landscape where N-fixing woody plants have encroached into grasslands during the past century. Results will offer new perceptions on the effects of woody encroachment on interactions between C, N, and P cycles in arid and semi-arid ecosystems across the globe, and enhance our ability to represent these interactions in linked biogeochemistry-climate models.