Climate-smart grazing to restore tropical soils

退化景观的恢复日益被视为一项可持续投资，引发了大规模的全球承诺，以恢复生态系统中的土壤功能。一个关键的前提是，优化放牧可以导致草原的净碳(C)固存，足以抵消牲畜介导的甲烷（CH4）排放。对具有多年生植被和“最佳”载畜率的草原的管理被作为气候智慧型土壤管理加以推广。然而，对于不同草地和牲畜管理方法对温室气体交换的净影响，特别是在热带地区，存在很大的不确定性。目前对土壤净碳吸收的估计可能被高估了100%2。相互作用的因素决定了草原是碳汇还是碳源，其中放牧压力、土壤类型和降雨起着基础性作用3。牲畜种类和放牧管理通过选择性放牧、排泄物沉积以及对土壤暴露和侵蚀的影响来控制土壤C和常量营养素的输入。模型可用于了解不同牲畜管理对土壤净温室气体交换的总体影响，但需要数据来减少集约化草地系统的不确定性。国际热带农业中心最近从1980年代以来进行的草地调查和实验中汇编了土壤和植物生物量数据，并要求进行研究合作，以发展代表净排放系数关键控制的简单模型。该数据集包括广泛的土壤质地、气候带和放牧方式。该项目旨在利用这一资源，结合系统审查和一些实证工作，开发简单的统计和过程模型，以说明热带草原恢复在何处以及如何提供净碳封存。这将有助于综合景观规划。项目描述本项目将整理来自拉丁美洲热带草原的经验证据，以确定在哪些条件下草原成为碳汇，因此可能适合景观恢复。这项研究将把实地水平和机械研究与实验室操作相结合，以参数化和测试现有的土壤模型。研究将分四个阶段进行：文献综述和模型检验，以估计土壤和放牧管理对土壤c的影响大小。学生将对热带草原进行系统的文献综述，并访问哥伦比亚CIAT的广泛实验数据库。结果将用于建立简单的生态系统模型，如N14CP5，保留一些数据集用于模型测试。统计分析将用于评估主要因素和相互作用。现场工作。学生将开发实验和/或调查设计，以填补在评估现有数据库后变得明显的知识空白。CIAT可以在哥伦比亚大草原提供不同土壤质地、深度和有机质含量的农田，并提供季节性、永久性和轮牧管理。监督小组将协助进行实验设计和方法，以评估植被（CO2）和土壤（CO2和CH4）中的碳储量（至1米深）和通量。3. 操作实验。根据数据回顾和建模研究的结果，学生将选择控制碳分解的关键因素，如土壤质地、常量养分输入（主要是氮和磷）和水势，并进行实验室孵化，旨在填补剩余的知识空白。场景分析。所开发的模型和关系将用于评估在受到土壤和管理限制的情况下，退化土地在哪些地方可以实现净碳封存。

Problem statementThe restoration of degraded landscapes has increasingly been promoted as a sustainable investment, sparking large-scale global commitments to restore soil functions in ecosystems. A key premise is that optimisation of grazing can lead to net carbon (C) sequestration in grasslands, sufficient to offset livestock-mediated methane (CH4) emissions. Management of grasslands with perennial vegetation and 'optimal' stocking rates is promoted as climate-smart management of soils1. However, there is much uncertainty as to the net effects on greenhouse gas exchange of different grassland and livestock management approaches, particularly in tropical regions. Current estimates of net C uptake in soils may be overestimated by 100%2. Interacting factors determine whether grasslands become sinks or sources of C, among which grazing pressure, soil type and rainfall play fundamental roles3. Livestock species and grazing management control soil C and macronutrient inputs, via selective grazing, excreta deposition, and effects on soil exposure and erosion. Models can be applied to understand overall impacts of different livestock management on soil net greenhouse gas exchange4, but require data to reduce uncertainties for intensive grasslands systems. The Centre for International Tropical Agriculture (CIAT) have recently compiled soil and plant biomass data from grassland surveys and experiments carried out since the 1980s, and have requested research collaboration to develop simple models that represent key controls on net emission coefficients. The dataset includes a wide range of soil textures, climatic zones and grazing practices. The project aims to use this resource, together with a systematic review and some empirical work, to develop simple statistical and process models that represent where and how restoration of tropical grasslands can provide net C sequestration. This will contribute to integrated landscape planning. Project descriptionThis project will collate empirical evidence from tropical grasslands of Latin America that identifies under which conditions grasslands become C sinks, and therefore potentially suitable for landscape restoration. The research will combine field level, and mechanistic research with laboratory manipulations to parameterise and test existing soil models. The research will be organised in four phases:1. Literature review and model testing to estimate the size of the effects of soils and grazing management on soil C. The student will conduct a systematic literature review on tropical grasslands, and access an extensive database of experiments at CIAT in Colombia. Results will be used to set up simple ecosystem models such as N14CP5, withholding some datasets for model testing. Statistical analyses will be used to assess major factors and interactions.2. Field work. The student will develop experimental and/or survey designs to fill knowledge gaps that become evident after assessing the existing database. CIAT can offer field sites in the Llanos of Colombia with contrasts in soil texture, depth and organic matter contents, and with seasonal, permanent, and rotational grazing management. The supervision team will assist with experimental design and methods for assessing carbon stocks (to 1 meter depth) and fluxes in vegetation (CO2), and soil (CO2 and CH4). 3. Manipulation experiments. Based on outcomes of the data review and modelling study, the student will select key factors controlling C decomposition such as soil texture, macronutrient inputs (mainly N and P) and water potential, and conduct laboratory incubations aimed at closing remaining knowledge gaps.4. Scenario analyses. The models and relationships developed will be used to assess where net C sequestration can be achieved in degraded land, subject to soil and management constraints.