The Amazon Fertilisation Experiment (AFEX)

陆地生态系统目前吸收了人类排放到大气中的二氧化碳的四分之一，从而减缓了气候变化的速度。在这种背景下，亚马逊雨林极其重要，每年吸收超过5亿吨碳。这比美国和中国排放量的总和还要多。然而，我们对亚马逊森林生产力如何控制的了解有限，这降低了我们预测未来随着大气二氧化碳浓度持续上升和气候变化发生什么的能力。生态学的主要范式之一是，热带生态系统的生产力受到磷有效性的限制，热带生态系统发生在古老的、高度风化的土壤上。这与更温和的生态系统形成对比，后者的生产力已被证明受到氮素供应的限制。然而，磷素模式还没有得到详细的测试，因为在热带森林中的养分调控研究很少，而且还没有在亚马逊森林中进行大规模的研究。这是一个主要问题，因为亚马逊河流域大部分地区的土壤养分利用率大大低于巴拿马，巴拿马是唯一正在进行的热带低地热带雨林肥力试验的地点。因此，巴拿马的调查结果可能不能代表亚马孙大片地区，因此，我们对土壤肥力在控制热带森林生产力方面的作用的了解是不完整的。在亚马逊地区测试磷范例至关重要，原因有两个。首先，亚马逊东部和中部是土壤肥力最低的地区，被认为面临气候变化不利影响的风险，一些科学家预测会出现大范围的枯萎。这些森林的弹性被认为高度依赖于树木是否能够增加其生长，以应对大气中二氧化碳浓度的上升，而这种能力可能取决于它们的生长目前受到土壤养分供应的限制程度。其次，越来越多的证据表明，生态系统对全球变化的反应可能不同，这取决于限制其生产力的营养物质。因此，在亚马逊地区建立第一个大规模养分调控研究应该是生态系统和气候变化研究的最优先事项之一。我们将做到这一点，操纵亚马逊中部森林中代表盆地最常见土壤类型的氮、磷和阳离子的有效性，并将量化关键森林过程的反应。我们将确定对光合作用、植物呼吸、生物质生产和周转以及分解的影响，最终使我们能够采取全生态系统的方法来确定碳储存是如何受到影响的。我们产生的新知识和理解将用于改进Amazon在联合英国陆地环境模拟器(Jules)中的流程表示。这将是热带森林功能的多养分控制首次被纳入动态全球植被模型，从而能够更真实地模拟亚马逊碳循环对环境变化的响应。这将提高我们预测亚马逊雨林在21世纪将如何变化的能力，以及这将对区域和全球气候变化的速度产生什么影响。总而言之，我们的项目将解决一个基本的生态问题，并将极大地提高我们对这个问题的理解，这个问题在很大程度上导致了对21世纪气候变化速度的预测的不确定性；即，如何控制地球上最重要的自然碳汇之一--亚马逊雨林的生产力。

Terrestrial ecosystems currently absorb one quarter of the carbon dioxide that Humankind releases into the atmosphere, thus reducing the rate of climate change. In this context, Amazon rainforest is extremely important, absorbing more than half a billion tonnes of carbon per year. This represents more than the combined emissions from the USA and China. However, we have limited understanding of how the productivity of Amazon forests is controlled, and this reduces our ability to predict what will happen in the future as atmospheric CO2 concentrations continue to rise and the climate changes.One of the main paradigms in ecology is that the productivity of tropical ecosystems, which occur on old, highly-weathered soils, is limited by the availability of phosphorus. This contrasts with more temperate ecosystems whose productivity has been shown to be limited by nitrogen availability. However, the phosphorus paradigm has not been tested in detail as there have been very few nutrient manipulation studies in tropical forests, and no large-scale study has been carried out in Amazon forest. This is a major issue because soil nutrient availability in most of Amazonia is substantially lower than in Panama, the location of the only ongoing fertilisation experiment in tropical lowland rainforest. Thus, the Panama findings may not be representative of large areas of Amazonia, and, therefore, our understanding of the role soil fertility plays in controlling tropical forest productivity is incomplete. Testing the phosphorus paradigm in Amazonia is critical for two reasons. Firstly, eastern and central Amazonia, the area which contains the lowest fertility soils, is considered to be at risk from the adverse effects of climate change, with widespread dieback predicted by some scientists. The resilience of these forests is considered to be highly dependent on whether trees are able to increase their growth in response to rising atmospheric CO2 concentrations, and this ability is likely to depend on the extent to which their growth is currently limited by soil nutrient availability. Secondly, there is growing evidence that the response of ecosystems to global change may differ depending on which nutrient limits their productivity. Therefore, establishing the first large-scale nutrient manipulation study in Amazonia should represent one of greatest priorities for ecosystem and climate change research. We will do just that, manipulating nitrogen, phosphorus and cation availability in central Amazon forest, at a site representative of the most common soil type in the Basin, and will quantify the response of key forest processes. We will determine the impacts on photosynthesis, plant respiration, biomass production and turnover, and decomposition, ultimately allowing us to take a full-ecosystem approach to establish how carbon storage has been affected. The new knowledge and understanding which we generate will be used to improve Amazon process representation in the Joint UK Land Environment Simulator (JULES). This will be the first time that multi-nutrient control of tropical forest function has been included in a dynamic global vegetation model, allowing for more realistic simulation of the response of the Amazon carbon cycle to environmental change. This will improve our ability to predict how the Amazon rainforest will change during the 21st century and what the implications will be for rates of regional and global climate change.In summary, our project will address a fundamental ecological question and will improve greatly our understanding of an issue that contributes substantially to uncertainty in predictions of rates of 21st century climate change; namely, how the productivity of one of the most important natural carbon sinks on the planet, the Amazon rainforest, is controlled.