The Amazon Fertilisation Experiment (AFEX)

目前，陆地生态系统吸收了人类排放到大气中的二氧化碳的四分之一，从而降低了气候变化的速度。在这种情况下，亚马逊雨林极其重要，每年吸收超过5亿吨的碳。这比美国和中国的排放量加起来还要多。然而，我们对亚马逊森林的生产力是如何被控制的了解有限，这降低了我们预测未来大气中二氧化碳浓度持续上升和气候变化会发生什么的能力。生态学的一个主要范例是，热带生态系统的生产力发生在古老的、高度风化的土壤上，受到磷的可用性的限制。这与温带生态系统形成对比，温带生态系统的生产力已被证明受到氮供应的限制。然而，磷范式尚未得到详细的检验，因为在热带森林中很少有养分操纵研究，在亚马逊森林中也没有进行过大规模的研究。这是一个主要问题，因为亚马逊大部分地区的土壤养分利用率远远低于巴拿马，而巴拿马是唯一正在进行的热带低地雨林施肥试验的地点。因此，巴拿马的发现可能不能代表亚马逊河流域的大部分地区，因此，我们对土壤肥力在控制热带森林生产力方面所起作用的理解是不完整的。由于两个原因，在亚马逊地区测试磷范式至关重要。首先，土壤肥力最低的亚马逊东部和中部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面积的枯死。这些森林的恢复力被认为高度依赖于树木是否能够对不断上升的大气二氧化碳浓度作出反应而增加其生长，而这种能力很可能取决于它们的生长目前在多大程度上受到土壤养分供应的限制。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因限制其生产力的营养物质而异。因此，在亚马逊地区建立第一个大规模的营养操纵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，在亚马逊流域最常见的土壤类型的一个代表性地点，操纵亚马逊森林中部的氮、磷和阳离子的可用性，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、生物量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联合英国陆地环境模拟器（JULES）中的亚马逊流程表示。这将是第一次将热带森林功能的多养分控制纳入动态全球植被模型，从而可以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，以及对区域和全球气候变化速度的影响。总之，我们的项目将解决一个基本的生态问题，并将大大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21世纪气候变化速度预测的不确定性；也就是说，如何控制地球上最重要的天然碳汇之一——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.