Stephan Hättenschwiler博士（Centre d’Ecologie Fonctionnelle et Evolutive, CEFE, CNRS, Montpellier, France）在功能生态学，全球气候变化，化学计量学，生物多样性等领域研究达到世界顶尖水平，尤其是对森林生态系统的研究，被法国国家科学研究院（CNRS，类似中科院）称为“The Man of the Woods”。Stephan Hättenschwiler博士定于22至28日来我们实验室进行学术访问，访问期间欢迎各位老师同学和Stephan Hättenschwiler教授探讨生态学科学各种问题。并定于23日（星期四）上午九点于生命科学学院A316（仙林校区）做相关讲座，欢迎老师同学们参加，讲座涉及如下内容：
1. Biodiversity – Ecosystem Functioning: a view from the decomposer system
Recycling of elements during the decomposition of dead organic matter contributes critically to the functioning of the Earth’s ecosystems. The chemical diversity of plant tissues as the single major source of dead organic matter, and the extraordinary diversity of decomposer organisms, make the understanding of the role of this diversity in decomposition challenging. In this presentation, I will explore the significance of biodiversity across trophic levels for decomposition and elemental cycling based on a range of studies from our group and collaborators. Experimental work from Amazonian rainforests, Mediterranean garrigue, but also across broad climatic gradients up to subarctic latitudes including terrestrial and aquatic ecosystems show that decomposition changes with increasing biodiversity. These changes can be explained by functional characteristics of plant litter and decomposers rather than by species richness. Moreover, ecological stoichiometry provides a powerful conceptual framework to understand the underlying mechanisms for observed biodiversity effects across forest ecosystems.
2．Amazonian trees compete for nutrients by starving decomposers
Coastal tropical rainforests of the Amazon grow on some of the oldest and most nutrient impoverished soils on Earth. Phosphorus (P) availability is particularly low, increasing the competition between plants and soil heterotrophs for this key element. However, decomposer organisms apparently are not controlled by the very low P concentrations in plant leaf litter in these forests. Instead, decomposers respond strongly to variations in litter carbon (C) compounds, such as non-structural carbohydrates and phenolics that are left behind in senescing leaves. These results suggest plant-imposed C limitation of decomposer organisms in the studied tropical rainforest. Microbial biomass data in decomposing leaf litter support the C limitation hypothesis, as we observed the lowest microbial biomass in the tropical rainforest compared to other forest ecosystems across a broad latitudinal gradient. Moreover, if standardized for the large temperature differences, decomposition in tropical forests is much slower than for example in temperate forests. The particular C quality trait syndrome of tree leaf litter in nutrient-poor tropical rainforests may have evolved to increase plant access to limiting nutrients by enforcing energy starvation of decomposers. This largely unexplored aspect of interactions between C and nutrient cycling may have strong implications for global change impacts on the biogeochemistry of tropical rainforests. The C chemistry driven plant-decomposer feedbacks may also contribute to a better understanding of niche partitioning among tropical tree species and their co-existence in the highly diverse tropical rainforests.
3．Linking P and C cycling in the decomposer system of an Amazonian rainforest
Coastal tropical rainforests of the Amazon grow on some of the oldest and most nutrient impoverished soils on Earth. Phosphorus (P) availability is particulary low, increasing the competition among organisms for this key element. Accordingly, we found that microbial decomposer communities from a tropical forest of French Guiana sequester leaf litter P at higher rates than leaf litter N with clear shifts in microbial biomass C :N :P stoichiometry. Apparent P limitation of microorganisms was tested with a two-year fertilization experiment in the field where we added the key elements C, N, and P in all possible combinations. We measured a 47% higher biomass and substantially increased activity of soil microbial communities that grew in P-fertilized plots compared to plots without P fertilization. These responses were amplified with a simultaneous C fertilization suggesting P and C co-limitation of soil microorganisms. Stimulated microbial activity under P fertilization led to strongly reduced soil organic C concentrations after two years. Supplementary laboratory incubations of soil samples from our experimental site confirmed higher respiratory C loss and higher dissolved organic carbon (DOC) leaching from P fertilized soils. Our data suggest that microbial soil communities in the studied tropical forest respond strongly to increased P availability leading to a substantial breakdown of soil organic matter (SOM). We conclude that P availability has a strong control on SOM dynamics in the studied tropical rainforests.Tropical forest soils may shift from C sinks to sources much more rapidly than previously thought.
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