Ecophysiological mechanisms controlling the balance between C3 and C4 species in grasslands, with emphasis on responses to nitrogen loading

具有C3或C4光合作用模式导致植物碳氮经济的内在差异。因此，在C3和C4物种共存的草原和稀树草原，C3/C4平衡强烈影响农艺性状和生物地球化学循环。因此，影响这种平衡的环境和（或）管理变化对（农业）生态系统功能具有重要影响。目前大多数草原经历的人为氮负荷率的增加可以大大减少甚至消除C4物种。为什么会这样，这在多大程度上是一种普遍的反应，以及放牧是否会改变这种反应，目前还不清楚。我们的目标是阐明C3和C4物种之间平衡变化的生态生理机制，特别是对氮负荷的响应。为此，该项目的目的是，首先验证氮负荷对里约热内卢de la Plata草原（南美洲温带-亚热带）C3/C4组成的季节趋势的影响，评估与放牧制度可能的相互作用。其次，本研究试图验证以下假设：氮负荷导致植物向光限制生长方向转移，因此C3/C4平衡的变化反映了C3和C4个体捕获和利用光的特定能力，并且放牧制度对光照竞争的减弱限制了冠层发育，从而使施肥驱动的变化最小化。为了验证这些假设，需要分析混合林分中C3和C4个体碳捕获的形态发生和生理决定因素的内在差异和异速生差异的作用和权衡。碳收益将通过稳态13C/12C田间标记来量化，而碳稳定同位素的天然丰度将用于划分C3和C4植物对有机碳库、呼吸性CO2外排和食草动物饮食的贡献。

Possessing the C3 or C4 mode of photosynthesis leads to intrinsic differences in plant carbon and nitrogen economies. Therefore, in grasslands and savannas where C3 and C4 species coexist, the C3/C4 balance strongly influences agronomic properties and biogeochemical cycles. Environmental and/or managerial changes affecting this balance hence hold important consequences for (agro)ecosystems function. The increase in anthropogenic nitrogen loading rates most grasslands experience nowadays can drastically reduce, even eliminate, C4 species. Why is this so, to what extent it is a general response, and whether grazing alters it, is not known. Our goal is to elucidate ecophysiological mechanisms underpinning changes in the balance between C3 and C4 species, in particular in response to nitrogen loading. To this end, the project aims at, first, verifying the effect of nitrogen loading on the seasonal trend of the C3/C4 composition of the Rio de la Plata grasslands (temperate-subtropical South America), assessing possible interactions with grazing regime. Second, it seeks to test the hypotheses that nitrogen loading results in a displacement towards more light-limited growth, and therefore changes in the C3/C4 balance reflect specific abilities of C3 and C4 individuals to capture and use light, and that attenuation of light competition by grazing regimes which restrict canopy development minimizes fertilization-driven changes. Testing these hypotheses imports the analysis of the role of, and trade-offs between, intrinsic and allometric differences in morphogenetic and physiological determinants of carbon capture by C3 and C4 individuals in mixed stands. Carbon gain will be quantified by steady-state 13C/12C field-labeling, while natural abundance of carbon stable isotopes will serve to partition the contribution of C3 and C4 plants to organic carbon pools, respiratory CO2 efflux, and herbivore¿s diets.