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

拥有C3或C4模式的光合作用导致了植物碳和氮经济的内在差异。因此，在C3和C4物种共存的草原和稀树草原上，C3/C4平衡强烈地影响着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)生态系统的功能具有重要影响。如今，大多数草原经历的人为氮素负荷率的增加可能会大幅减少甚至消灭C4物种。为什么会这样，这在多大程度上是一种普遍的反应，以及放牧是否改变了这一点，目前尚不清楚。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拉普拉塔草原(温带-亚热带南美洲)C3/C4组成的季节性趋势的影响，评估可能与放牧制度的相互作用。其次，它试图检验这样的假设：氮素负荷导致向更多光限制生长的转移，因此C3/C4平衡的变化反映了C3和C4个体捕捉和利用光的特定能力，以及限制冠层发育的放牧制度对光竞争的减弱将化肥驱动的变化降至最低。检验这些假设意味着分析混交林中C3和C4个体在形态发生和生理决定因素上的内在差异和异速生长差异的作用和权衡。碳增量将通过稳定状态的13C/12C场标记来量化，而碳稳定同位素的自然丰度将用于划分C3和C4植物对有机碳库、呼吸二氧化碳排放和食草动物S饲料的贡献。

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.