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

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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.