Ecophysiological mechanisms controlling the balance between C3 and C4 species in grasslands, with emphasis on responses to nitrogen loading
Final Report Abstract
We found that the balance between species with the C3 and C4 photosynthetic modes −i.e. cool-season and warm-season species, respectively− is strongly responsive to the exclusion of grazing in temperate and subtropical campos and pampas of Argentina and Uruguay (the so called 'Rio de la Plata grasslands', in South America). Grazing exclusion led to a rapid and drastic dominance of C3 vegetation, which overtopped C4 plants. Indeed, C4 species virtually dissapeared in the temperate site. While nitrogen loading accelerated this response, it did not change it qualitatively. Moreover, nitrogen loading had a very minor effect upon the C3/C4 composition of grasslands that were either cut six to eight times a year or kept short (8 - 14 cm) by continuous grazing. These results are based on data from field experiments with nitrogen loading (0 vs. 100 kg N/ha/y) and defoliation treatments (continuous grazing vs. cutting vs. grazing exclosure), carried out at three humid sites placed along a 1,300 km North-South transect: a temperate one (37°S), and two subtropical ones (31°S and 29°S), on which we measured the seasonal evolution of the C3/C4 composition of green vegetation, litter and roots over two to four years using the natural abundance of the stable isotope 13C. On these experimental plots, we used a mobile 13C-labeling facility that we developed and built to measure the daily carbon gain of C3 and C4 individuals growing together in dense field canopies, in each season (spring, summer, autumn and winter). We found that the amount of carbon assimilated per unit leaf area was not much lower in C4 species than in C3 ones during the autumn-to-winter transition (i.e. the cool-season). But C4 plants had very little leaf area per unit plant mass, and therefore their carbon gain was low due, mostly, to a morphological constrain. While this was evident in both subtropical and temperate sites, its magnitude was larger in the latter. The relative performance of C3 vs. C4 species regarding the carbon gain of individuals was similar in cut and continuously grazed grasslands. Therefore, the way in which the canopy was defoliated affected little the C3/C4 balance. But the exclusion of grazing led to rapid shading of C4 plants by their taller C3 neighbors, and to a dramatic reduction of their carbon gain in all seasons. In this case, the lower carbon gain of C4s was largely due to a reduced assimilation of carbon per unit leaf area (shading effect). These results are important to understand how C3 and C4 species coexist in grasslands, and what variations can be brought about by changes in grassland management (e.g. fertilization and grazing regimes) or by global change (e.g. nitrogen deposition). The expansion of C4 vegetation into cooler environments (cool-season, temperate areas) seems to be limited not by the photosynthetic performance of C4 leaves but by the ability of C4 species to use assimilated carbon to expand and sustain leaf area at moderately low temperatures (~10 °C). In this context, the presence of herbivores (which might have been thought detrimental for the maintenance of C4 leaf area) actually reduced the competitive ability of the dominant C3 neighbours, and thus prevents overtopping and competititve exclusion of C4 vegetation.
Publications
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(2008). Consequences of changes in the C3/C4 composition of natural grasslands on potentially mineralizable nitrogen. In: Resúmenes del 21° Congreso Argentino de la Ciencia del Suelo, Asociación Argentina de la Ciencia del Suelo. CD-ROM
Murray F., Lattanzi F.A., Berone G.D., Videla C. & Pizzio R.
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(2010). Carbon gain of C3 and C4 grasses in a dense canopy in the field. Grassland Science in Europe 15, 48-50
Berone G.D., Lattanzi F.A. & Schnyder H.
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(2012). 13C-labeling shows the effect of hierarchy on the carbon gain of individuals and functional groups in dense field stands. Ecology
Lattanzi F.A., Berone G.D., Feneis W. & Schnyder H.
