Total and aboveground radiation use efficiency in C₃ and C₄ grass species influenced by nitrogen and water availability

Absorbed solar radiation and radiation use efficiency ( RUE ) can be used to estimate net primary productivity of terrestrial ecosystems. In ecosystems dominated by grasses, belowground productivity cannot be neglected in terms of carbon balance because of the high proportion of biomass allocated to roots. The objective of this study was to quantify total RUE ( tRUE ), which includes both below and aboveground biomass of two C 3 ( L olium perenne and D actylis glomerata ) and one C 4 ( C ynodon dactylon ) grass species, under four treatments with contrasting water and nitrogen availabilities. The ratios between tRUE and aboveground RUE ( aRUE ) for species and treatments were analyzed. The tRUE was calculated from measurements of incoming photosynthetically active radiation ( PAR ), the fraction of PAR intercepted and shoots and roots productivity in pot experiments. The highest tRUE values in the three species were found in the treatment without growth limitations (4.32–6.93 g MJ −1 ), while the lowest tRUE values were observed under water and nutrient deficits conditions (2.62–2.85 g MJ −1 ). Contrary to predictions from the optimization theory, one of the C 3 grass species allocated relatively high biomass to the roots when water availability was high while for the C 4 grass species the shoot:root ratios and the root mass fraction did not change under resource limitation conditions compared to ample resource availability. tRUE exhibited small changes in the C 4 species with variations in resource availability while it did decrease substantially for the C 3 species when at least one of the resources was limited. These results highlight belowground biomass importance in calculating RUE of grasses.