Root biomass and soil δ13C in C3 and C4 grasslands along a precipitation gradient

Many studies have investigated the aboveground distributions of C-3 and C-4 grasses along climatic gradients because they illustrate complex interactions between abiotic and biotic controls on ecosystem functions. Yet few studies have examined belowground components of these distributions, which may present very different patterns compared with aboveground measures. In this study, we surveyed grass species cover and collected soil and root samples from field plots at 100-150-m elevation intervals along a climatic gradient in Hawai'i. We examined how the relationship between soil carbon isotopic composition (delta C-13), a proxy for C-4 dominance, and % C-4 cover changed along a climatic gradient. We also evaluated root biomass to determine if belowground dominance reflects aboveground patterns under climate variation. Results showed that soil delta C-13 under predicted C-4 dominance in wetter sites. The relationship between % C-4 cover and soil delta C-13 became more negative with increasing mean annual precipitation (MAP) based on a linear mixed-effects model (F (1,34) = 12.25, P < 0.01). Soil delta C-13 in wetter sites indicated a larger C-3 contribution than estimated by aboveground cover, which was in part due to C-3 root biomass increasing (P < 0.05), whereas C-4 root biomass did not change along the precipitation gradient. C-3 and C-4 grasses appear to allocate disproportionately belowground; thus, a different understanding of C-4 ecological dominance (biomass or productivity) may emerge when considering both the above and belowground components. Our results show that belowground allocation and interpretation of soil delta C-13 need to be more carefully considered in global vegetation and carbon models and paleoecological reconstructions of C-4 dominance.