Decreased root turnover in a diverse grassland community maintains belowground overyielding

Background/Question/Methods

Evidence is accumulating that the positive diversity-productivity relationship also occurs belowground. However, results on root mass are based on measurements of root standing biomass which have built up over several years, and contrast with the measurements taken aboveground: net annual shoot production. Increased standing root biomass in mixtures compared to monocultures is the result of the balance between root production and root turnover. We currently have no information about root turnover in diverse communities compared to monocultures, although this is essential for understanding the biodiversity effects on ecosystem functions as nutrient availability and carbon sequestration. Here we report on a biodiversity experiment with four grassland species and address whether belowground overyielding is the result of increased production, decreased mortality or a combination of both processes, in one or more species. In order to do so, root growth and disappearance was monitored monthly with a portable scanner in minirhizotron tubes (placed horizontally at 18 cm depth) in the growing season over a four year period. The experiment was carried out in the Nijmegen Phytotron (www.ru.nl/phytotron)

Results/Conclusions

Root production took place in the growing seasons, with a major increase in root length in the first year. In this year’s cohort, 4-species mixtures produced nearly twice the amount of roots than the average of the monocultures, whereas observed root production in subsequent years did not differ from expected. Root mortality took place throughout the year. Only a small proportion of the living roots died within a given year (0-33%), evidencing low turnover of roots in these grassland communities and root lifespans far longer than 1 year. Mortality rates differed widely among species (2-19% per year on average), with the most productive grass species (Festuca rubra) having the lowest root mortality. For each of the yearly root cohorts, observed root mortality in species mixtures (<2%) was much lower than expected from monocultures, suggesting decreased root turnover in mixed communities. Our results show that mixing of plant species has large positive effects on root production and especially root longevity. We suggest that roots may live longer in mixtures because their own species-specific soil pathogens are diluted. If our findings have general validity for species-rich grasslands, the enhanced carbon input observed with increasing biodiversity is not caused by increased root turnover.