The amounts and dynamics of nitrogen transfer to grasses differ in alfalfa and white clover-based grass-legume mixtures as a result of rooting strategies and rhizodeposit quality

Abstract

Differences in the ability to fix and transfer N have been shown between perennial legume species. However, the traits responsible for such variations are largely to be identified. This study aimed at comparing the dynamics of N transfer from alfalfa and white clover and test whether their differences resulted from difference in fixation, legume proportion, population dynamics or tissue composition. A three-year field experiment and a greenhouse experiment were carried out. Nitrogen fixation and transfer were assessed trough 15 N dilution and difference methods. Both experiments confirmed significant differences between legumes regarding N transfer capacity. Although alfalfa cumulated twice as much biomass and fixed nitrogen, it transferred smaller amounts of N (59 versus 147 kg N.ha−1 over three years in the field) under a delayed dynamic. The amounts of nitrogen transferred were related to recent decreases in legume population density. Moreover, root tissue composition differed; white clover had a higher proportion of fine roots with a lower C/N and lignin content. This resulted in more rapid N release from the severed roots of clover. The traits controlling plant persistency, root material turnover and residue quality may better explain N transfer dynamics than overall legume production and fixation.