Nitrogen addition increases the production and turnover of the lower-order roots but not of the higher-order roots of Bothriochloa ischaemum

Global nitrogen deposition alters grassland ecosystems. Whether added nitrogen changes root production and turnover by root orders is unclear. We compared the root dynamics across four root orders of Bothriochloa ischaemum treated with nitrogen addition (0-10 g N m(-2) year(-1)). The higher order roots exhibited lower production, turnover, number, length, and biomass, indicating a hierarchical system of B. ischaemum. At whole root system level, nitrogen addition increased length production, biomass production and turnover. At root order level, nitrogen addition increased length production, biomass production, and turnover of the first two order roots but not of the third- and fourth-order roots. Nitrogen addition reduced root biomass, and the belowground to aboveground biomass ratio, supporting the functional equilibrium hypothesis. The increased root production, turnover and decreased root number, length and biomass were mainly attributed to the increasing ammonium and nitrate nitrogen. Nitrogen addition increased the length production (7-30%), biomass production (10-34%) and turnover (8-35%) of the first two order roots but not of the higher order roots compared with the control pots. The discrepancy in root characteristics and their responses to nitrogen availability among root orders should be considered in establishing root dynamic models.