FINE-ROOT DEMOGRAPHY IN ALFALFA (MEDICAGO-SATIVA L)

In perennial forages like alfalfa (Medicago sativa L.), repeated herba ge removal may alter root production and mortality which, in turn, cou ld affect deposition of fixed N in soil. Our objective was to determin e the extent and patterns of fine-diameter root production acid loss d uring the year of alfalfa stand establishment. The experiment was cond ucted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA , using horizontally installed minirhizotrons placed directly under th e seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N-2 fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercia lly-available cultivar; Ineffective Agate, which is a non-N-2-fixing n ear isoline of Agate; a new germplasm that has few fibrous roots and s trong tap-rooted traits; and a new germplasm that has many fibrous roo ts and a strongly branched root system architecture. Video images coll ected biweekly throughout the initial growing season were processed us ing C-MAP-ROOTS software. More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root pro duction was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than oth er germplasms. In all germplasms, about 7% of the fine roots at each d epth developed into secondarily thickened roots. By the end of the fir st growing season, greatest fine root mortality had occurred in the up permost depth (43%), and least occurred at 40 cm (36%). Survival of co ntemporaneous root cohorts was not related to soil depth in a simple f ashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm dept hs in the 2-week period following herbage removal. Median life spans o f these early-season cohorts ranged from 58 to 131 days, based on fitt ed exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter med ian life spans than early-season cohorts. Similar patterns of fine roo t mortality did not occur at the second harvest. Germplasms differed i n the pattern, but not the ultimate extent, of fine root mortality. Fi ne root turnover during the first year of alfalfa establishment in thi s experiment released an estimated 830 kg C ha(-1) and 60 kg N ha(-1), with no differences due to N-2 fixation capacity or root system archi tecture.