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Combined Nitrogen and Vegetative Regrowth of Symbiotically-grown Alfalfa¹

Few studies clearly compare N source effects on herbage yield of alfalfa (Medicago sativa L.) grown with Rhizobium over several harvest cycles. Therefore, the effects of combined N on nodulation, growth, N accumulation, and development of alfalfa were studied over four harvest/regrowth cycles with a genotype cloned from the nondormant cv. ‘Dohfari.‘ Ramets inoculated with Rhizobium meliloti Dang. strain 102F28 and irrigated with 0,2,8, or 16 mM N provided as NH₄NO₃, NO₃^-, or NH₄⁺ were grown in vermiculite in a naturally illuminated greenhouse in two replicate experiments from March through June and July through October. All combined N treatments enhanced herbage dry weight and total crude protein above the nil N control during the first growth period. The promotive effect of combined N decreased with each harvest until during the fourth regrowth, plants totally dependent on Rhizobium for reduced N produced herbage and crude protein yields equivalent to plants supplied 8 or 16 mM N as NH₄NO₃. Plants grown with 2mM N, a more reasonable N concentration for the soil solution, were more productive than nil N plants during the first two growth cycles but had no advantage during the third and fourth cycles regardless of the form of combined N used. Nitrate suppressed nodule development more than either NH₄NO₃ or NH₄*. Greater availability of combined N increased the soluble sugar:starch ratio in the root and crown tissues. No significant differences were observed between the two experiments except that plants grown later in the year (July to October) took longer to flower during each regrowth cycle.

The data indicate that the Rhizobium symbiosis cannot produce sufficient reduced N for optimum alfalfa growth during the first few harvest/regrowth cycles. During subsequent regrowth cycles, however, plants totally dependent on Rhizobium for N compounds had overcome the period of N-limited growth and produced as much herbage and crude protein as plants grown with levels of combined N that might be expected under normal field conditions.

Key Words: Rhizobium meliloti Dang. • N fertilization • Crude protein • N-limited growth • Medicago sativa L

² Graduate research assistant and agronomist, respectively. Present address of the senior author: Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication January 2, 1981.

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