Seasonal Response of Uninoculated Alfalfa to N Fertilizer: Soil N, Nodule Turnover, and Symbiotic Effectiveness of Rhizobium meliloti1
- M. B. Jenkins and
- P. J. Bottomley2
Information exists on the effects of fertilizer N upon alfalfa (Medicago sativa L.) grown either in the greenhouse or during the establishment year in the case of field-grown plants. Very little information has been obtained by using fertilizer N as a means to assess the performance of symbiotic N, fixation during postestablishment growing seasons. ‘Anchor’ alfalfa was established from uninoculated seed on three soils (two Xerollic Durargids and one Xerollic Camborthid) of limited depth (0.6 m) in central Oregon. After the year of establishment, alfalfa was harvested annually in three cuttings. Fertilizer N (NH4NO3,) was applied at rates of np to 230 kg N ha−1 In mid-April and immediately after the first and second harvests. Most of the responses to added Noccurred in the third harvest. Significant increases were observed in dry weight in three of seven cuttings, in total reduced N in four of seven cuttings, and in the percentage of reduced N in the herbage in two of seven cuttings. At least one of these responses was enhanced with added N in five of the seven third harvest cuttings. Increases were not seen in the first harvest cuttings and only in two of seven second harvest cuttings. Mineral N levels in the soil profiles under the non-fertilized plants at two of the sites declined during the growing season. This decline (35 and 40 kg N ha−1) approximated the difference between the total reduced N in the herbage of N-fertilized and non-fertilized plants in five of six cases (40 to 70 kg N ha−1). Symbiotic effectiveness tests of isolates of Rhizobium meliloti taken from nodules of 10-week-old plants during the establishment year showed that 20% of the isolates were ineffective, 16% were of intermediate effectiveness, and 64% were highly effective. In the 3rd year, although no ineffective nodule isolates were recovered, nodule numbers declined from 66 to 11 per plant between the 6rst and third harvest and no isolates recovered from nodules at the thii harvest were highly effective. The influence of nodule turnover, nodule occupants of suboptimum effectiveness, and the contribution of soil N on alfalfa production during postestablishment seasons requires more attention.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
Copyright © . .