Growth and Nitrogen-Fixing Responses of Subterranean Clover to Application and Subsequent Removal of Ammonium Nitrate

Considerable variation exists in recommendations for the use of N during establishment of forage legumes. Abundant literature documents the inhibition of N₂ fixation by applied N, but few experiments determined the consequences of its subsequent removal. In this study the effects of NH₄NO₃ on the growth and N₂ fixation of subterranean clover (Trifolium subterraneum L.) were investigated. Plants were grown in a greenhouse from seed with a modified Hoagland's solution containing 0, 2, 4, 6, 8, and 12 mM NH₄NO₃ for 80 d. The plants were then divided into two groups, the N treatments continued for one (+ N) group and the other group given an N-free Hoagland's solution (−N) for an additional 21 d. Total dry wt. (DM), apparent N₂ fixation (ANF), leaf area (LA), and shoot N concentration were measured and levels of photosynthetically active radiation (PAR) were monitored continuously during daylight hours. Per plant DM accumulation increased exponentially over 101 d. Neither DM nor LA accumulation was influenced by NH₄NO₃ at 50 and 80 d. At 101 d, both DM and LA were greater (P < 0.01) for the 2 mM treatment than for the control, and each declined linearly (P < 0.01 for ± NPM and −NLA; P < 0.05 for +NLA) with increasing NH₄NO₃ levels. Differences in slopes were not significant (P > 0.05). Leaf area ratio (LAR) was not influenced by NH₄NO₃ treatments at any of the sampling dates. Nitrogen per plant was greater than the control (P < 0.01) for both ± N at 2 mM, and it declined linearly (P < 0.05 for +N; P < 0.01 for −N) with increasing NH₄NO₃ levels. A reduction in per-plant ANF was significant (P < 0.01) over all NH₄NO₃ levels at 50 and 101 d and over 6 to 12 mM levels at 80 d. Plants previously grown at 6 to 12 mM levels (−N) recovered much of their ANF ability at 101 d, while ANF for + N plants declined (P < 0.01) across all N levels. Expression of ANF on a per unit DM or LA basis improved interpretation of these data. Nitrogen concentration of shoots for all treatments except 12 mM −N was similar to the control at 101 d. Overall, N yields per plant were best explained by DM accumulation from 80 to 101 d. The ability to rapidly recover N₂-fixing ability following depletion of inorganic soil solution N provides opportunity for early season use of N fertilizer in annual range systems. Contribution from the Dep. of Agronomy and Range Sci., Univ. of California, Davis.