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.

The biological yield potential of winter annual range legumes is influenced by the timing of rainfall adequate to ensure germination. If rains are late, the establishment phase of growth must occur at cold soil and air temperatures. On low‐P soils, this combination of abiotic factor constraints can limit symbiotic N fixation. We conducted three controlled‐environment experiments to compare the seedling growth of rose clover (Trifolium hirtum All., ‘Hykon’) and subterranean clover (T subterrateum L., ‘Mt. Barker’, hereafter referred to as subclover) on a P‐deficient soil (Sobrante‐Las Posas association), simulating winter conditions of cold soils and low light. Comparative growth responses, especially intraplant dry matter partitioning and nodulation, were observed for several soil P availability levels. A sunlit phytotron provided a common shoot environment of seasonal (fall/winter) ambient light intensity and photoperiod, with a programmed smooth diurnal temperature cycle of 20 C maximum and 5 C minimum. Three independently‐controlled water baths maintained soil temperature experiments at a constant 5 C, a constant 10 C, and a diurnal 5 C to 10 C alternation. Phosphorus levels were 0, 45, 90, and 180 kg/ha equivalents. Growth intervals (5 ± 0.5 trifoliolate‐leaf stage), varied between 58 and 114 days. Leaf area of subclover was greater than that of rose clover, and responded more to increasing P levels. Leaf areas tended to increase with higher soil temperatures. Nodule weight was positively correlated with leaf area; nodule weight:leaf area regressions suggested that nodule development was adequate at 5 C and that nodule development was primarily related to time rather than to morphological development of the shoot. Nodule weights were similar for the two species, and low at 0 P. The results suggest that while the practical soil threshold temperature for overall growth is near 5 C, some intraplant growth processes may proceed, but only at adequate, availability levels of mineral nutrients. These relationships may be particularly important where range annual legumes are being introduced as part of a more general range improvement program.

Sustained productivity in multiple‐species, perennial grass‐legume pastures depends in part on achieving and maintaining an appropriate species (SP) balance. Nitrogen levels for the grass component and consistent opportunity for rooting and nodulation of new legume stolons are important. Using an outdoor pot experiment, this study investigated perennial regrowth responses of orchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), ladino clover (Trifolium repens L.) and strawberry clover (T.fragiferum L.) to differing soil textures (ST) obtained by addition of sand and peatmoss, together with N in split applications of 60 kg ha−1 to provide rates of 60,120, and 180 kg ha−1. Of particular interest were maintenance of a favorable (e.g., 1:1) grass‐legume balance and identification of the effects of alteration in ST on legume root and nodule development (RND). The four SP were established as split plots of transplanted propagules on whole plots of N and ST treatments in a completely random design. At 30‐d intervals dry matter (DM) was sampled for each species. Legume stolons were observed for leaf appearance rate (LAR) and were subsampled once during the 120‐d experiment to determine RND. Differences in soil texture had no significant influence (P > 0.05) on DM yields or on legume LAR or RND, but significant (P < 0.001) effects on DM were observed for N and SP and for N × SP interactions. Increasing N significantly (P < 0.05) reduced LAR and RND. Percent N recovery was highest (72%) at 60 kg ha−1 N but legume dominance was avoided only at 180 kg ha−1 N, (N recovery = 12%). Successive split applications of N are useful in managing grass‐legume balance and DM yield in closely‐grazed perennial pastures, but at N levels effective in achieving these objectives a low recovery of applied N is likely.

Winter annual legumes have low fall and early winter forage yields in California's Mediterranean‐type climate. A 2‐year field study was conducted to determine the effects of late summer irrigation on seedling development, forage yield potential, and management problems of subterranean clover (Trifolium subterraneum L.), rose clover (Trifolium hirtum All.), and bur clover (Medicago polymorpha Gaertn). A mixture of three annual grasses, slender wild oats (Avena barbata Pott. ex Link), ‘Blando’ brome (Bromus mollis L.), and annual ryegrass (Lolium multiflorum L.), and a natural stand of indigenous species were included in the study for comparison. By irrigation prior to fall rains, we subjected six successive seedings, 2 weeks apart, to higher temperatures and longer fall‐growth periods than usual. Rates of morphological development of subclover and rose clover were measured until the seven‐leaf stage. In the 1972‐73 season, rose clover showed the largest differences in growth rates between planting dates. Rose clover from the 23 October planting required 90 more days to reach the seven‐leaf stage than when planted 29 September. Early‐flowering subclover from the 23 October planting grew more rapidly than did mid‐flowering subclover and rose clover. In the 1973‐74 season, late‐flowering sub‐clover had the fastest growth rate following 7 August irrigation; there were no differences between clovers following 20 September irrigation. Forage yields for August and September plantings were similar but were greater than those from the October planting. Plants irrigated in August were subjected to daylengths which promoted fall flowering. Summer insect problems, primarily with beet armyworm (Spodoptera exigua Hübner), were associated with August plantings. Subterranean clover produced more dry matter and had fewer problems than did the other legumes tested.

We sought to determine the individual contributions of ambient root and shoot temperatures to rate of plant development, dry matter accumulation, and root/shoot ratio. Subterranean clover (Trifolium subterraneum L., subsp. yaminnicum Katznelson and Morley)' plants were grown from seed to the 7‐ to 10‐ leaf stage in sand‐nutrient culture. Combinations of 10 and 20 C were used in the following basic chronological sequence: (i) plants were grown at 20 C from seed to the two‐leaf stage; (ii) root, shoot, or overall temperature was then lowered to 10 C and growth was allowed to progress until the five‐leaf stage; and (iii) temperature was returned to 20 C and growth was continued to the 7‐ to 10‐ leaf stage.Growth was expressed as a regression of trifoliolate leaf number on days accumulated (y = bx + a) within each of the three growth intervals. For intervals (i) and (iii), b = 0.32 and 0.44, respectively. For (ii), b = 0.20 (root temp. 10 C), and 0.15 (overall temp. 10 C). Correlations (rxy) = 0.99 in all instances. Plant weights were more closely related to final stage of plant development than to temperature treatment. Root/shoot ratios averaged 0.44. Shoot ambient temperature appeared to be less important than root ambient temperature as a determinant of growth rate.