Root excretions into the medium of hydroponically grown N2-fixing alfalfa were studied by quantitative and qualitative measurement of nitrogenous compounds in frequently collected aliquots of the culture solutions. Considerable amounts of several nitrogenous compounds were excreted from the nodulated roots. Ammonia, glutamate, serine, alanine, and aspartate were the main compounds excreted and were similar in proportion to the soluble nitrogenous compounds in nodule and root tissues, with the exception of asparagine. Asparagine was present in very high concentrations in the plant tissues and also in the root xylem exudates destined for the shoots, but it was not detected in the solution bathing the roots. 15N2 was also supplied to the nodulated roots of alfalfa and the flow of the label was followed both in the plant tissues and in the culture solution. Results showed that the amounts of nitrogenous compounds excreted to the medium were related to their formation following N2 fixation and also indicated that the recently fixed nitrogen was the main source of the excreted N compounds.

Summary Three legume species (alfalfa, red clover, and birdsfoot trefoil) in combination with five grass species (timothy, bromegrass, red fescue, tall fescue, and orchardgrass) were used to study N transfer in mixtures, using the 15 N dilution technique. The advantage of grass-legume mixtures was apparent, Total herbage and protein yields of grasses in mixtures were higher than those alone, especially at the later cuts. This benefit of mixed cropping is mainly due to N transfer from legumes to associated grasses. N2-fixation and N transfer by alfalfa rated highest, red clover intermediate, and birdsfoot trefoil lowest. The importance of each pathway of N transfer from legumes appeared to differ between species. Alfalfa and red clover excreted more N than trefoil, while the latter contributed more N from decomposition of dead nodule and root tissue. The greatest advantage from a grass-legume mixture, with respect to the utilization of N released from the legume, varied with early maturing tall fescue (Kentucky 3 l), orchardgrass (Juno), and bromegrass (Tempo), to intermediate timothy (Climax), and least with late maturing red fescue (Carlawn).

The inclusion of legumes in grass pastures often increases grass yields due to the contribution of the N fixed by the legume to the associated grass. This benefit was evaluated in field experiments under different management conditions at Ottawa, Ontario, Canada, on a Manotick mixed sandy loam soil (Typic Dystrochrett) labeled with 15N. Timothy (Phleum pratense L.) and alfalfa (Medicago sativa L.) were grown in pure stands and in mixtures at two proportions (1:1 and 1:2) and harvested at three alfalfa maturity stages (early, mid‐, and late bloom). Nitrogen fixed by alfalfa and N transfer from alfalfa to timothy were calculated from the differences between the isotopic compositions of the two species in mixture and timothy in pure stand. The highest amounts of N in alfalfa shoots originating from N fixed were 192 and 215 kg N ha−1 yr−1 in the first and second year, respectively, when harvested at mid‐bloom (four cuts). The inclusion of timothy in alfalfa stands increased the proportion of N fixed by alfalfa. Nitrogen transfer from alfalfa to associated timothy contributed up to 22% (first year) and 30% (second year) of the total N yield of timothy and amounted to up to 13 kg N ha−1 yr −1. This transfer increased with progressive cuts and with an increased proportion of alfalfa in the mixture. Growing alfalfa in mixture with timothy at a high proportion to timothy is an ideal system for maximum N transfer and for reducing N fertilizer usage.

Perennial legume such as alfalfa have the capacity to sustain shoot regrowth and some nodule N~-fixation after removal ('cutting') of shoots which contain practically all of the plant's photosynthetic capacity. The role of the roots in supporting these processes has not been fully described. Measurements were made of the nodules' responses to removal of shoots from 8-week-old seedlings in terms of N2-fixation, as nitrogenase activity (NA) measured as acetylene reduction, dark CO 2 fixation, measured as in vitro phosphoenolpyruvate carboxylase (PEPC) activity, and total non-structural carbohydrate (NSC) content. These properties decreased and recovered in that sequence, which suggests that nodule NSC supported the substrate requirements of NA and PEPC immediately after cutting. The utilization and redistribution or root carbon and nitrogen, prelabeled with ~4C and I5N, were also followed after cutting 8-week-old alfalfa seedlings. In the first 2 weeks of regrowth 12% of root C and 25% of root N were transferred for incorporation into new shoots. Up to 40% of the root C was used for plant respiration to support 28 days of shoot regrowth and N2-fixation. The decline of N2-fixation was slower after cutting and its minimum activity rose up 45% of pre-cut activity as root reserves were built up with plant age. Therefore, the stored reserves of nodulated roots play an important role in support of N,-fixation after cutting.

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