Nitrogen Sources for Bean Seed Production '

Beans (Phase()las valgtais L.) often respond to N fertilization; however, N fertilization is not practiced for maximum seed production in southern Idaho. This suggests that the symbiotic relationship and/or soil N sources can provide most of the N needed by this legume. Our objective was to evaluate the relative contribution of the symbiotic-nonsymbiotic N sources by studying the effects of N fertilization on the symbiotic N. fixation and seed yields under field conditions. Experiments were conducted on silt loam soils belonging to the Portneuf series (Xerollic Calciortnids). An acetylene reduction (AR) method was used to determine the effect of N fertilization treatments on the relative seasonal N, (AR) fixation. The symbiotic N, fixation was also estimated by the equation, N. = N„„ — (N, + N„, N.) —aN., where N„, is the accumulated N uptake measured near physiological maturity, N, and N,, are the amounts of soil NO,-N in the root zone before planting and near physiological maturity, N„ is the N mineralized from soil organic N sources, and a is the recovery of the N fertilizer (N,) applied. Estimates of the N fertilizer recoveries were obtained from two experiments using 15N-depleted (NH4), SO,. The symbiotic N, relationship contributed up to 90 kg N/ha, which was 40 to 50% of the total N found in bean plants near physiological maturity. The amount of symbiotic N. fixed decreased as the available soil N or fertilizer N increased, and increased as the N required by the individual cultivars increased. The response to N fertilization depended upon the cultivar, as well as on the N_available from soil sources. Measured fertilizer N recoveries ranged from 7 to 33%. An average of 52% of the total N uptake near physiological maturity was taken up after the maximum symbiotic NAAR) rate occurred; while the seed contained an average of 60% of the total N uptake. A low N fertilization rate (< 50 kg Nilia) when the soil N, was low (<50 kg N/ha) ensured an early vi us plant growth but did not always increase seed yields. Higher N fertilization rates may be required on soils with lower amounts of mineralizable N. Additional index words: Acetylene reduction, Fertilizer N utilization, N fixation, N uptake, l'haseolus outwit. EANS (Phaseolus vuigaris L.) utilize inorganic soil N or applied fertilizer N and N2 fixed by a symbiotic relationship with Rhizobium phaseoli. Both the inorganic and symbiotic N sources seem necessary for maximum yields of seed legumes (5, 10); however, as the soil N or fertilizer N increases, that fixed by the symbiosis decreases (3, 8). A relatively low amount of available N during the initial plant development generally enhances noduladon and plant growth (7). Recent studies indicate that foliar N applications during seed development may be beneficial to some legumes (6, 17), however, no consistent soybean [Glycine max (L.) Merr.] yield increases have occurred from several N application methods and N sources (23). In contrast, beans respond to N fertilization (2, 4, 11, 20, 26). This indicates that their symbiotic N2 fixation process generally does not provide sufficient N for maximum yields. The symbiotic N2 fixation capacity of beans is reported to be 40 kg N/ha as compared with 100 kg N/ha for soybeans (3, 9). Soybeans obtain 25 to 60% of their N requirements from the symbiotic fixation process. Dry and garden beans are normally grown for seed production in southern Idaho without N fertilization and are nodulated from indigenous soil Rhizobium populations. Nitrogen fertilization is only recommended when large amounts of crop residue are being returned to the soil or if it is the first bean crop Contribution from USDA, SEA-AR; Univ. of Idaho College of Agric. Res. and Ext. Ctr., Kimberly, cooperating. Received 5 June 1980. Soil scientist, Snake River Cons. Res. Ctr., Kimberly, ID 83341; crop physiologist, Univ. of Idaho Res. and Ext. Ctr., Kimberly; and soil scientists, U.S. Dep. of Agric., Fort Collins, Coto., and Kimberly, Idaho, respectively. 660 Purchased by the U. S. Der.p9rini::nq of Agricultur. Eat (Maki Dm WESTERMANN ET AL.: N SOURCES FOR BEAN SEED PRODUCTION 561 Table 1. CuWears, soil N amounts, N fertilization rates and estimated efficiencies, 1ST uptake, estimated seasonal N I, and seed 3rNilds for different N treatments in each experiment. Experiment and cultivar Nit 1ST Nmt Nit at Nupt Estimated seasonal Seed yielctt N,(AR) N,t