Nodulation Responses from Nitrogen Applied to Soybean Half-Root Systems

Interrelationships between combined and symbiotic N in soybeans (Glycine max L. Merr.) are complex. To further clarify the potential role of combined N in field production, we tested the hypothesis that adding N to one-half of a soybean root system would have no effect on nodule formation or nodule growth on untreated roots of the same plant. Roots of greenhouse plants were divided into two equal parts and transplanted into different pots of low-N field soil when plants were 17 days old. Half of the root system received no nitrogen (—N), and the other (+N) received 0, 30, 60 or 120 ppm in Experiment I and 0, 15, 30, 60, 120 or 240 ppm N in Experiment II. Nitrogen reduced nodule number and weight in +N pots. In —N pots nodule numbers were nearly constant, but nodule weights differed significantly at the end of each experiment (44 and 57 days from planting, respectively). On half-root systems whose companion roots received intermediate rates of N, nodule weights in —N pots were up to 50% more than those for controls (neither half received N) in each experiment. However, for the highest rate of N in each experiment, nodule weights in —N pots were equal to or significantly lower than those from control pots. In —N pots of the 240 ppm treatment in Experiment II, nodule weights were 40% less than for controls. This is interpreted as a nonlocalized inhibition of nodule development. Nitrogen increased root weight in +N pots in both experiments, but increased root weight in —N pots only in the longerduration Experiment II. Nitrogen also increased total plant weight, but N added to similar soil in a field experiment did not increase seed yield. In another field experiment, side-dressing one vs. both sides of field-plot rows with N did not influence seed yield and had no detectable effect on localizing nodule development. Thus, responses to N by greenhouse seedlings were not transferred to seed-yield differences in field plots.