Nitrogen application during rhizoma peanut shoot emergence affects planting-year nitrogen fixation

Rhizoma peanut (Arachis glabrata Benth.; RP) is an important perennial forage legume in the US Gulf Coast region, but it is vegetatively propagated from rhizomes and relatively slow to establish. There are reports that N fertilization enhances RP establishment, but other evidence suggests plant N content and establishment rate may not benefit. Understanding the effect of N fertilizer on RP biological N-2 fixation will help elucidate this response. Our objective was to determine the effect of RP genotype and N fertilization during shoot emergence on N-2 fixation and N accumulation during the year of planting. Replicated field experiments were conducted in adjoining fields; one was planted in 2016 and the other in 2017. Treatments were the factorial combinations of two RP entries (decumbent germplasm Ecoturf and upright cultivar 'UF Tito') and three N rates (0, 40, and 80 kg N ha(-1) applied 10 wk after planting; 1% of total N as N-15) arranged in a randomized complete block design with four replications. Across N rates, upright cultivar UF Tito generally had greater N concentration and content in aboveground and belowground plant parts compared with decumbent germplasm Ecoturf. Nitrogen fertilization generally reduced the percentage of N derived from atmosphere and the amount of fixed N (BNF) during the year of planting, but it did not affect overall N concentration or content. These data suggest RP increased soil N uptake to compensate for reduced BNF following N fertilization, indicating starter N application had no measurable benefit for legume establishment under these conditions.