Differential modulation response and biomass yield of Alexandria clover as affected by levels of inorganic nitrogen fertilizer

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Abstract
Two greenhouse trials were performed to assess the effect of applying various levels of inorganic nitrogen (N) fertilization in yield biomass and nodulation response of clover (Trifolium alexandrinum L.). A 2x2x4 complete factorial resulted in 16 treatments that were tested in a completely randomized design with four replications. Factor A consisted in two levels of inoculum (inoculated with a commercial strain of Rhizobium and no inoculation), Factor B consisted in two times of harvesting (six and nine weeks after seeding), and Factor C consisted in four N fertilization rates applied as urea [CO(NH2)2] (45%N). The first experiment tested the application of 0, 30, 60, and 90 kg N ha−1, and according to the results, a second experiment evaluated the application of 5, 10, 15, and 20 kg N ha−1. The soil used as a growing medium was a sandy loam with a pH of 6.2 without salt problems nor sodium (Na) hazard. After handclipping top biomass, roots were carefully washed, obtained and a count of nodules was made. Dry matter for both top biomass as well as root biomass was determined. Crude protein (CP) content, calcium (Ca), and phosphorous (P) were estimated in laboratory for top biomass. Treatments in the six week harvest periods and treatments without inoculation did not present nodules. Inoculated treatments after nine weeks showed that the amount of nodules (29, 25, 16, and 12 nodules) decreased when increasing N fertilization. However, herbage weight increased (2.600, 3.8437, 4.3187, and 4.3206 g) in an evident linear trend. Number of nodules was stabilized in treatments of the second trial (30, 27, 27, and 25 nodules) as well as herbage yield (2.710, 2.850, 2.905, and 3.0105 g). Crude protein content showed a linear trend in the first experiment, but no differences were noted in the second experiment. Calcium and P concentrations were not affected by treatments. These results showed that application of 10 to 15 kg N ha−1 permitted a maximum of both nodule formation as well as herbage and root weight in Alexandria clover.