532 Nitrogen Fertilization Effect on Biomass Yield of Six Different Sorghum bicolor Varieties Dimitrios Bartzialis 1 , Kyriakos D. Giannoulis 2 , Elpiniki Skoufogianni 1 , George Charvalas 1 , Asterios Kritsanos 1 , Leonidas Siokas 1 , Nicholaos G. Danalatos 1 1 University of Thessaly, Dept of Agriculture Crop Production and Rural Environment, Laboratory of Agronomy and Applied Crop Physiology, Fytokoy Str., 38446 Volos, Greece 2 University of Thessaly, Dept of Agriculture Crop Production and Rural Environment, Laboratory of Agronomy and Applied Crop Physiology, Fytokoy Str., 38446 Volos, Greece; e- mails: kgiannoulis@uth.gr; kyriakos.giannoulis@gmail.com Abstract. Sorghum belongs to the Poaceae family and it is used for its high biomass production. A field experiment was conducted using a factorial split- plot design with three replicates and twenty-four plots per replication. The main factor was the different varieties (V1: Buffalo grain, V2: Elite, V3: Big Kahuna, V4: 25K1009, V5: 4264 and V6: 5D61) and the sub-factor the different nitrogen fertilization levels (N1: 0, N2: 80, N3: 160 and N4: 240 kg ha-1, using urinary ammonia 40-0-0). In the case of fresh biomass weight, the variety "25K1009" produced the higher yield, followed by "Big Kahuna" and "Elite" with 66.9, 65.27 and 63.03 t ha -1 respectively, which are statistically significantly higher compared to the yield produced from the other three varieties. The same was also found in the case of the dry biomass weight. The increase in the fresh biomass weight was higher between the higher n- application (160 and 240 kg N ha -1 ) reaching the 9 t ha -1 . Keywords: Sorghum; biomass; yield; nitrogen. 1 Introduction The mechanization of agriculture led to the intensification of agriculture and the over-exploitation of land. This practice has resulted in a reduction in soil fertility and a consequent increase of the cultivation cost and the reckless overuse of fertilizers. Various suitable systems are used for soil remediation or maintenance of soil’s fertility as well as the increase of productivity, such as crop rotation with legumes, inoculated crops, co-crops, green or organic fertilizers, etc. (Ball et al., 2005). It has been reported that corn - soybean rotation can increase corn yield up to 5-30% (Marburger et al., 2015; Stanger & Lauer 2008). Crop rotation along with fallow and green manure are the oldest and most substantial agricultural practices. The change of crops with the system of crop rotation or the intermediate cultivation helps to reduce harmful enemies that appear in the monoculture system such as insects, diseases and weeds (Castellazi et al., 2008; Pantoja et al., 2015). These cultivation practices affect the nitrogen cycle in soil, and they consist the major source of