Economic evaluation of biological nitrogen fixation in flooded rice cultivation in subtropical lowlands

This study aimed to assess the economic viability of biological nitrogen fixation in flood rice cultivation in subtropical lowlands. A field experiment of seven treatments was carried out during the cropping seasons 2015/16, 2017/18, 2018/19 and 2019/20 at the Lowland Experimental Station of Embrapa, in Pelotas, RS, Brazil. The evaluated treatments were: (1) negative control (without N and inoculant); (2) positive control with recommended N-fertilization (120 kg N ha-¹, without inoculant) (standard recommendation); (3) inoculant 1 (accessions CMM 174 + CMM 175 + CMM 179); (4) inoculant 2 (accessions CMM 176 + CMM 197 + CMM 205); (5) combination of reduced N-fertilizer doses (90 kg N ha-¹) + inoculant 1; (6) combination of reduced N-fertilizer doses (90 kg N ha-¹) + inoculant 2; and (7) commercial inoculant [Azospirillum brasilense (strains Ab-V5 and Ab-V6)]. A partial budget was used to compare evaluated treatments with the standard recommendation regarding nitrogen fertilisation. Inoculant 2 composed of bacterial accessions CMM 176 (Rhizobium sp.), CMM 197 (Bacillus sp.) and CMM 205 (Aeromicrobium sp.) combined with reduced mineral nitrogen fertilisation [90 kg N ha-¹ (applied in top dressing)] demonstrates the best agroeconomic efficiency in the production of irrigated rice in subtropical lowlands with the cultivar BRS Pampa.