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Oryza sativa L is a grass of high nutritional value and high consumption worldwide. In Cuba, crop production does not respond to high demand, so the development of strategies aimed at increasing its yield is a priority for agriculture. This work aims to characterize associative diazotrophic bacteria with plant growth promoting activity in O. sativa. The bacterial isolates and control strains studied fix atmospheric nitrogen, produce phytohormones and solubilize inorganic phosphorus. It is demonstrated that the bacterial inoculation favors the development of the root system and the aerial part of the O. sativa plants, with a greater increase in the LP5 cultivar which could be related to a greater plant-bacterial specificity. Likewise, the relative contribution derived from BNF is in the range of 12 to 33% in plants of cultivar LP5 and 10 to 30% in Perla de Cuba. Analysis of the sequences of the gene encoding the 16S rRNA allowed the identification of the bacterial species Azospirillum brasilense, Pseudomonas putida and Enterobacter sacchari. These results have application and practical importance in the design of ecologically sustainable strategies in O. sativa. [ABSTRACT FROM AUTHOR]

The nitrogen (N) use efficiency can be determined by the isotopic dilution or by the difference method. The objectives were to measure the destination of foliar fertilizer and to compare the Nitrogen Recovery Efficiencies (NRE) between the mentioned methods. A field experiment was conducted in the southwest of the Buenos Aires Province (Las Oscuras). Nitrogen rates of 0, and 50 kg N ha-1 were applied at sowing and 0 and 25 kg N ha-1 at anthesis. The enriched urea was applied in anthesis and post-anthesis in the microplots. Yields were low with no response to the initial or foliar fertilization: 2100 and 2154 kg ha-1 for 0 and 50N without foliar N and 2160 and 2303 kg ha-1with foliar N application. In the isotopic plots, wheat accumulated 83 kg in aboveground biomass and 52 kg N ha-1 in grain without a significant impact of initial or foliar fertilization. Nitrogen from the foliar fertilizer was accumulated 20% in aerial dry matter (ADM) and 12% in the grain, with a recovery of 6 kg N ha-1. Accumulated N in straw ranged 10 kg N ha-1 and unrecovered N varied from 7,2 to 8,3 kg N ha-1 (30%). The NRE varied from 30% to 54 according to the difference method for the initial 0 and 50N treatments in ADM and grain, with an accumulation of 2,4 to 6 kg N ha-1. Isotopic determined NRE was of 70% in ADM and 25 % in grain. Again, grain accumulation was of 6 kg N ha-1. The differences between the methods suggested the need to identify the factors that influence these estimates. [ABSTRACT FROM AUTHOR]

The intensification of conventional agriculture in the southeastern Buenos Aires province (Argentine) produced soil fertility degradation. No-tillage (SD) agriculture and the inclusion of pastures (PP) in crop rotations, represent management alternatives to reduce conventional tillage (LC) effects. A lab experiment was carried out with the aims of: a) to study nitrogen mineralization (TMB), nitrification (TNB) and consumption (TCB) gross rates in a Mollisol under LC, SD and PP; and b) to quantify the potentially mineralizable nitrogen through anaerobic incubation (NAN) and its relation with mineralization rates. The studied soil was a Typic Argiudoll - Petrocalcic Paleudoll complex which had been under conventional tillage for 17 yr until 1994 when it was converted to three different soil management systems: continuous PP and continuous cropping under SD, and under LC. Gross processes were estimated through the 15N dilution technique, adding 15N-labeled (NH4),SO4 (10% 15N at. exe.) four times during an 35-d incubation period and NAN was determined through short anaerobic incubations. Gross N mineralization rates ranged between 0.17 and 1.36 mg N kg-1 day-1. The soil under PP treatment presented higher TMB than cropping managements. Gross consumption rates ranged between 0.47 and 3.21 mg N kg-1 day-1, being nitrification the most important consumption process for all three management systems (80%). Cumulative mineralized N (P= 28.6 mg kg-1, SD= 19.7 mg kg-1 and LC= 10.8 mg kg-1) was different among management systems, whereas cumulative consumed N were not (P= 72.6 mg kg-1, SD= 56.9 mg kg-1 and LC= 74.1 mg kg-1). The soil under PP presented highest NAN values (93.4 mg kg-1) that was twice as much as soil under LC (41.4 mg kg-1) or SD (46.6 mg kg-1). Lineal correlation coefficients between TMB and NAN were statistically significant for all the measurement times and ranging between 0.47 and 0.97. After five years from management installation, PP produced an increase in TMB relative to the cropping managements, whereas the same period under SD did not improve the TMB relative to LC. Gross consumption rates did not present differences among management systems and neither PP nor SD improved TNB respect to LC. [ABSTRACT FROM AUTHOR]

The decomposition of organic matter in soil, and the accompanying mineralization and immobilization of inorganic N, are key processes in the soil-plant cycle. It is hypothesized that moisture regime may also play an important role on these processes. Two laboratory studies were carried out to investigate the effect of moisture regime on gross mineralization and immobilization of N, in an Ultisol soil, which was incubated at 25°C, under two moisture regimes, field capacity (CC) (40% w/w) and 85% of field capacity (34% w/w). Using mirror image 15N isotope dilution techniques, it was possible to determine gross nitrification and mineralization rates. The rates of gross mineralization and nitrification were greater with 85% CC, and the direct nitrification rate in this soil was extremely rapid and efficient, specially with 85% CC. The immobilization and the amount of the biomass were greater at CC. The N transformations varied with moisture regime. [ABSTRACT FROM AUTHOR]