ABSTRACT No-tillage (NT) has been largely adopted in Brazil as a strategy for soil conservation, but for the last decade, there have been governmental incentives for its adoption arising from its potential for soil C accumulation. Notwithstanding, the soil mulch formed from crop residues favors the maintenance of soil moisture and nutrients in the upper soil layers, which stimulates soil microbial activity and may increase the potential for nitrous oxide (N2O) emissions. In addition, double-cropping systems in the same year are typical in Brazil and the impact on the fraction of fertilizer N lost as N2O needs to be evaluated. This study aimed to assess the influence of soil tillage and N fertilization on N2O emissions in a wheat-soybean succession system as commonly practiced in southern Brazil. The experiment was carried out at Embrapa Soybean research station located in Southern Brazil. Treatments were conventional tillage (CT) and no-tillage (NT), with and without nitrogen fertilization for the wheat and no N fertilizer for the soybean. Closed-static chambers were used to monitor N2O fluxes for two consecutive years. Together with gas monitoring, soil samples were also taken and analyzed for mineral N, soil moisture and labile carbon. Soybean yields were higher under NT, which seemed to be the result of a higher soil water availability that helped to overcome extended periods without rainfall. Soil N2O emissions were similar between CT and NT, with just a tendency for higher emissions under NT. The highest emissions occurred from the soybean crop. In the second year under NT, the emissions from the soybean crop were higher when preceded by N-fertilized wheat, but the converse was true under CT. None of the soil variables consistently correlated with N2O emissions, with mineral-N as the best predictor in the second wheat cycle and soil moisture in the first soybean cycle. Calculated emission factors were not statistically different between CT and NT and consistently lower than the IPCC default of 1 %. The calculated N2O emission intensity by relating N2O emission to grain yield showed an environmental advantage of NT compared to CT by presenting a 44 % reduction in soybean and similar values for fertilized wheat.

RESUMO: Os objetivos deste estudo foram determinar taxas de deposição e decomposição de serapilheira em pastagens de Marandu (Urochloa brizantha cv. Marandu) sob a) três formas de suprimento de nitrogênio (N), b) em diferentes estágios de pastejo rotativo e c) comparar a constante de decomposição (‘k’) derivada de “litterbags” com os valores derivados das estimativas de serapilheira existente e depositada (técnica SED). Os três tratamentos de suprimento de N foram: sem ou com adubação nitrogenada (zero ou 150 kg N ha-1 ano-1) ou com a leguminosa Desmodium ovalifolium. Não houve diferenças significativas (p<0,05) entre a serapilheira existente e as taxas de deposição e decomposição da serapilheira entre os três tratamentos de N. A taxa de decomposição da serapilheira foi estimada usando a técnica SED para os 7 dias de pastejo e dois períodos subsequentes de 14 dias em cada ciclo de 35 dias. A taxa de decomposição da serapilheira foi (p<0,05) maior no segundo período de descanso (dias 21 a 35) em 0,089 g g-1 dia-1, do que no período de pastejo (0,038 g g-1 dia-1) e no primeiro período de repouso (0,040 g g-1 dia-1). A meia-vida média da serapilheira foi de 12 dias pela técnica SED, enquanto a estimativa dos “litterbags” foi de 136 dias. Os resultados mostram que as estimativas fornecidas pelos “litterbags” subestimam severamente a decomposição em relação à técnica SED, e preveem um acúmulo de serapilheira a longo prazo que não é observado.

ABSTRACT: The objectives of this study were to determine the rates of plant litter deposition and decomposition in Marandu pastures (Urochloa brizantha cv. Marandu) under a) three forms of nitrogen (N) supply, b) at different stages of rotational grazing and c) to compare the single-exponential decay constant (‘k’) derived from litterbags with values derived from estimates of deposited and existing litter (DEL technique). The three N supply treatments were: without or with N fertilization (zero or 150 kg N ha-1 yr-1) or with the legume Desmodium ovalifolium. There were no significant differences (p<0.05) between existing litter and rates of litter deposition and decomposition between the three N supply treatments. The litter decomposition rate was estimated using the DEL technique for the 7-day grazing periods and two subsequent 14-day periods in each 35-day grazing cycle. The litter decomposition rate was (P<0.05) higher for the second rest period (days 21 to 35) at 0.089 g g-1 day-1, than for the grazing period (0.038 g g-1 day-1) and for the first rest period (0.040 g g-1 day-1). The mean half-life of the litter was 12 days using the DEL technique while the estimate from the litterbags was 136 days. Results showed that estimates provided by litterbags severely underestimate the decomposition in relation to the DEL technique and predict a long-term accumulation of litter which is not observed.

RESUMO A cana-de-açúcar é uma cultura de grande destaque na economia, em razão da produção de açúcar, etanol e energia. Tecnologias que possam contribuir para o aumento da produtividade e qualidade da cultura com mínimos danos ao meio ambiente são necessárias. Objetivou-se avaliar a produção de ácido indol acético de cinco estirpes de bactérias diazotróficas e o efeito da inoculação delas na brotação de duas variedades de cana-de-açúcar, RB867515 e IACSP95-5000. A produção de auxina foi determinada pelo teste colorimétrico, usando o reagente de Salkowski. Para avaliar a germinação, foi conduzido um experimento em casa de vegetação, utilizando-se delineamento experimental em blocos ao acaso com quatro repetições e sete tratamentos: controle não inoculado; inoculação mista com as cinco estirpes e inoculação individual com Gluconacetobacter diazotrophicus (Gd) estirpe BR11281T(PAL-5T), Herbaspirillum seropedicae (Hs - BR11335 = HRC54), Herbaspirillum rubrisubalbicans (Hr - BR11504 = HCC103), Burkholderia tropica (Bt - BR11366T = PPe 8 T) e Azospirillum amazonense (Aa - BR11145 = CBAMc). As bactérias mais eficientes na produção de auxina foram Hs e Hr, declinando 48 h após o crescimento. Hr, Aa e Bt aumentaram o índice de velocidade de germinação e o número de brotações nas duas variedades. Na var. RB867515, a velocidade de germinação ainda foi positivamente influenciada pela inoculação mista, sendo o mesmo observado pela inoculação de Gd na var. IACSP95-5000.

ABSTRACT Sugarcane is one of the most important crops in the economy due to production of sugar, ethanol, and energy. Technologies are necessary to improve its yield and quality with minimum damage to the environment. We evaluated the indole acetic acid production and germination in the sugarcane varieties RB867515 and IACSP95-5000 after inoculation with five strains of diazotrophic bacteria. The production of indole acetic acid was assayed through the colorimetric method using the Salkowski reagent. To evaluate germination, an experiment was conducted in a greenhouse using a randomized block experimental design with four replications and seven treatments: uninoculated control; mixed inoculation with the five strains; and individual inoculation with Gluconacetobacter diazotrophicus (Gd) estirpe BR11281T (PAL-5T), Herbaspirillum seropedicae (Hs - BR11335 = HRC54), Herbaspirillum rubrisubalbicans (Hr - BR11504 = HCC103), Burkholderia tropica (Bt - BR11366T = PPe 8 T) e Azospirillum amazonense (Aa - BR11145 = CBAMc). The most efficient bacteria for production of indol acetic acid were Hs and Hr, and production declined after 48 h of growth. Hr, Aa, and Bt also improved the germination speed index and number of stalks sprouted in both varieties. Furthermore, in the var. RB867515, germination speed was positively affected by the mixed inoculation, and the same effect was observed from Gd in the var. IACSP95-5000.

O aumento da eficiência produtiva da cana-de-açúcar com tecnologias que possibilitem aumentar seu balanço energético é uma necessidade do setor sucroenergético, por se tratar de uma cultura estratégica para o país. Entre as diversas alternativas atualmente estudadas, o inoculante com bactérias diazotróficas vem se evidenciando uma tecnologia com elevado potencial. Diante desse contexto, este estudo teve como objetivo avaliar a eficiência do inoculante bacteriano no desempenho agronômico e na nutrição nitrogenada da cana-de-açúcar. O ensaio foi realizado no campo experimental da Embrapa Agrobiologia, em Seropédica, RJ. O delineamento experimental foi em bloco ao acaso, em esquema fatorial 2 × 3 (duas variedades e três tratamentos), com quatro repetições, totalizando 24 parcelas. As variedades utilizadas foram a RB867515 e RB72454. Os tratamentos foram: inoculação com bactérias diazotróficas, controle nitrogenado com 120 kg ha-1 de N e controle absoluto (sem inoculação e sem N). O inoculante foi composto por cinco estirpes de cinco espécies de bactérias diazotróficas. As variáveis avaliadas foram a produtividade de colmos, o acúmulo de matéria seca e N total da parte aérea das plantas e a contribuição da fixação biológica de N (FBN), utilizando como referência a abundância natural de 15N da própria cana-de-açúcar não inoculada. O inoculante bacteriano aumentou a produtividade de colmos na variedade RB72454 de maneira similar à adubação com 120 kg ha-1 de N, nas colheitas de cana-planta e primeira soqueira; no entanto, não houve modificação na contribuição da FBN advinda da inoculação, dando evidências de que os benefícios do inoculante para a cana-de-açúcar podem ser provenientes do efeito da promoção de crescimento de plantas.

The sugarcane industry, a strategic crop in Brazil, requires technological improvements in production efficiency to increase the crop energy balance. Among the various currently studied alternatives, inoculation with diazotrophic bacteria proved to be a technology with great potential. In this context, the efficiency of a mixture of bacterial inoculant was evaluated with regard to the agronomic performance and N nutrition of sugarcane. The experiment was carried out on an experimental field of Embrapa Agrobiologia, in Seropédica, Rio de Janeiro, using a randomized block, 2 × 3 factorial design (two varieties and three treatments) with four replications, totaling 24 plots. The varieties RB867515 and RB72454 were tested in treatments consisting of: inoculation with diazotrophic bacteria, N-fertilized control with 120 kg ha-1 N and absolute control (no inoculation and no N fertilizer). The inoculum was composed of five strains of five diazotrophic species. The yield, dry matter accumulation, total N in the shoot dry matter and the contribution of N by biological fixation were evaluated, using the natural 15N abundance in non-inoculated sugarcane as reference. The bacterial inoculant increased the stalk yield of variety RB72454 similarly to fertilization with 120 kg ha-1 N in the harvests of plant-cane and first ratoon crops, however the contribution of biological N fixation was unchanged by inoculation, indicating that the benefits of the inoculant in sugarcane may have resulted from plant growth promotion.