Graphical abstractSchematic diagram of urea dissolution, diffusion and hydrolysis in the soil. (a) Without an inhibitor, hydrolysis is fast (dark blue color) causing NH3/NH4+ accumulation and increasing the pH close to the soil surface around the fertilizer granule, driving NH3 volatilization. As the ammonia species are less mobile in soil, diffusion is limited. (b) The inhibitor maintains urea unhydrolyzed for some time. Urea has no electrical charges and diffuses easily into the soil solution. When the effect of the inhibitor phases down and urea starts to hydrolyze, both the pH and the NH3/NH4+ concentrations are lower (light blue color) as a result of dilution. Part of the urea is incorporated into the soil before hydrolysis; the NH3 produced inside the soil is retained by the negative charges of colloidal material and losses are reduced even if no rain or irrigation incorporates urea into the soil.

No abstract

Core Ideas The volatilization losses averaged 31.0% of applied N for urea and 14.8% for NBPT‐treated urea. NBPT‐treated urea showed a potential yield increase of 5.3% for major crops. The effect of NBPT in reducing volatilization losses were reduced under high N rates. NBPT had a limited effect on increasing yield in coarse‐textured soils and for NBPT rates >1060 mg kg−1. The urease inhibitor N‐(n‐butyl) thiophosphoric triamide (NBPT) slows urea hydrolysis, reduces NH3 volatilization loss, and enhances N availability to plants. Even though most studies have proved the potential of NBPT‐treated urea to reduce NH3 loss, the benefits to increase crop yield have been less consistent, mainly because N is not always the limiting factor. A meta‐analysis was carried out to evaluate the effect of soil properties (e.g., soil pH, soil texture, soil organic C [SOC]), N rate, and NBPT concentration on NH3 volatilization loss and crop yield when comparing urea with NBPT‐treated urea. Regression analysis indicated cumulative NH3 loss of 31.0 and 14.8% of applied N for urea and NBPT‐treated urea, respectively, a 52% reduction in NH3 loss by using the urease inhibitor. The use of NBPT delayed NH3 loss. It took 4.8 and 8.3 d for 50% of the total NH3 loss to occur for urea and NBPT‐treated urea, respectively. The meta‐analyses indicated that when compared with urea, NBPT‐treated urea reduced NH3 volatilization loss across all soil pH classes, soil texture classes, SOC contents, N rates, and NBPT concentrations. The meta‐analysis indicated an average crop yield increase of 5.3% for NBPT‐treated urea compared with urea. This trend was observed for all classes of soil pH, SOC content, and N rate, but yield increases were limited in coarse‐textured soils and NBPT rates >1060 mg kg−1.

Land area devoted to sugarcane (Saccharum spp.) production in Brazil has increased from 2 million to 10 million ha over the past four decades. Studies have shown that, from an environmental perspective, the transformation of nitrogen (N) fertilizers into N 2 O gases can offset the advantages gained by replacing fossil fuels with biofuels. Our objectives here were to review recent developments in N management for sugarcane-biofuel production and assess estimates of N use efficiency (NUE) and N losses based on future scenarios, as well as for life-cycle assessments of bioenergy production. Approximately 60 % of N-based fertilizer applied to sugarcane fields in Brazil is recovered by plants and soils, whereas N losses to leaching and N 2 O emissions can average 5.6 and 1.84 % of the total applied N, respectively. Maintenance of trash, rotation with N-fixing legume species, and optimization of byproducts usage have potential for reducing the N requirements of sugarcane cultivation in Brazil. Moreover, the development of sugarcane genotypes with higher NUEs, along with management systems that consider soil capacity of mineralization, is required for improving the NUE of sugarcane. Strategies to maintain N as NH 4 + in sugarcane-cropped soils also have the potential to reduce N losses and enhance NUE. The development of secondgeneration biofuels is important for increasing biofuel production while simultaneously maintaining N rates and improving NUE, and sugarcane systems in Brazil show potential for sustainable biofuel production with low N rates and limited N 2 O losses. Reducing N rates in sugarcane fields is thus necessary for improving sugarcane-based biofuel production and reducing its environmental impacts.

No abstract

The area under mechanized sugarcane (Saccharum spp.) harvesting is expanding in Brazil, increasing the return of trash to the soil. The main questions regarding this management are: (i) after adopting unburned mechanical harvesting, how long will it take to observe decreases in fertilizer requirements, (ii) what will be the magnitude of this decrease and, (iii) the impact in the short run of removing trash for energy purposes in the nutrient cycling? This study aimed to build an N prediction model for long term assessment of the contribution of sugarcane crop residues to sugarcane nutrition and to evaluate the cycling of other nutrients derived from crop residues. Keeping crop residues over the soil will increase soil N stock and N recovery by sugarcane, reaching equilibrium after 40 years with recovery of approximately 40 kg ha -1 year -1 of N. Removing trash for energy production will decrease the potential reduction in N fertilizer requirement. Of the total nutrients in the trash, 75 % of the K 2 O (81 kg ha -1 year -1 ) and 50 % of the N (31 kg ha -1 year -1 ) are in the tops, indicating the importance of maintaining tops in the soil to sustain soil fertility. Because the input data employed in the simulations are representative of the conditions in Southeast Brazil, these results might not be definitive for situations not represented in the experiments used in the study, but the model produced is useful to forecast changes that occur in the soil under different trash management.

SUMMARYFew studies on sugar cane have evaluated the root system of the crop, in spite of its importance. This is mainly due to the difficulty of evaluation and high variability of results. The objective of this study was to develop an evaluation method of the cane root system by means of probes so as to evaluate the mass, distribution and metabolically active roots related to N fertilization at planting. For this purpose, an experiment was conducted in an Arenic Kandiustults with medium texture in Jaboticabal/SP, in a randomized block design with four replications and four treatments: control (without N) and 40, 80 and 120 kg ha -1 of N applied in the form of urea in the planting furrow of the cane variety SP81 3250. One week before harvest, a urea-15 N solution was applied at the cane stalk base to detect active metabolism in the root system. Trenches of 1.5 m length and 0.6 m depth were opened between two sugar cane rows for root sampling by two methods: monoliths (0.3, 0.2 and 0.15 m wide, deep and long respectively) taken from the trench wall and by probe (internal diameter 0.055 m). For each method, 15 samples per plot were collected. The roots were separated from the soil in a sieve (2 mm mesh), oven-dried (at 65 °C) and the dry matter was measured. Root sampling by probes resulted in root mass that did not differ from the evaluation in monoliths, indicating that this evaluation method may be used for sugar cane root mass, although neither the root distribution in the soil profile nor the rhizome mass were efficiently evaluated, due to the small sample volume. Nitrogen fertilization at planting did not result in a greater root accumulation in the sugar cane plant, but caused changes in the distribution of the root system in the soil. The absence of N fertilization led to a better root distribution in the soil profile, with 50, 34 and 16 % in the 0-0.2, 0.2-0.4 and 0.4-0.6 m layers, respectively; in the fertilized treatments the roots were concentrated in the surface layer, with on average 70, 17 and 13 % for the same layers. The metabolically active roots were concentrated in the center of the cane stool, amounting to 40 % of the total root mass, regardless of N fertilization (application of 120 kg ha -1 N or without N).Index terms: active roots, root sampling methods. RESUMO: DISTRIBUIÇÃO DO SISTEMA RADICULAR DA CANA-DE-AÇÚCAR, EM FUNÇÃO DA ADUBAÇÃO NITROGENADA, AVALIADOS POR DOIS MÉTODOS: MONÓLITO E SONDAPoucos estudos com cana-de-açúcar avaliaram o sistema radicular da cultura, apesar de sua importância. Esse fato é devido, em grande parte, à dificuldade de avaliação e à elevada variabilidade dos resultados. Objetivou-se com este trabalho avaliar um método de avaliação do sistema radicular da cana, por meio de sondagem, a fim de avaliar a massa, a distribuição e as raízes metabolicamente ativas em função da adubação nitrogenada de plantio. Para isso, foi conduzido experimento em um Latossolo Vermelho distrófico típico textura média, em Jaboticabal/SP, no esquema de blocos ao acaso com quatro repetições...

A still unclear question related to sugarcane cropping refers to the low response of the planted cane to nitrogen fertilization. Two experiments were carried out in areas under a Typic Hapludox, located in Pirassununga, São Paulo State, Brazil, and an Arenic Kandiudult, located in Jaboticabal, São Paulo State, Brazil, with the objective to evaluate planted cane response to nitrogen fertilization at planting. The experimental design was organized as random blocks and treatments consisted of three N rates (40, 80, and 120 kg ha -1 Nurea) and a control without N. Nitrogen fertilizers were applied to the bottom of the planting furrow and then incorporated into the soil. During the maximum growth stage, +1 leaf samples were collected from all experimental plots to evaluate the crop nutritional status. In the Pirassununga experiment, N fertilization at planting increased N, K, Mg, and S contents in the leaves and increased the stalk yield, without effect in the technological attributes. Conversely, no stalk yield response was observed at the Jaboticabal experiment, but N fertilization benefited the stalk technological attributes. The N rates increased the sugar yield per hectare in both experiments. The highest margin of agricultural contribution was obtained at the rate of 40 kg ha -1 N. Key words: Saccharum spp., nitrogen, urea Produção de colmos e atributos tecnológicos da cana planta relacionados com a adubação nitrogenada RESUMO: Uma questão não esclarecida na cultura da cana-de-açúcar se refere à baixa resposta da cana planta à adubação nitrogenada de plantio. Nesse sentido, foram desenvolvidos dois experimentos em áreas cultivadas sobre um Latossolo Vermelho-Amarelo Distrófico (Pirassununga, SP) e um Latossolo Vermelho Distrófico (Jaboticabal, SP), com objetivo de avaliar a resposta da cana planta a adubação nitrogenada de plantio. O delineamento experimental foi em blocos ao acaso, e os tratamentos foram três doses de N (40, 80 e 120 kg ha -1 na forma de uréia) mais um tratamento controle sem adição de N. Os fertilizantes nitrogenados foram aplicados no fundo de sulco de plantio e incorporados ao solo. Durante a fase de máximo crescimento da cultura, foram coletadas amostras de folhas +1 em todas as parcelas experimentais para a determinação do estado nutricional. Em Pirassununga, a adubação nitrogenada de plantio aumentou as concentrações de N, K, Mg e S nas folhas diagnósticas e a produção de colmos, sem apresentar efeito nos atributos tecnológicos. Por outro lado, em Jaboticabal não houve resposta em produtividade, mas houve efeito nos atributos tecnológicos. Obtiveram-se incrementos na produção de açúcar por hectare em ambas as áreas em função da fertilização nitrogenada. A maior margem de contribuição agrícola foi obtida com a dose de 40 kg ha -1 de N.