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Sugarcane straw removal for bioenergy production will increase substantially in the next years, but this may deplete soil organic carbon (SOC) and exacerbate greenhouse gas (GHG) emissions. These aspects are not consistently approached in bioenergy life cycle assessment (LCA). Using SOC modeling and LCA approach, this study addressed the life cycle GHG balance from sugarcane agroindustry in different scenarios of straw removal, considering the potential SOC changes associated with straw management in sugarcane-cultivated soils in Brazil. Long-term simulations showed SOC losses of up to -0.5 Mg ha-1 yr-1 upon complete straw removal, whereas the moderate removal had little effects on SOC and the maintenance of all straw in the field increased SOC accumulation by up to 0.4 Mg ha-1 yr-1. Our analysis suggests that accounting for SOC changes in LCA calculations could lower the net GHG benefits of straw-derived bioenergy, whose emissions intensity varied according to soil type. Overall, SOC depletion induced by complete straw removal increased the life cycle GHG emissions of straw-derived bioenergy by 26 % (3.9 g CO2eq MJ-1) compared to a scenario without taking SOC changes into account. Straw removal for cellulosic ethanol could be effective for mitigating GHG emissions relative to gasoline, but it was not advantageous for bioelectricity generation depending on the energy sources that are displaced. Therefore, straw-induced change of SOC stocks is a critical factor to model life cycle GHG emissions of straw-derived bioenergy.

RESUMEN

Crop residues decomposition are controlled by chemical tissue components. This study evaluated changes on plant tissue components, separated by the Van Soest partitioning method, during cover crop decomposition. The Van Soest soluble fraction was the first to be released from the crop residues, followed by cellulose and hemicellulose. Lignin was the crop residue component that suffered the least degradation, and for certain crop residue types, lignin degradation was not detected. The degradation of the main components of crop residues (soluble fraction, cellulose, hemicellulose and lignin) is determined by the chemical and structural composition of each fraction.

A decomposição de resíduos culturais é controlada pela composição química do tecido vegetal. O objetivo deste estudo foi avaliar as alterações que ocorrem nos componentes do tecido vegetal, separados pelo fracionamento de Van Soest, durante a decomposição de plantas de cobertura. A fração solúvel foi a primeira a ser liberada dos resíduos culturais, seguida pela celulose e hemicelulose. A lignina foi o componente dos resíduos culturais de menor degradação, sendo que em alguns resíduos culturais não foi possível detectar a degradação deste componente. A degradação dos principais componentes dos resíduos culturais (fração solúvel, celulose, hemicelulose e lignina) é determinada pela composição química e estrutural de cada uma destas frações.

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Non-legume bioenergy crops can be fertilized with animal manures instead of mineral fertilizers, but the simultaneous application of carbon (C) and nitrogen (N) with manures can increase nitrous oxide (N2O) emissions. On the other hand, manure could increase soil organic C stocks and partly offset greenhouse gas (GHG) emissions and global warming potential (GWP) of crop systems. We performed a two-year study in a biofuel cropping system with sunflower and canola to examine the effects of manure fertilization on grain yields and N use efficiency of crops, and on GWP and GHG intensity (GHGI) in no-till soils under subtropical conditions. The GWP and GHGI were calculated from measured methane (CH4) and N2O emissions and soil organic C stock change, and from estimated carbon dioxide emissions associated with agricultural inputs and farm operations. The following treatments were tested: (i) mineral fertilizer (MF); (ii) poultry manure (PM); (iii) pig deep-litter (PDL); and (iv) no-N control. The application rate of each treatment was adjusted to provide 60 kg available N ha-1 to crops. Grain yield and N accumulated by sunflower and canola were greater in fertilized treatments than in the control, and did not differ among N sources. However, crop N use efficiency was on average 50% lower with manures than MF. CH4 emissions were not affected by N sources, but N2O emissions increased as follows: control (1.37) < MF (2.04) < PDL (4.12) < PM (4.95 kg N ha-1). On the other hand, soil organic C stocks increased more rapidly with manures than MF, resulting in significantly lower GWP and GHGI with manures than MF after two years. These results indicate that animal manures can replace MF as the main source of N to non-legume oil crops and reduce net GHG emissions in biofuel cropping systems under subtropical conditions.

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ABSTRACT: This research aimed to evaluate the mineralization of nitrogen (N) and to define the efficiency index (EI) of N after the addition of organic fertilizers based on cattle manure on the soil under laboratory conditions. A completely randomized statistical design with four replicates was used. The treatments were set as follows: T1) Soil (control); T2) Soil + vermicompost of cattle manure (CMV); T3) Soil + cattle manure and straw compost (CMS); and T4) Soil + cattle manure (CM). Experimental units were constituted by acrylic flasks with 5 x 5cm (height x diameter). Each flask was added with 135g of wet soil and 2.20, 2.45 and 2.27g of CMV, CMS and CM, respectively. Treatments were incubated at 25°C and the amount of mineral N from the soil (N-NH4+ and N-NO2- + N-NO3-) was determined at the beginning of the experiment and after 7, 14, 28, 56 and 112 days of incubation. The highest concentration of N-NO2- + N-NO3- in the soil were observed within the CMS treatment. The EI of N was of 27, 23 and 22% for CMS, CMV and CM, respectively. The mineralization of N from organic fertilizers based on cattle manure occurs on its vast majority within the first 28 days after its addition to the soil. The EI of N from the organic fertilizers based of cattle manure was higher for CMS>CMV>CM and achieved only 80% of what expected for organic fertilizers derived from cattle manure.

RESUMO: Este trabalho teve como objetivo avaliar em condição de laboratório a mineralização do nitrogênio (N) e determinar o índice de eficiência (EI) do N após adição de fertilizantes orgânicos a base de esterco bovino no solo. Foi utilizado o delineamento inteiramente casualizado com quatro repetições. Os tratamentos avaliados foram: T1) Solo (testemunha); T2) Solo + vermicomposto de esterco bovino (CMV); T3) Solo + composto de palha e esterco bovino (CMS); e T4) Solo + esterco bovino (CM). As unidades experimentais foram frascos de acrílico com 5cm de altura e 5cm de diâmetro. Em cada frasco, foram adicionados 135g de solo úmido e 2,20, 2,45 e 2,27g de CMV, CMS e CM, respectivamente. Os tratamentos foram acondicionados em incubadora a 25°C e os teores de N mineral do solo (N-NH4+ e N-NO2- + N-NO3-) foram determinados na data de instalação, aos 7, 14, 28, 56 e 112 dias de incubação. Os maiores teores de N-NO2- + N-NO3- no solo foram observados no tratamento CMS. O EI do N foi de 27, 23 e 22% para CMS, CM e CMV, respectivamente. A mineralização do N dos fertilizantes orgânicos a base do esterco bovino ocorre em sua grande maioria nos primeiros 28 dias após sua adição ao solo. O EI do N dos fertilizantes orgânicos a base de esterco bovino foi maior para CMS>CMV>CM e atingem apenas 80% do esperado para fertilizantes orgânicos oriundos de esterco bovino.