ABSTRACT Global warming potential (GWP) of rice paddies depends on straw management. This study evaluated methane (CH4) and nitrous oxide (N2O) emissions and soil C stocks to determine GWP and yield-scaled GWP under different strategies and intensities of rice straw management in a subtropical climate. We hypothesized that decreasing soil management intensity and straw incorporation in the soil would reduce GWP. Methane fluxes were substantially higher during the rice growing season than in the off-season, while the opposite was observed for N2O fluxes. The cumulative emissions of CH4 during the growing season among the straw management strategies evaluated ranged from 165.8 to 586.0 kg ha-1. Annual CH4 emissions were lower when soil and straw received some type of management compared to no-tillage. Daily N2O fluxes ranged from -2.8 to 201.7 g ha-1 day-1; cumulative N2O emissions during the off-season ranged from 455.2 to 2816.5 g ha-1. During the off-season, strategies to reduce N2O emissions include post-harvest straw incorporation using a disc harrow, winter straw removal, and ryegrass cropping. Soil organic C stocks ranged from 35.96 to 38.36 Mg ha-1. Straw management using a disc harrow reduced soil organic C stocks, with more adverse effects than straw removal. Soil and rice straw management did not affect rice grain yield, with an average of 10.4 Mg ha-1. Methane emissions were the main component of GWP in all straw management systems. The contribution of N2O emissions to GWP was small and mostly (>85 %) determined by off-season emissions. Yield-scaled GWP ranged from 0.64 to 1.06 Mg CO2eq Mg-1 yield and was lower when soil and straw management systems occurred shortly after the rice harvest. Our results indicate that soil and straw management immediately after rice harvest reduces CH4 emissions, GWP, and yield-scaled GWP.

RESUMO: 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.

ABSTRACT: 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.

ABSTRACT Cover crops have numerous benefits when used in a no-till system. Understanding the processes of decomposition and N release of summer cover crops (SCC) may help select species and management to be used in cropping systems. This study aimed to evaluate C and N loss of SCC shoots. Six SCCs were evaluated: velvet bean (Mucuna aterrima), pearl millet (Pennisetum americanum), dwarf pigeon pea (Cajanus cajan), sunn hemp (Crotalaria juncea), showy rattlebox (Crotalaria spectabilis), and jack bean (Canavalia ensiformis). The experiment was conducted for two years under no-till in soil classified as Typic Hapludalf (Argissolo). The C and N remaining of crop shoots were evaluated for 140 days using litter bags and the results were fitted with a simple nonlinear regression model. Elevated air temperature and rainfall volume accelerated C and N loss in the first year. Carbon and N loss was characterized by a rapid initial phase followed by a slower one. Jack bean had the highest C loss rates, while the lowest rates were found in pearl millet. Velvet bean and pearl millet had the lowest N loss rates in the first year, together with showy rattlebox in the second year. The rates of C (kC) and N (kN) loss were positively correlated with total N and water-soluble C and N in crop shoots. During the 35 days after cover crop termination, SCCs released 35 to 137 kg ha-1 N in 2010 and 5 to 66 kg ha-1 N in 2011. Velvet bean and showy rattlebox showed more gradual N release, which may favor the synchronization between N release and N uptake by the succeeding crop. The sunn hemp was the legume species that combined higher remaining residues on the soil surface, releasing less N but preventing soil erosion.

ABSTRACT The DayCent ecosystem model, widely tested in upland agroecosystems, was recently updated to simulate waterlogged soils. We evaluated the new version in a paddy rice experiment in Southern Brazil. DayCent was used to simulate rice yield, soil organic carbon (SOC), and soil CH4 fluxes. Model calibration was conducted with a multiple-year dataset from the conventional tillage treatment, followed by a validation phase with data from the no-tillage treatment. Model performance was assessed with statistics commonly used in modeling studies: root mean square error (RMSE), model efficiency (EF), and mean difference (M). In general, DayCent slightly underestimated rice yields under no-tillage (by 0.07 Mg ha-1, or 9.2 %) and slightly overestimated soil C stocks, especially in the first years of the experiment. A comparison of observed and simulated CH4 daily fluxes showed that DayCent could simulate the general patterns of soil CH4 fluxes with slight discrepancies. Daily soil CH4 fluxes were overestimated by 0.43 kg ha-1 day-1 (12 %). Growth-season CH4 emissions under no-tillage were also somewhat overestimated (11 % or 45.29 kg ha-1). We conclude that DayCent simulated SOC, rice yield, and CH4 with some inaccuracies, but the overall performance was considered adequate. However, the model failed to represent the observed potential of no-tillage to mitigate CH4 emissions, possibly because model algorithms could not capture the actual field conditions derived from no-tillage management, such as soil redox potential, plant senescence, and surface placement of plant residue.

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.

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 A incorporação conjunta de lodo de esgoto (LE) e palha ao solo pode aumentar a mineralização do carbono da palha e a imobilização microbiana de nitrogênio. Essa hipótese foi avaliada em dois experimentos de laboratório em que o LE foi incorporado ao solo na presença e na ausência de palha de aveia. Um tratamento com somente palha incorporada e um tratamento-controle apenas com solo também foram avaliados. Durante 110 dias, foi avaliada a liberação de CO2 e o N mineral no solo, após a aplicação dos materiais orgânicos. A mineralização acumulada do C atingiu valor de 30,1 % no LE e de 54,7 % na palha. Quando esses materiais orgânicos foram incorporados juntos ao solo, a mineralização do C da palha não foi alterada. Aproximadamente 60 % do N orgânico presente no LE foi mineralizado em 110 dias. Esse índice de mineralização do N é duas vezes maior do que aquele definido pela Resolução 375/2006 do Conselho Nacional do Meio Ambiente (Conama). A incorporação combinada de LE e palha no solo provocou imobilização microbiana de N de 5,9 kg Mg-1 de palha de aveia (média de 3,5 kg Mg-1). Os resultados deste estudo indicaram que o LE não aumentou a mineralização do C da palha, porém a dose de LE deve ser ajustada para compensar a imobilização microbiana de N induzida pela palha.

ABSTRACT The combined incorporation of sewage sludge (SS) and oat straw (OS) to the soil can increase straw carbon mineralization and microbial nitrogen immobilization. This hypothesis was tested in two laboratory experiments, in which SS was incorporated in the soil with and without OS. One treatment in which only straw was incorporated and a control with only soil were also evaluated. The release of CO2 and mineral N in the soil after organic material incorporation was evaluated for 110 days. The cumulative C mineralization reached 30.1 % for SS and 54.7 % for OS. When these organic materials were incorporated together in the soil, straw C mineralization was not altered. About 60 % of organic N in the SS was mineralized after 110 days. This N mineralization index was twice as high as that defined by Resolution 375/2006 of the National Environmental Council. The combined incorporation of SS and OS in the soil caused an immobilization of microbial N of 5.9 kg Mg-1 of OS (mean 3.5 kg Mg-1). The results of this study indicated that SS did not increase straw C mineralization, but the SS rate should be adjusted to compensate for the microbial N immobilization caused by straw.

O objetivo deste trabalho foi avaliar o efeito da aplicação de doses crescentes de xisto retortado (XR) sobre características biológicas indicadoras da qualidade do solo. Foram realizados experimentos, em Argissolo Vermelho distrófico arênico, em condições de laboratório e de campo. Em laboratório, os tratamentos consistiram da aplicação ao solo de sete diferentes doses de XR (0, 300, 450, 600, 750, 1.500 e 3.000 kg ha-1). Em campo, os tratamentos foram compostos por quatro diferentes doses de XR (0, 750, 1.500, 3.000 kg ha-1) combinadas à adubação mineral recomendada, avaliados em dois cultivos de feijão (Phaseolus vulgaris L.), no sistema plantio direto. Avaliaram-se: a evolução de CO2, o carbono da biomassa microbiana (CBM), a atividade enzimática do solo e o teste de ecotoxicidade. A aplicação de doses crescentes de XR melhorou a atividade microbiológica do solo, por reduzir a emissão de CO2 sem causar variação no CBM e sem provocar impactos negativos sobre a atividade enzimática do solo. Os resultados obtidos com as enzimas em condições de campo, após duas aplicações de XR, aliados aos de CBM, do quociente metabólico (qCO2) e do teste ecotoxicológico, em condições de laboratório, indicam que o uso do XR não provoca a degradação biológica do solo.

The objective of this study was to assess the effect of increasing doses of retorted oil shale (ROS) application on the biological property indicators of soil quality. Experiments were carried out, under field and laboratory conditions, on a Hapludalf soil. In the laboratory, the treatments consisted of the application of seven different ROS rates (0, 300, 450, 600, 750, 1,500 and 3,000 kg ha-1) to the soil. In the field, the treatments were composed by four different ROS rates (0, 750, 1,500 and 3,000 kg ha-1) combined with the recommended mineral fertilizer rate, evaluated in two croppings of common bean (Phaseolus vulgaris L.), under no-till system. CO2 evolution, microbial biomass carbon (MBC), soil enzyme activity and the ecotoxicity test were evaluated. Application of increasing ROS rates improved the soil microbial activity by decreasing CO2 emission and causing no variations in MBC nor negative impacts on the soil enzymatic activity. The results obtained with the enzymes under field conditions, after two ROS applications, allied to MBC results, and to the metabolic quocient (qCO2) and to the ecotoxicity test, under laboratory conditions, show that the use of ROS cause no soil biological degradation.