Cover crops in rotation or intercropped with annual crops are important strategies to increase C and N input in agricultural soils. However, these practices may also enhance soil N 2 O emissions. The effect on N 2 O emissions may be dependent upon the biochemical composition of cover crop residues. A 47-d incubation study was conducted to determine soil N 2 O emissions following the addition of residues from three summer legume species [pigeon pea (Cajanus cajan L. Millsp.), cowpea (Vigna unguiculata L. Walp.), lablab bean (Lablab purpureus)], one winter legume [vetch (Vicia sativa L.)], one winter monocotyledon [black oat (Avena strigosa Schreb.)], and maize (Zea mays L.) under two water-filled pore space levels (40 and 70% WFPS). Short-term peaks of N 2 O fluxes were observed after the addition of all crop residues, but were much greater under 70 than 40% WFPS (5.2 and 5133 μg N-N 2 O kg -1 soil for 40 and 70% WFPS, respectively). Under both WFPS, significantly higher peaks were detected after the application of N-rich legume residues (7.7 and 3,356 μg N-N 2 O kg -1 soil under 40 and 70% WFPS, respectively) than after the application of grass residues (2.8 and 1,777 μg N-N 2 O kg -1 soil under 40 and 70% WFPS, respectively). Cumulative soil N 2 O produced under 70% WFPS was approx. 110 times greater than under 40% WFPS. Soil N 2 O emissions increased linearly as residue N content increased. Soil N 2 O emissions also increased linearly as the content of the recalcitrant compounds lignin and polyphenols increased, because residues with high N content also had high lignin and polyphenols content. When the content of C and recalcitrant compounds were expressed on an N basis, soil N 2 O emission decreased linearly as residue C/N, lignin/N, polyphenol/N, and (lignin + polyphenols)/N ratios increased. Indices that include recalcitrant compounds on N basis may be useful for the selection of cover-crop species with the least impact on soil N 2 O emissions. However, our results show that these ratio indices had similar correlation coefficients compared to C/N ratio, indicating that C/N ratio is an efficient index to predict soil N 2 O emission following cover crop application.

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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 CO 2 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.

Resumo -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 CO 2 , 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 CO 2 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 (qCO 2 ) e do teste ecotoxicológico, em condições de laboratório, indicam que o uso do XR não provoca a degradação biológica do solo.Termos para indexação: biomassa microbiana, ecotoxicidade, enzimas do solo, mineralização do carbono, qualidade do solo. Microbial and enzymatic activities in the soil after application of retorted oil shaleAbstract -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. CO 2 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 CO 2 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 (qCO 2 ) and to the ecotoxicity test, under laboratory conditions, show that the use of ROS cause no soil biological degradation.