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A B S T R A C TAgriculture can be either a source or sink of atmospheric CO 2 depending on soil management. The application of swine slurry in conventional tilled soils could enhance soil CO 2 emission depleting soil organic C stocks. However, the use of recalcitrant C-rich organic fertilizers in no-till soils can offset soil CO 2 emission promoting soil C sequestration. This hypothesis was tested by evaluating short-term CO 2 -C emissions from a Rhodic Nitisol under contrasting soil disturbance levels (disturbed (DS) and undisturbed soil (US)) top-dressed with mineral or organic fertilizers (urea (UR), raw swine slurry (RS), anaerobically digested swine slurry (ADS), and composted swine slurry (CS)). Soil CO 2 emission was evaluated for 64 days using static chambers where gas samples were collected and analysed by photoacoustic infrared spectroscopy. Soil water-filled pore space (WFPS), temperature and meteorological data were concomitantly registered and a first-order exponential decay model was used to assess the decomposition of organic fertilizers and CO 2 emissions induced by soil disturbance. Soil CO 2 -C emission was correlated with soil temperature, while limiting soil aeration impaired CO 2 -C efflux when WFPS >0.6 cm 3 cm À3 . Disturbance increased soil CO 2 -C efflux (36.3 AE 2.2 kg CO 2 -C ha À1 day À1 ) in relation to US (33.3 AE 1.6 kg CO 2 -C ha À1 day À1 ). Extra labile C input through RS amendment induced an increased soil CO 2 -C efflux for a longer period (t 1/2 = 16.9 and 9.6 days in DS and US treatments, respectively), resulting in higher CO 2 -C emissions than soil amended with other fertilizers. The recalcitrant C input by ADS and CS had limited effect on soil CO 2 -C emissions. CS presented a genuine potential for substantial soil organic C accumulation while offsetting increased CO 2 -C emissions in comparison to RS amended soils.

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Utilizando-se os registros de temperatura mínima do ar de nove estações meteorológicas situadas na região da Depressão Central do Rio Grande do Sul, calculou-se as probabilidades de ocorrerem dias com temperatura do ar ≤13°C, 15°C e 17°C. Foram considerados os períodos mensal e decendial dos meses de dezembro, janeiro, fevereiro e março, época em que essas temperaturas podem prejudicar as lavouras de arroz em floração. Também foram calculadas as probabilidades de ocorrência de dias consecutivos com temperatura igual ou menor que os níveis térmicos mencionados. Verificou-se que o número total de dias com temperatura baixa adere à distribuição Binomial Negativa enquanto que as seqüências aderem melhor à distribuição de Poisson. Verificou-se também que as probabilidades de ocorrência de dias consecutivos e do total do número de dias com temperatura do ar igual ou abaixo dos níveis térmicos estudados é menor do 2° decêndio de janeiro ao 2° decêndio de fevereiro embora possam ocorrer dias frios também nesses meses. Dessa forma, para minimizar as perdas de rendimento devido a problemas de fecundação das flores, causados pelo frio, as cultivares de arroz e as épocas de semeadura devem ser escolhidas para que a floração aconteça nesse período.

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