CO2 Emission in No-till Area after Sewage Sludge Application.

The experiment has as main aim to assess the CO₂ emissions resulting from application of sewage sludge from a long-time in area under no-till, in Botucatu, State of São Paulo, Brazil. Two types of sewage sludge (centrifuged and biodigested) and four rates applied on soil surface (0, 2, 4 and 8 Mg ha^-1) have been evaluated, with four replicates. The experiment has been carried out since 2002 and the sewage sludge is applied every 2 years. The last application of sewage sludge was performed in September 2013. The soybean was sowed in October 2013 and CO₂ emission from soil was measured at 1, 3, 8, 15, 30, 60 and 120 days after sowing (DAS), using an Automated Flux CO₂ Soil System, model LI-8100A. At the same dates, soil moisture and temperature were measured. The type of sludge affected the CO₂ emission only at 3 DAS, when the higher CO₂ flux to the atmosphere occurred in the centrifuged sludge plots (1.81 µmol m^-2 s^-1). In the others evaluations no difference between sludges was observed. The highest carbon flux to the atmosphere occurred to 60 DAS (19.3 µmol m^-2 s^-1), and might be related to the decomposition rate of the sludges. However, regardless of the sludge, the rate applied has affected the magnitude of the CO₂ emission at 1, 3, 15 and 60 DAS. At 1, 3 and 15 DAS, the greater values of CO₂ flux occurred with the rate of 8 Mg ha^-1. At 60 DAS, the behavior of CO₂ emission was quadratic and the highest flux occurred with the calculated rate of 5.06 Mg ha^-1. There are several factors that can affect the CO₂ emission from soil. In our case, the soil moisture seems to have been decisive, since it has shown a strong and significant correlation with CO₂ emission (r = 0.74**). The soil temperature has shown a weak and non-significant correlation (r = -0.22) with soil CO₂ emission and it seems to have no relation with the results.