The sugar and bioethanol industry generate large amounts of filter cake and vinasse, residues that are applied to sugarcane fields as conditioners and organic fertilizers. However, these may be significant sources of greenhouse gases emissions to the atmosphere. This study assessed the impact of sugarcane straw biochar on the emissions of CO
2, CH
4and N
2O promoted by filter cake and vinasse applied to soil, and its effects on the chemical properties and bacterial communities of a Typic Hapludox and a Quartzipsamment. A laboratory incubation was conducted for 100 days with both soils under five treatments: vinasse and filter cake amendment (FV), plus biochar at 10 (FV + B10), 20 (FV + B20) and 50 (FV + B50) Mg·ha
-1, and a control. Soil pH, available P and exchangeable base contents increased with biochar added to sandy soil. Mineral N decreased with biochar addition to both soils. The FV treatment increased CO
2 emissions by 5-fold and 2.4-fold in sandy and clayey soils, respectively, compared to the control. Moreover, FV +B10 increased CO
2 emissions by 4% and 6.4% in sandy and clayey soils, respectively, compared to FV. Cumulative N
2O emissions in FV were 537% and 125% higher in sandy and clayey soils, respectively, compared to the control. Nevertheless, increasing biochar amendment rates reduced N
2O emissions from 24% to 34% in sandy soil, and from 14% to 56% in clayey soil. CH4 emissions were negligible. The effects of filter, vinasse and biochar amendments on soil amelioration were closely related to its buffering capacity. Temporal changes on bacterial community structure were more pronounced in the sandy soil compared to clayey, and indicated that N
2O emission mitigation in clayey soil was directly related to biotic mechanisms, while abiotic mechanisms caused by biochar played a more important role in mitigating N
2O emissions in sandy soil.
The intensification of green cane harvesting has led to a greater deposition of leaves and tips on soil surface, ranging between 10 and 20 Mg∙ha−1 of dry matter, and the amount of sugarcane crop residues generated in Brazil is estimated in 175 million Mg∙yr−1 [1] . Against the claiming demand to use this biomass for bioenergy generation, the Brazilian sugarcane sector has considered the partial removal of the post-harvest residues from soil surface without harming sustainability and yields [2] . On the other hand, the sugar and bioethanol industry generate large amounts of filter cake and vinasse, residues that are applied to sugarcane fields as conditioners and organic fertilizers [3] .
Vinasse is an acidic (pH ≈ 4.5) nutrient-dense effluent that is produced at a rate of approximately 13 L for every liter of ethanol. Filter cake is a nutrient-rich solid residue from the filtration of sugarcane juice, produced in an average of 8 kg per ton of processed sugarcane [4] . Despite vinasse and filter cake benefits to conservation agriculture [3] , these residues may be significant sources of greenhouse gases (GHG), mainly nitrous oxide (N2O) and methane (CH4) [5] [6] [7] .
In this context, one of the proposed means to reduce GHG emissions in agriculture is through the use of biochar (charcoal derived from the pyrolysis of biomass). Despite the benefits of biochar applications to soil [8] - [16] , studies regarding its combination with other organic residues are still limited. Positive interactions between biochar and organic residues can be expected due to the biological activation of biochar and reduced organic fertilizer mineralization, leading to synergisms between biochar and organic residues [17] .
According to [18] , the combination of biochar with poultry manure reduced N losses by volatilization and produced high quality composts. [17] showed that biochar addition to soil in combination with organic fertilizer can stabilize com- post-derived organic matter (OM) and increase soil C sequestration, as well as improve soil fertility over the sole biochar or organic/mineral fertilizer application. Biochar-amended soils have also shown to reduce CO2emissions [19] in response to vinasse application.
Under the current scenario of climate change, the combination of biochar with organic residues may be an approach to improve nutrient cycling and to fulfill non-agronomic purposes, such as reduction of GHG emissions. The aim of this study was to assess the effects of applying sugarcane straw biochar combined with vinasse and filter cake on the emissions of CO2, CH4 and N2O, chemical properties and bacterial community composition of two contrasting soils (i.e. clayey and sandy tropical soils). It was hypothesized that: i) The effects of biochar amendments on soil amelioration is closely related to soil buffering capacity; ii) biochar suppresses GHG emissions from filter cake and vinasse applied to soils as a function of its application rate; and iii) soil-biochar interactions cause temporal changes in bacterial communities both directly and indirectly, affecting niche-microbe interactions related to N2O emission mitigation. For testing these hypotheses an incubation experiment was conducted under controlled environmental conditions (i.e., temperature and moisture), with and without application of vinasse and filter cake combined with addition of biochar at different rates in two contrasting forest soils.
2. Materials and Methods2.1. Biochar Production and Characterization
The feedstock for biochar production was straw collected from a sugarcane field within a mill located in Piracicaba, State of Sao Paulo, Brazil. A recently harvested area (i.e., 7 days after unburned mechanized harvesting) was selected since it presented a largeamount of fresh post-harvest residues on soil surface (≈ 10 Mg∙ha−1 of dry matter).
Before pyrolysis, the straw particles were cut into fragments of 5 ± 1 cm. Then, the reactor was cleaned under heating with air injection in order to remove impurities prior to allocation of the raw material. Approximately 3 kg of feedstock was manually placed into the sample port of the reactor, which consisted of 300- × 2400-cm steel cylinder (diameter × length) closed on one end with a circular steel plate.
The pyrolysis process was carried out under N2atmosphere, with a final temperature of 450˚C (∆ ≈ 20˚C) and heating rate of 10˚C∙min−1 for a retention time of 2 hours. The condensable gases were recovered on the other end of the reactor as a liquid (i.e. bio-oil). Non-condensable gases were exhausted to a water tank outside the processing unit to avoid their direct release to the atmosphere.
After completion of pyrolysis, the sample presented homogeneous carbonization and a volume reduction of 30% to 40%. The pyrolysis process yielded 30% of biochar, 40% of liquids (bio-oil) and 30% of gas, which is within the range observed in most studies for slow pyrolysis [20] . Chemical properties of the feedstock and final biochar are presented in Table 1.
2.2. Soils and Organic Residues
Two soils with contrasting texture, a Quartzipsamment (sandy) and a Typic Hapludox (clayey), were collected from two different native forest areas located, respectively, from near Anhembi town, State of Sao Paulo, Brazil (22˚43'31.1''S; 48˚01'20.2''W) and within the ESALQ campus (22˚42'05.1''S; 47˚37'45.2''W), Piracicaba, respectively; both located at the State of Sao Paulo, Brazil. Native vege-
Characterization of the feedstock (sugarcane straw) for biochar production, biochar, filter cake and vinasse used in the study
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