The predicted increase in greenhouse gas emissions (GHG) and the growing demand for manufactured goods [1] promote the adoption of soil management techniques that mitigate these emissions [2] and [3] . Soils can sequester and accumulate larger quantities of carbon than plant biomass and the atmosphere [4] . For the global carbon cycle, any activity that favors the decomposition and mineralization of organic material, with consequent carbon emission, should be avoided [1] .

Numerous studies have investigated carbon residence time in soil, as in charcoal form (“biochar”) [5] and [6] . Biochar is the product obtained from pyrolysis of various biomasses. This process occurs in the absence of oxygen (anoxic environment) or at a very low level (hypoxic environment), which produces condensable gases and vapor, as well as charcoal [7] . The pyrolysis temperature alters the proportion of fulvic and humic acids in biochar [5] , concentration of nutrients, such as phosphorous and nitrogen [8] , pH and porosity [9] . Aromatic and hydrophobic structures give stability, enhancing recalcitrance, and acidic groups give reactivity [4] , making biochar useful to increase chemical, physical and biological qualities of soils. In regard of plant biomass, hemicellulose is the first to be lost in the pyrolysis process, since it degrades at 200˚C. From 240˚C to 350˚C, cellulose is degraded, followed by lignin at 280˚C a 500˚C [10] .

There is a wide choice of raw materials that generate environmental problems upon their accumulation in the fields [11] and [12] . According to [13] , agricultural soils, enteric fermentation and animal waste, are responsible for 70% of GHG emissions in AFOLU areas (Agriculture, Forestry and Other Land Use), making necessary an appropriate management of these materials. For instance, sugar cane, planted on 8.8 million hectares in Brazil, which generates, approximately, 250 million tons of straw [14] , had recent laws prohibiting straw burning, which limits the management options for this residue [15] . The straw left in the field retards sprouting and tillering, reduces productivity [16] , and also affects the growth and development of sockets [16] . Since two thirds of biomass produced by sugarcane is considered bagasse and straw [17] , biochar production is an alternative for the management of this waste [18] . Furthermore, animal residues also have a large contribution in GHG emissions [19] , and are difficult in transport and store. Increased poultry production and concerns about the waste, poses the need for an environmentally secure deposit for this residue [20] .

Since biochar has higher carbon stability than the original raw material, it is relevant to GHG mitigation [6] [10] [11] and [12] . [21] concluded that pyrolysis of wheat straw would avoid the emission of 0.9 to 1.06 t CO₂eq per ton of dry weight, if the non-pyrolysed straw was allowed to decompose in the field. [22] predicted that the use of biochar could sequester 3.7 to 6.6 Pg CO₂eq by 2050, contributing 7 to 13% reduction in GHG emissions. [23] calculated a reduction of 0.7 to 1.3 t CO₂eq per ton of miscanthus, when the waste is used on biochar production. [24] compared biochar from corn and grass straw in the USA and demonstrated a reduction of 0.885 t CO₂eq per ton of dry weight in GHG emissions. [12] considered the energy used in pyrolysis and calculated that the incorporation of biochar into the soil would reduce emissions by 2.8 to 10.2 Mt CO₂eq by 2030 and 2.9 to 10.6 Mt CO₂eq by 2050. The variation in emissions between these values is influence by the type of raw material used to produce the biochars. This author [12] observed that the highest potential for GHG emission reduction occurred with forestry residues, followed by straw from cereals and pastures; the lowest potential was biochars derived from cattle manure. [25] measured CO₂ and CH₄ emissions and did not obtain a significant difference between the untreated soil and soil amended with biochar from wheat straw; however, a significant difference in N₂O emission was observed. [26] observed an increase in CH₄ emissions of 200 mg∙m⁻² when applying 20 Mg∙ha⁻¹ of biochar from forestry residues on an unfertile tropical soil. However, [27] observed a reduction of 51.1% in CH₄ emission from a waterlogged paddy soil when applied biochar from bamboo fragments and, a reduction of 91.2%, when biochar from rice husks was applied, likely due to a reduction in methanogenic.

Under tropical soil conditions, there are a limited number of published results on biochar and its impacts on GHG emissions. Few investigations in Brazil compare different materials and rates of applied biochar. Therefore, the objective of this study was to quantify and compare GHG emissions from a tropical sandy soil, which received either different amounts of biochars from sugar cane straw and poultry manure, pyrolysed in two temperatures, or their respective non-pyrolysed materials.

About 30% of the Brazilian territory is occupied by sandy soils [28] . With proper management and fertilization, these soils are intensively cultivated and are highly productive [29] . Samples from the 0-20 cm layer of a Typic Quartzipsamment soil type (Table 1) were collected from the Anhembi region of São Paulo State (22˚43'31.1''S e 48˚01'20.2''W) under natural vegetation. The samples were dried, sieved to 2 mm size and 50 g of soil were incubated with the raw materials and the respective biochars, in different treatments.