Pyrolysis of organic residues results in a highly stable and carbonaceous material defined as biochar [1] . Pyrolysis reaction in high temperatures and low oxygen concentration produces biochar high C content organized in aromatic and stable structures, defined as fixed C, not available for microorganisms’ degradation [2] . Particularly for wood derived biochars, this accumulation of C and release of less stable organic compounds, combined with lower feedstock macronutrient content, produces a highly and stable C containing biochar, ideal for increasing C content of soil [3] [4] . This supports the use of such biochar as a C sequestration strategy rather than a nutrient source. Biochar can contribute to the greenhouse gas (GHG) mitigation not only due to its C sequestration potential [5] but also displacing the use of fossil fuel, producing alternative energy source through pyrolysis process [6] . As a global warming mitigation strategy, application of biochar in soil also showed decreasing N₂O emissions. Evidence found in literature shows more than 14% decrease in N₂O emissions in biochar amended soil compared to soil-only [7] . However, results are inconclusive and display variations and the underlying mechanisms explaining the effect of biochar-soil interaction include biochar properties and soil biotic and abiotic conditions [8] .

Biochar produced from different feedstock type may, however, have varied concentrations of nutrients of agricultural interest. In this sense, animal manure derived biochar is shown to accumulate important elements, such as phosphorus (P), calcium (Ca) and magnesium (Mg) [9] [10] . Thus, animal manure derived biochar has higher potential to be used as a nutrient source in agricultural systems [11] . Macronutrients concentration in biochar increase during the pyrolysis process while volatile matter and water is released from biochar structure. These latter compounds are represented by organic acids, and as pyrolysis temperature increases, the release of such molecules and the accumulation of basic elements such as Ca and Mg are the drivers of high pH in biochars. These properties support the use of biochar as soil amendment, as liming agent and nutrient source [12] .

Higher soil aggregation was also observed for fine-textured soil where wood and animal derived biochar was added [5] , improving soil physical structure, aeration and moisture ratio, consequently an improved environment for root development. These mechanisms are often related to increased agricultural production; however, results vary due to biochar properties and its interaction with different environmental conditions [13] .

It is clear that the use of the biochar can vary according to its properties, which are defined as a function of the origin/type of biomass used and the variables related to the pyrolysis process, such as time and temperature. Several outcomes are observed from the interaction of biochar and soil particles [14] . These contrasting effects are caused by the various physicochemical properties of biochar combined with environmental conditions. Thus, elucidation of the effect of pyrolysis conditions and feedstock type on biochar structure and chemical properties provide basic information to support the understanding of the resultant interactions of biochar with soil. Moreover, this knowledge also enables the selection of feedstock type and production conditions according to the environmental conditions and desired amendments for particular situations.

The purpose of this study is to present potential uses for biochar in cultivated soils considering the variation on biochar agricultural properties and C sequestration potential, as an effect of pyrolysis temperature and feedstock type. In this sense, we specifically aim to 1) evaluate the effect of pyrolysis temperature and feedstock type on relevant agricultural properties and C sequestration potential of biochar and 2) investigate the effect of contrasting biochar on GHG emission applied in tropical soil from Brazil.

Selected feedstock comprised contrasting organic residues derived from agricultural production systems: poultry litter, rice hulls, sugar cane straw and sawdust.

Poultry litter (PL) was donated and collected from the poultry facility within the Department of Genetics at the University of Sao Paulo―“Luiz de Queiroz” College of Agriculture (USP-ESALQ). These poultry are part of a sustainable farming production project developed in the department, and the posture poultry are fed daily with grass. The manure sits on the ground of the facility and it is mixed with sawdust weekly. Clean rice hull (RH) was collected in the same facility where the material is used as bedding for broiler.

Sugarcane straw (SC) was collected from a commercial sugarcane field. The straw was left over the cultivated area after harvesting operation. The Department of Forestry Sciences, in the Wood Technology and Management Laboratory, at USP-ESALQ, provided sawdust (SD). Pre-treatment included drying at 45˚C for 24 h and ground to less than 1 mm particle size, followed by characterization analysis.

Prior to pyrolysis, selected feedstocks were dried at 105˚C to approximately 13% moisture (w/w) to improve the reactor efficiency. Biochars were pyrolyzed in a 60 L static reactor in N₂ saturated atmosphere with a heating rate of 10˚C·min⁻¹. The feedstock was placed individually in the reactor chamber and heated by six electrical resistances to the temperatures of 350˚C, 450˚C, 550˚C and 650˚C. Temperature was monitored by three sensors placed in the reactor, reaching its interior atmosphere close to the chamber. The reaction time varied according to each run and feedstock, and the completion was reached when the release of gases from the reactor stopped. The biochars were removed from the chamber 12 h after the reaction time was completed in order to avoid spontaneous combustion. The mass of all materials contained in the chamber reaction was determined in order to obtain biochar yield (Table 1).

Feedstocks were analyzed accordingly to the same methodologies used for biochar, concerning the determinations of pH, electrical conductivity (EC), cation exchange capacity (CEC), proximate and elemental analysis. Additionally, feedstock samples were evaluated in relation to their devolatization characteristics, through thermogravimetry analysis. Grind samples of 9 mg were placed in a crucible with N₂ gas flow with a heating rate of 10˚C·min⁻¹, from 25˚C to 900˚C (TGA-50, Shimadzu). Weight loss in respect to temperature increase was recorded.

After pyrolysis of feedstock, biochars were maintained within plastic bags tightly sealed. Prior to the analyses, air-dried biochars were ground with mortar and pestle and sieved to achieve particle size of 150 - 850 µm. Proximate and elemental analyses as well as pH and EC measurements were performed following the methods recommended by the International Biochar Initiative Guideline [15] . Measurements of pH and EC were performed in 20 ml of deionized water mixed for 90 min with 1.0 g of sample [16] . pH-meter (Digimed DM-23) and conductivity-meter (Digimed DM-32) were both previously calibrated with standard solutions. CEC was determined using 0.5 g of biochar and 1 g of feedstock. Samples were mixed with 100 ml of HCl (0.5 mol·L⁻¹) in an orbital mixer for 30 min. Samples were filtered in vacuum, while washed with 300 ml of deionized water divided in 10 aliquots of 30 ml each. The residual solution was discarded. Calcium acetate (0.5 mol·L⁻¹, pH = 7.0) was added to the solid sam-