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Biochar is a promising tool for an efficient and low environmental impact agriculture since can offer both soil carbon (C) sequestration and mitigation of nitrous oxide (N 2 O) emissions. The extent of biochar C stability after soil amendment and efficiency in reducing N 2 O emissions from an external nitrogen (N) source were accessed through laboratory incubations. A clay loam soil was amended with chicken manure (CM), sewage sludge (SS), eucalyptus sawdust (ES) and filter cake (FC) feedstocks and corresponding slow-pyrolysis (400 °C) biochars at 5 g C kg − 1 soil in combination with two N fertilizer rates (0 and 140 mg N kg − 1 soil). Carbon dioxide (CO 2 ) and N 2 O emissions were measured during 60 days. Biochars and feedstocks CO 2 emissions were described by an exponential first order kinetics model. For C mineralization an interaction effect was observed for feedstock source and organic amendment. Lower values of mineralizable C was found for biochars than corresponding feedstocks, except for ES. Carbon losses in 60 days of incubation totaled between 0.8 and 9.4% and 2.4 and 32% for biochars and feedstocks, respectively. Regarding to N 2 O emissions, only CM-biochar impacted emissions with a two-fold increase in non-fertilized soil. When NH 4 NO 3 was co-applied, biochars reduced fertilizer induced N 2 O emissions, reaching a seven-fold reduction in SS-biochar treatment. The fertilizer emission factor (EF) decreased with biochar amendments as well, varying between 0.01 and 0.08% of the fertilizer N emitted as N 2 O, which shows the biochar potential to reduce fertilizer induced N 2 O emissions, with major reduction by SS-biochar mitigating 87% of the soil-fertilizer emissions. Such potential could be explored by designing biochars based on feedstock chemical and structural properties, including a mixed feedstock source biochar that promotes C sequestration and mitigates N 2 O emissions. [ABSTRACT FROM AUTHOR]

• BC-based fertilizers with 51% of BC had 21% higher maize yield than urea. • Grain N was 16% higher with BC-based fertilizers than with mineral sources. • BC-based fertilizers replacing urea result in more favorable carbon balances. • BN 51/10 use promoted soil C sequestration and helped mitigate net greenhouse gases. The sustainable development of agriculture depends on increasing N use efficiency (NUE) and consequently reducing N losses from different sources, such as NH 3 volatilization, NO 3 − leaching, and N 2 O emissions. While the chemical and physical properties of biochar (BC) in fertilizers have been evaluated to increase NUE, a lack of information exists regarding the effects of BC amendments in tropical soils. We performed a one-year field experiment with tropical soil to evaluate the effects of BC-based N fertilizers (BN) on maize yield and on greenhouse gas (GHG) emissions. The treatments consisted of five fertilizers: ammonium nitrate (AN), urea (U), BN 51/10 (51% BC, 10% N), BN 40/17 (40% BC, 17% N), BN 29/20 (29% BC, 20% N), and a control (without N fertilizer). The N fertilizers (80 kg N ha−1) were broadcast 20 days after sowing. Yield, grain N uptake, NUE, ammonia volatilization, and GHG emissions were measured. The results demonstrated the potential of BNs to enhance the efficiency of the fertilizers. BN 51/10 and BN 40/17 had an average maize yield that was 26% higher than that of U, and BN 51/10 resulted in a NUE that was 12% higher than what was observed for U. Both the effects on yield and NUE were attributed to lower N release rates of the BN-amended fertilizers compared to that of the conventional soluble N sources. The BC-based fertilizers presented better environmental performance, and BN 51/10 showed the lowest emission intensity when C sequestration by BC was not considered, with a value that was 14% lower than that of the U treatment. When considering C sequestration by BC, the emission intensity of the C equivalents demonstrated that all BNs presented C sequestration that differed from that of the mineral N sources. BC-based nitrogen fertilizers may have promising applications for sustainable agricultural development by mitigating N losses and increasing C stocks. [ABSTRACT FROM AUTHOR]