Biofuel production from crop residues is widely recognized as an essential component of developing a bioeconomy, but the removal of crop residues still raises many questions about the sustainability of the cropping system. Therefore, this study reviews the sustainability effects of crop residues removal for biofuel production in terms of crop production, soil health and greenhouse gas emissions. Most studies found little evidence that residue management had long-term impacts on grain yield unless the available water is limited. In years when water was not limiting, corn and wheat removal rates ≥90% produced similar or greater grain yield than no removal in most studies. Conversely, when water was limiting, corn grain yield decreased up to 21% with stover removal ≥90% in some studies. Changes in soil organic fractions and nutrients depended largely on the amount of residue returned, soil depth and texture, slope and tillage. Reductions in organic fractions occurred primarily with complete stover removal, in the top 15-30 cm in fine-textured soils. Soil erosion, water runoff and leaching of nutrients such as total nitrogen (N) and extractable soil potassium decreased when no more than 30% of crop residues were removed. Stover management effects on soil bulk density varied considerably depending on soil layer, and residue and tillage management, with removal rates of less than 50% helping to maintain the soil aggregate stability. Reductions in CO 2 and N 2 O fluxes typically occurred following complete residue removal. The use of wheat straw typically increased CH 4 emissions, and above or equal to 8 Mg/ha wheat straw led to the largest CO 2 and N 2 O emissions, regardless of N rates. Before using crop residues for biofuel production, it should therefore always be checked whether neutral to positive sustainability effects can be maintained under the site-specific conditions.

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The concentration of greenhouse gases (GHGs) in the atmosphere has been increasing since the beginning of the industrial revolution. Nitrous oxide (N2O) is one of the mightiest GHGs, and agriculture is one of the main sources of N2O emissions. In this paper, we reviewed the mechanisms triggering N2O emissions and the role of agricultural practices in their mitigation. The amount of N2O produced from the soil through the combined processes of nitrification and denitrification is profoundly influenced by temperature, moisture, carbon, nitrogen and oxygen contents. These factors can be manipulated to a significant extent through field management practices, influencing N2O emission. The relationships between N2O occurrence and factors regulating it are an important premise for devising mitigation strategies. Here, we evaluated various options in the literature and found that N2O emissions can be effectively reduced by intervening on time and through the method of N supply (30–40%, with peaks up to 80%), tillage and irrigation practices (both in non-univocal way), use of amendments, such as biochar and lime (up to 80%), use of slow-release fertilizers and/or nitrification inhibitors (up to 50%), plant treatment with arbuscular mycorrhizal fungi (up to 75%), appropriate crop rotations and schemes (up to 50%), and integrated nutrient management (in a non-univocal way). In conclusion, acting on N supply (fertilizer type, dose, time, method, etc.) is the most straightforward way to achieve significant N2O reductions without compromising crop yields. However, tuning the rest of crop management (tillage, irrigation, rotation, etc.) to principles of good agricultural practices is also advisable, as it can fetch significant N2O abatement vs. the risk of unexpected rise, which can be incurred by unwary management.

Currently, biochars are produced from a wide range of feedstocks with a broad diversity in physicochemical characteristics. Therefore, a diverse agronomic response of crop plants to biochars application was expected. A preliminary ecotoxicological assessment is necessary before application of biochar to soil, even though biochar is a recalcitrant carbon considered as a promising soil amendment because of its ability to climate change mitigation by sequestration of carbon in the soil. Thus, a Petri dish germination test was conducted to assess the effects of six walnut shell biochar rates (i.e., 0, 10, 20, 40, 80, and 120 Mg ha−1) on seed germination and early growth of seedlings of fodder crops (Triticale cultivar X Triticasecale Wittmack and Pisum sativum sp. arvense L. varieties Taşkent and Özkaynak). A simple Petri dish bioassay method used to determine the effect of biochar rates on seed germination. Germination rate decreased with both higher and lower rate of biochar application. Results showed that the germination rate and growth indices were dependent on plant species. The seed germination rate of all three species was stimulated at the 40 Mg ha−1 rate, but Taşkent mung bean occurred at the highest rate of 120 Mg ha−1. Significantly higher germination rate and growth indices observed with the 40 and 80 Mg ha−1 biochar rates, respectively. Biochar application generally increased seed germination at rates ≤ 40 Mg ha−1 and seedling growth indices at rates ≤ 80 Mg ha−1. This rapid test can be used as the first indicator of biochar effects on seed germination rate and early growth of seedlings. Farmers could use this test before investing in biochar application.

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