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Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N 2 O). Previous meta-analyses have linked various biochemical properties of crop residues to N 2 O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N 2 O emissions from specific residues. Here we combine comprehensive databases for N 2 O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N 2 O emissions. On average, crop residue incorporation increased soil N 2 O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N 2 O emissions. Crop residue effects on N 2 O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N 2 O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N 2 O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N 2 O emissions, whereas mature residues with opposite characteristics had marginal effects on N 2 O. The most important crop types belonging to the immature residue group - cover crops, grasslands and vegetables - are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N 2 O emissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

The emission of nitrous oxide (N 2 O), a strong greenhouse gas, during crop residue decomposition in the soil can offset the benefits of residue recycling. The IPCC inventory considers agricultural N 2 O emissions proportional to the amount of nitrogen (N) added by residues to soils. However, N 2 O involves several emission pathways driven directly by the form of N returned and indirectly by changes in the soil induced by decomposition. We investigated the decomposition factors related to N 2 O emissions under controlled conditions. Residues of sugar beet (SUB), wheat (WHT), rape seed (RAS), potato (POT), pea (PEA), mustard (MUS), red clover (RC), alfalfa (ALF), and miscanthus (MIS), varying by maturity at the time of collection, were incubated in two soils (GRI and SLU) at 15 °C with a water-filled pore space of 60%. The residues contained a wide proportion range of water-soluble components, components soluble in neutral detergent (SOL-NDS), hemicellulose, cellulose, and lignin. Their composition drastically influenced the dynamics of C mineralization and soil ammonium and nitrate and was correlated with N 2 O flux dynamics. The net cumulative N 2 O emitted after 60 days originated mostly from MUS (4828 ± 892 g N-N 2 O ha -1 ), SUB (2818 ± 314 g N-N 2 O ha -1 ) and RC (2567 ± 1245 g N-N 2 O ha -1 ); the other residue treatments had much lower emissions (<200 g N-N 2 O ha -1 ). For the first time N 2 O emissions could be explained only by the residue content in the SOL-NDS, according to an exponential relationship. Residues with a high SOL-NDS (>25% DM) were also non-senescent and promoted high N 2 O emissions (representing 1-5% of applied N), likely directly by nitrification and indirectly by denitrification in microbial hotspots. Crop residue quality appears to be valuable information for accurately predicting N 2 O emissions and objectively weighing their other potential benefits to agriculture and the environment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)