N2O emissions from decomposing crop residues are strongly linked to their initial soluble fraction and early C mineralization
Lashermes, Gwenaëlle | Recous, Sylvie | Alavoine, Gonzague | Janz, Baldur | Butterbach-Bahl, Klaus | Ernfors, Maria | Laville, Patricia
The emission of nitrous oxide (N₂O), a strong greenhouse gas, during crop residue decomposition in the soil can offset the benefits of residue recycling. The IPCC inventory considers agricultural N₂O emissions proportional to the amount of nitrogen (N) added by residues to soils. However, N₂O 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 N₂O 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 N₂O flux dynamics. The net cumulative N₂O emitted after 60 days originated mostly from MUS (4828 ± 892 g N-N₂O ha⁻¹), SUB (2818 ± 314 g N-N₂O ha⁻¹) and RC (2567 ± 1245 g N-N₂O ha⁻¹); the other residue treatments had much lower emissions (<200 g N-N₂O ha⁻¹). For the first time N₂O 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 N₂O 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 N₂O emissions and objectively weighing their other potential benefits to agriculture and the environment.
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