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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 ( 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.

International audience; Dew retting of fiber crops, such as hemp or flax, in the field after harvest promotes the microbial biodegradation of the tissues surrounding cellulosic fibers, which helps preserve the quality of fibers during their extraction and valorization for industry. This bioprocess is currently the bottleneck for plant fiber valorization because it is empirically managed and its controlling factors have not been properly quantified. A novel multiscale model representing tissue and polymer biodegradation was developed to simulate microbial growth on the stem during retting. The model was evaluated against experimental hemp retting data. It consistently simulated the mass loss of eight plant polymers belonging to two tissues of the stem outer layer, i.e., parenchyma and fiber bundles. Microbial growth was modeled by Monod equations and modulated by the functions of temperature and moisture. This work provides a tool for gaining more insights into microorganism behavior during retting under local climate conditions.

Dew retting, a selective microbial degradation of industrial hemp (Cannabis sativa L.) stems occurring after harvest under local climatic conditions, is an important field process for plant fibre uses. We investigated how the crop harvest time, which depends on the hemp valorization strategy envisaged by the farmer, affected the initial stem quality and the subsequent microbial degradation dynamics of retting. We used simulated rains and dews to carry out retting under laboratory conditions for 42 days at 15 °C with hemp stems harvested at different times (flowering and seed maturity stage). The microbial colonization and changes in the bast tissue architecture were tracked from day 0 to day 42 using colorimetry, surface infrared spectroscopy, chemical analysis, and scanning electron microscopy. The early changes in the microbial colonization and color of the stem surface and the degradation of the bast tissue parenchyma followed the same pattern during retting for the two stem qualities. However, the kinetic of these processes was 7–14 days faster for the flowering stems, which had higher initial soluble and lower lignin contents than the mature stems. The results suggest a promising potential use of colorimetry and surface infrared spectroscopy data as candidate indicators for the dew retting progress in the field. Besides the results of this study indicate that differences in hemp stem chemical composition due to different crop harvest time significantly impacted microbial colonization, biofilm formation and eventually dew retting duration.