Natural Nitrogen Isotope Ratios as a Potential Indicator of N2O Production Pathways in a Floodplain Fen

Assessing the environmental sustainability of different soil disinfestation methods used in solar greenhouse vegetable production systems

Overuse of fertilizers and irrigation and continuous monocropping is increasingly jeopardizing vegetable production in solar greenhouses as it causes serious soil degradation and the spread of soil-borne diseases. As a countermeasure, the practice of anaerobic soil disinfestation (ASD) has been recently introduced, which is carried out during the summer fallow period. However, ASD may increase N leaching and greenhouse gas emissions when large amounts of chicken manure are applied. This study assesses how the use of different amounts of chicken manure (CM) combined with rice shells (RS) or maize straw (MS) affects soil O₂ availability, N leaching, and greenhouse gases (GHG) emissions during and following the ASD period. Application of RS or MS alone effectively stimulated long-lasting soil anaerobiosis without major stimulating effects on N₂O emission and N leaching. Seasonal N leaching and N₂O emissions were in the ranges of 144-306 and 3-44 kg N ha^-1, respectively, and were strongly increasing with increasing rates of manure application. Combining high rates of manure application with additional incorporation of crop residues further increased N₂O emissions by 56 %-90 % as compared to the standard practice of farmers (1200 kg N ha^-1 CM). About 56 %-91 % of seasonal N₂O emissions occurred during the ASD period, whereas N leaching mainly occurred in the cropping period (75 %-100 %). Our study shows, that for priming ASD incorporation of crop residue is sufficient and that the addition of chicken manure for ASD is not needed and should be reduced or even prohibited as it does not improve yields but stimulates the emission of the strong GHG N₂O.

High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

Constructed wetlands (CW) can efficiently remove nitrogen from polluted agricultural run-off, however, a potential caveat is nitrous oxide (N₂O), a harmful greenhouse gas and stratospheric ozone depleter. During five sampling campaigns, we measured N₂O fluxes from a 0.53 ha off-stream CW treating nitrate-rich water from the intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N₂ flux using the HeO₂ incubation method. Both inlet nitrate (NO₃^-) concentrations and N₂O emission varied significantly between the seasons. In the Autumn and Winter inlet concentrations were about 11 mg NO₃^--N L^-1, and < 6.5 mg NO₃^--N L^-1 in the Spring and Summer. N₂O emission was highest in the Autumn (mean ± standard error: 9.7 ± 0.2 μg N m^-2 h^-1) and lowest in the Summer (wet period: 0.2 ± 0.3 μg N m^-2 h^-1). The CW was a very weak source of N₂O emitting 0.32 kg N₂O-N ha^-1 yr^-1 and removing around 938 kg NO₃^--N ha^-1 yr^-1, the ratio of N₂O-N emitted to NO₃^--N removed was 0.033%. The automated and manual chambers gave similar results. From the potential N₂O formation in the sediment, only 9% was emitted to the atmosphere, the average N₂ N ₂O ratio was high: 89:1 for N₂-N_potential: N₂O-N_potential and 1353:1 for N₂-N_potential: N₂O-N_emitted. These results indicate complete denitrification. The focused principal component analysis showed strong positive correlation between the gaseous N₂O fluxes and the following environmental factors: NO₃^--N concentrations in inlet water, streamflow, and nitrate reduction rate. Water temperature, TOC and DOC in the water and hydraulic residence time showed negative correlations with N₂O emissions. Shallow off-stream CWs such as Rampillon may have good nitrate removal capacity with low N₂O emissions.