Effect of crop residue incorporation and crop residue quality on soil N2O emissions and respiration - A laboratory measurement approach

Crop residues are a significant source for soil N₂O emissions and major component affecting the C storage in arable soils. The balance between C sequestration and N₂O emissions is delicate and depends on the type of residues and its management. Thus, residue management might be a feasible option to reduce the GHG footprint of crop production. However, the mitigation potential of residue management is highly variable and strongly affected by the crop residue quality (C and N content, C:N ratio, concentrations of lignin, cellulose and solutes), field management (incorporation depth, amount applied) as well as soil physical and soil biogeochemical properties. In the frame of the EU-ERAGAS project RESIDUEGAS, we investigated the impact of different crop residue qualities on soil respiration and reactive N fluxes as well as soil ammonium (NH₄⁺) and nitrate (NO₃^-) concentrations in order to test and possibly improve existing IPCC emission factors for GHG emissions from crop residue management.

In this study, we used sieved and homogenized soil columns of 8 cm height and 12 cm diameter filled with arable soil taken from a site near Gießen, Germany. Soil columns were incubated in the laboratory for 60 days at constant soil temperature (15°C) and water-filled pore space (60 %). Residues from nine different crops (oilseed rape, winter wheat, field pea, maize, potato, mustard, red clover, sugar beet, ryegrass) were re-wetted according to field moisture level and incorporated over approx. 0-4 cm topsoil layer one week after soil re-wetting and start of the measurements. The CO₂, N₂O (as well as NO and NH₃) fluxes were measured automatically using a dynamic chamber approach. Soil samples were additionally analyzed for soil NH₄⁺ and NO₃^- concentrations at specific time steps during the experiment.

Re-wetting of the dry soil immediately resulted in a sharp increase of soil N₂O and CO₂ emissions, which, however, was less pronounced than peak emissions following residue incorporation. Those were 4-5 times higher as compared to soil cores without residue amendment. Elevated emissions were short-lived and declined to background levels within 10 days for N₂O and within 30 days for CO₂. However, a small but significant period of higher than background N₂O emissions was observed in the second half of the incubation period, which might be directly related to the decomposition of slower decomposable organic matter such as lignin and cellulose from crop residues. Generally, the emission magnitude was strongly affected by the crop residue quality, with highest N₂O as well as CO₂ emissions being calculated for residues with a narrow C:N ratio. However, C:N ratio was not the single explaining factor. The range of calculated emission factors (fraction of cumulatively emitted N₂O-N to crop residue N input) over a 60 day period was larger than the range given by IPCC in 2006.