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Report, 2021

Effects of the quality, amount and spatial distribution of arable and ley crop residues on field N2O emissions

Ernfors, Maria; Bleken, M.A.; Rittl, T.; Nadeem, S.; Hansen, S.; Jensen, Erik Steen

Abstract

We present results from a 2-year field study in southern Sweden, on annually cropped fields, and two 1-year field studies in southern Norway, on temporary grassland. Gas fluxes were measured over the winter season, from early or mid-autumn to late spring. Residues of different qualities (ley with clover, ley without clover, winter wheat and sugar beet) were studied in combinations with management options (aboveground residues removed, retained or doubled). In selected treatments, the effects of an increase in pH were investigated. The outcomes of the studies imply that sugar beet residues, ley herbage containing red clover, and possibly other “immature” residues, produce higher N2O emissions at higher residue application rates - their removal could therefore be considered as a mitigation option, although the effect may not always be large. For wheat residues, and likely also other “mature” residues, the results indicate that removing residues may increase N2O emissions, although inconsistencies and effects well beyond the initial few months call for further study. It is clear from our results that residue quality strongly affects N2O emission patterns and should not be neglected. Our results add to previous evidence that moderately cold winter conditions with frozen soil at ~0°C can elicit strong N2O production, if there is sufficient substrate for heterotrophic respiration, as in the case of temporary grassland renewal. The slightly warmer conditions, with very little snow cover, in the Swedish study, provided a contrast with relatively low winter emissions. Based on the low emission factors for ley roots observed, and the difficulties in distinguishing background from root emissions and estimating amounts of root N, one may consider an alternative approach to the current IPCC methodology, not assigning any specific emission to roots, but instead include them in an estimate of background emissions. We could demonstrate that liming a low-pH soil can be an effective N2O emission mitigation tool, even when the nitrogen source is primarily organic and nitrification is an essential step prior to denitrification.

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Publisher: ResidueGas