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Residue removal for biofuel production may have unintended consequences for N 2 O emissions from soils, and it is not clear how N 2 O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field-scale N 2 O flux over 5 years (2005)(2006)(2007)(2010)(2011) from an annual crop rotation to evaluate how N 2 O emissions are influenced by no-till (NT) compared to conventional tillage (CV), and how crop residue removal (RÀ) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N 2 O flux between tillage practices at the onset of the growing season, but CT had 1.4-6.3 times higher N 2 O flux than NT overwinter. Nitrous oxide emissions were higher due to RÀ compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N 2 O emissions. The potential for higher N 2 O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event, dissolved organic C (DOC) was negatively correlated to peak N 2 O flux (r = À0.93). This suggests that lower N 2 O emissions with R+ vs. RÀ may reflect more complete stepwise denitrification to N 2 during winter and possibly relate to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter.Abbreviations CT = conventional tillage DOC = dissolved organic C MBC = microbial biomass C NT = no-till R+ = crop residue returned RÀ = crop residue removed SMN = soil mineral N.

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