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Understanding the production pathways of potent greenhouse gases, such as nitrous oxide (N2O), is essential for accurate flux prediction and for developing effective adaptation and mitigation strategies in response to climate change. Yet there remain surprising gaps in our understanding and precise quantification of the underlying production pathways – such as the relationship between soil moisture and N2O production pathways. A powerful, but arguably underutilized, approach for quantifying the relative contribution of nitrification and denitrification to N2O production involves determining 15N2O isotopomers and 15N site preference (SP) via spectroscopic techniques. Using one such technique, we conducted a short-term incubation where N2O production and 15N2O isotopomers were measured 24 h after soil moisture treatments of 40 % to 105 % water-filled pore space (WFPS) were established for each of three soils that differed in nutrient levels, organic matter, and texture. Relatively low N2O fluxes and high SP values indicted nitrification during dry soil conditions, whereas at higher soil moisture, peak N2O emissions coincided with a sharp decline in SP, indicating denitrification. This pattern supports the classic N2O production curves from nitrification and denitrification as inferred by earlier research; however, our isotopomer data enabled the quantification of source partitioning for either pathway. At soil moisture levels < 53 % WFPS, the fraction of N2O attributed to nitrification (FN) predominated but thereafter decreased rapidly with increasing soil moisture (x), according to FN=3.19-0.041x , until a WFPS of 78 % was reached. Simultaneously, from WFPS of 53 % to 78 %, the fraction of N2O that was attributed to denitrification (FD) was modelled as FD=-2.19+0.041x ; at moisture levels of > 78 %, denitrification completely dominated. Clearly, the soil moisture level during transition is a key regulator of N2O production pathways. The presented equations may be helpful for other researchers in estimating N2O source partitioning when soil moisture falls within the transition from nitrification to denitrification. [ABSTRACT FROM AUTHOR]

Residue removal for biofuel production may have unintended consequences for N2O emissions from soils, and it is not clear how N2O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field-scale N2O flux over 5 years (2005-2007, 2010-2011) from an annual crop rotation to evaluate how N2O emissions are influenced by no-till ( NT) compared to conventional tillage ( CV), and how crop residue removal (R−) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N2O flux between tillage practices at the onset of the growing season, but CT had 1.4-6.3 times higher N2O flux than NT overwinter. Nitrous oxide emissions were higher due to R− 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 N2O emissions. The potential for higher N2O 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 N2O flux ( r = −0.93). This suggests that lower N2O emissions with R+ vs. R− may reflect more complete stepwise denitrification to N2 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. [ABSTRACT FROM AUTHOR]

After cole crop harvest, over 400 kg N ha may remain in the field as crop residues and soil mineral N. Thus, methods to reduce potential post-harvest N losses are needed. Urea with 5 % N excess was incorporated in mini-plots to produce N enriched broccoli ( Brassica olecerea var italica L.). The fate of above-ground crop residue-derived N (N) and below-ground residual fertilizer or root biomass N (N) were studied from broccoli harvest (Aug and Sept 2011) to spring wheat ( Triticum durum L.) harvest (July 2012), with and without an amendment of used cooking oil. The N remained mostly as organic N, was not influenced by the amendment, and was resistant to post-harvest losses. With the oil amendment, soil mineral N was reduced by 19 kg ha and microbial biomass N was increased by 21 kg ha 2 weeks after broccoli harvest, indicating immobilization of N and reduced potential N losses. At spring wheat harvest, amended soil had greater total, organic, and mineral N compared to the unamended control, by 44, 43, and 0.75 kg ha, respectively. The amendment increased the recovery of N in the soil total N pool by 209 % at spring wheat harvest, and it did not affect spring wheat yields or plant N content. It is possible that the amendment facilitated the incorporation of N into organic compounds, which were less susceptible to losses. Growers should consider applying used cooking oil at harvest to minimize potential N losses and to increase the soil organic N fraction. [ABSTRACT FROM AUTHOR]