Shcherbak, I. 2013. Production and movement of N2O in the full soil profile. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Nitrous oxide (N2O) is a major greenhouse gas and cultivated soils are the dominant anthropogenic source. In this dissertation, I examine some aspects of N2O where knowledge is lacking: diffusion of N2O through the soil profile; production of N2O in soils below the A or Ap horizon in relation to irrigation, tillage, and fertilization; and patterns of N2O response to nitrogen (N) fertilizer rate.

In Chapter 2,I measure diffusion by comparing single and inter-port diffusivity determinations using sparse sampling after sulfur hexafluoride (SF6) and N2O tracer injections at KBS in SW Michigan. In general, the sparse method provided accurate measurements of soil diffusivity. Injection port diffusivities of SF6and N2O had poorer agreement in the summer (r2=0.49) than in the fall (r2=0.96),likely due to less uniform soil moisture in summer. The low N2O to SF6diffusivity ratio (0.67 compared to 1.82 in free air) suggests that water solubility of N2O plays a significant role in retarding its movement in the soil. Movement of SF6is not obscured by dissolution in water, making SF6a superior tracer compared to N2O. Results show it is possible to estimate N2O diffusivity with sparse measurements; accuracy can be improved with knowledge of soil moisture and texture in the immediate vicinity of the ports.

In Chapter 3,I estimate the influence of crop and management practices on subsoil N2O production in intensively managed cropping systems in a series of experiments also at KBS. N2O concentrations showed a saturating increase with depth except immediately after fertilization and in the winter when concentrations were highest in the surface horizon. Variability of N2O concentrations declined with depth, in agreement with more constant soil conditions. Total N2O fluxes from direct measurements and estimates by the concentration gradient method showed good agreement, with correlations ranging from 0.4-0.7. N2O production in subsoil horizons is significant, with over 50% of total N2O produced in moderately fertilized rainfed treatments. In highly fertilized sites where added N exceeded plant N requirements only a small fraction of total N2Owas produced in lower horizons. Dry conditions deepened the maximum N2O production depth. Results show that the fraction of total N2O produced in subsoil is controlled by the N and moisture status of the soil profile and not by tillage.

Knowledge of a more precise fertilizer N2O emission response could improve global and regional N2O assessments and help to design more efficient mitigation strategies. Evidence now suggests that the emission response is not linear, as assumed by IPCC methodologies, but rather exponentially increases with fertilization. In Chapter 4, I performed a meta-analysis to test the generalizability of these findings. I selected published studies with at least three N fertilizer rates otherwise identical. From 78 available studies (231 site-years),I calculated the change in N2O emission factors (ΔEFs) as the change in the percentage of applied N converted to N2O emissions. I found that ΔEF grew with N additions for synthetic fertilizers, for a majority of the crop types examined, and for soils with high organic carbon content, low mean annual temperatures, or low pH. Nitrogen-fixing crops had a significantly higher ΔEF than non-fixing crops. My results suggest a general trend of exponentially increasing N2O emissions as N fertilizer rates increase to exceed crop N needs. Use of this knowledge in global and regional greenhouse gas inventories could provide a more accurate assessment of fertilizer-derived N2O emissions and help further close the global N2O cycle.

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