Date

March 2008

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Environmental and Biomolecular Systems

Institution

Oregon Health & Science University

Abstract

Nitrate is the predominant form of nitrogen (N) used by agricultural crops, but can contaminate water resources or denitrify to the greenhouse gas nitrous oxide (N2O). Trends toward larger livestock farms and new legislation in developed countries have made N management increasingly important in manured fields. Recalcitrant organic N compounds in manure applied to agricultural fields mineralize slowly, contributing nitrate (NO3 [superscript -]) to the soil for years after application. With long-term manure application, the soil recalcitrant organic N pool may grow to the point that its annual mineralization generates significant masses of NO3 [superscript -]. Some of this recalcitrant organic N mineralization will occur during the cool season when crop N uptake is low and NO3 [superscript -] is especially prone to loss. A simple model, RONDIMS, was developed to estimate the increase of recalcitrant organic N in manured soils over time. Using input values from the literature, RONDIMS predicted that recalcitrant organic N would approach a steady-state in 25 years if all crop N was supplied by manure. RONDIMS also predicted that cool season recalcitrant organic N mineralization in manured fields could be an important contributor to leached N. In the RONDIMS simulations, cool season N mineralization was significant whether or not manure was applied at the correct rate and time. This project used an in situ methodology, Ion Exchange Resin / Soil Cores (IER/SC), to measure cool season recalcitrant organic N mineralization in manured soils from Oregon's Willamette Valley. In five of seven soils studied, cool season organic N mineralization was equal to or greater than 20 mg N kg [superscript -1] soil by early February. By 18 May, all soils tested had mineralization exceeding 20 mg N kg [superscript -1]. This project compared two methods of IER/SC: disturbed and undisturbed. Of 13 samples, three had N mineralization significantly greater (p < 0.10) in disturbed, as compared to undisturbed, cores. However, disturbed cores had less variable N mineralization, which allows the use of fewer cores in mineralization studies. Thus, this study indicates a trade-off in the two IER/SC methods: disturbed cores have lower variability but also have artificially increased N mineralization in some situations.

Identifier

doi:10.6083/M4125QK1

School

OGI School of Science and Engineering

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