We measured atmospheric nutrient deposition as wet deposition and dry deposition to dry and wet surfaces. Our analyses offer estimates of atmospheric transport of nitrogen (N), phosphorus (P) and silicon (Si) in an agricultural region. Annual dry and wet deposition (ha −1 year −1 ) was 0.3 kg of P, 7.7 kg of N, and 6.1 kg of Si; lower than or similar to values seen in other landscapes. N:P and Si:N imply that atmospheric deposition enhances P and Si limitation. Most P and soluble reactive P (SRP) deposition occurred as dryfall and most dry-deposited P was SRP so would be more readily assimilable by plant life than rainfall P. Dry deposition of N to wet surfaces was several times greater than to dry surfaces, suggesting that ammonia (NH x ) gas absorbtion by water associated with wet surfaces is an important N transport mechanism. Deposition of all nutrients peaked when agricultural planting and fertilization were active; ratios of NH x :nitrate (NO x ) reflected the predominant use of NH x fertilizer. Wet deposition estimates were consistent over hundreds of km, but dry deposition estimates were influenced by animal confinements and construction. Precipitation wash-out of atmospheric nutrients was substantial but larger rain events yielded higher rates of wet deposition. Methodological results showed that local dust contaminated wet deposition more than dry; insects, bird droppings and leaves may have biased past deposition estimates; and estimating dry deposition to dry plastic buckets may underestimate annual deposition of N, especially NH x .

Atmospheric nutrient (nitrogen, phosphorus, and silica) loading and transport through precipitation and dry deposition is one of the least understood and may be one of the most important pathways of nutrient transport in agricultural landscapes. The purpose of this project was to fill three essential information gaps: (I) to characterize both nitrogen and phosphorus deposition, (II) to determine contributions of wet deposition (W) and drydeposition to dry-and wet-surfaces (DD and DW, respectively), and (III) to characterize the spatial and temporal variation of this deposition across Iowa. We measured nutrient deposition from July-September 2003 at six sites representing a range of landscape characteristics common in Iowa, and at one site for one year. Comparisons were made among types of deposition measures, as well as among sites. We found that dry deposition can be as or more important than wet deposition. More of the P and N occurred in the dissolved inorganic form for DW than DD. Little of the P in wetfall is in the dissolved inorganic form, whereas most of the TN in wetfall occurred in the inorganic form. Sites showed no significant differences for W, and differences for DD and DW could be explained by characteristics of site location. Two sites were tested for differences in long-distance versus local nutrient transport. Sites were 3.2 km apart, with a 22 m vertical difference. We found no significant differences, indicating long-distance transport for this site may be more important that local inputs as a source of nutrients. Annual loading rates for W+DD and W+DW, and average concentrations in rainfall were calculated. Temporal trends were made graphically, which showed an increase in deposition rates in April, and decreasing rates by November, consistent with spring planting and fall harvests which disturb the soil and release nutrients to v the atmosphere. These results indicate that atmospheric deposition may be an important source of nutrients to water bodies, and should be included in studies of nutrient budgets. VI ACKNOWLEDGEMENTS I would like to extend my sincere thanks to my advisor, Dr. John A. Downing, for his patience and support throughout my time as a graduate student. He always made himself available for my many questions. I would like to thank my committee for their time and patience during the completion of my thesis. Many thanks go to the Limnology Lab at Iowa State University for help with analyzing samples, and especially to Tim Blake, for all the time he spent in a plane collecting samples around the state. Thanks also to the pilot, Terry Mayberry, for getting us where we needed to go. A special acknowledgement goes to the crew at LODA electronics for putting in overtime to build the automated samplers when we needed them. Cory Heilmann was a source of statistical knowledge, for which I am grateful. My family was the epitome of patience throughout my time as a graduate student. I am thankful for their support. Daelyn Woolnough, Dr. Thomas Hrabik, and Grace Hrabik provided emotional s...