Effect of converting row crop to prairie on nutrient concentration in shallow groundwater and soil properties

Alteration of the Iowa landscape transformed millions of hectares of tallgrass prairie into highly productive fields of primarily corn (Zea mays L.) and soybean [Glycine max. (L.) Merr.]. Introduction of native prairie in contour strips and at the footslope within row crop fields has been shown to reduce nutrient export from fields thus reducing adverse environmental effects. Inclusion of prairie within row crop fields provided an opportunity to modify soil properties to similar conditions prior to row crop use via organic matter addition and deep rooting. Nutrient concentrations in shallow groundwater beneath row crop have been shown to be elevated compared to nutrient concentrations in shallow groundwater beneath native vegetation. The first study detailed in this thesis compared concentrations of nitrate-nitrogen and phosphorous in groundwater beneath four treatments: 100% row crop, 10% footslope prairie strip (PS), 10% contour coupled with footslope PS, and 20% contour coupled with footslope PS. Maximum annual nitrate-nitrogen fluxes (kg ha-1) in the top 2 m beneath the soil surface in order from largest to smallest were 100% row crop, 10% foot slope PS, 10% PS in contours with footslope cover, and 20% PS in contours with footslope cover. Maximum annual fluxes (kg ha-1) for phosphorous were in decreasing order 10% footslope PS, 10% contour with footslope cover, 100% row crop, and 20% contour with footslope cover. In the 100% row crop treatment, it was possible phosphorous was exported with runoff instead of deposited with sediment at footslopes with phosphorous-releasing conditions. The second study reviewed soil data collected from 6 sites in 5 distinct locations throughout Iowa. A subset from sites with similar soil types was reviewed to determine the effects of reversion to native prairie from row crop for a chronosequence of 0, 10, 25, and 37 years. The remaining 3 sites with differing soil types were analyzed for 0 and 2 year trends. Soil properties measured from all sites were total nitrogen (TN), total carbon (TC), pH, bulk density, aggregate size distribution, and particulate organic matter (POM) associated carbon and nitrogen. In general, both carbon and nitrogen increased while maintaining a similar TC:TN. Bulk density decreased with time and pH did not follow a distinct pattern. After 10 years in prairie, macroaggregate fractions increased significantly and were maintained over time. Carbon and nitrogen content within aggregate fractions increased significantly while maintaining the TC:TN ratio. Within the POM fractions, TC and TN did not express a general increasing trend though the TC:TN ratio increased. Conservatively, prairie litter and dead roots annually provided 1950 kg C ha-1 and 2250 kg C ha-1 more than corn/soybean and continuous corn rotations, respectively. Annually prairie litter contained 53 kg N ha-1 and 57 kg N ha-1 more than corn/soybean rotation and continuous corn, respectively. High variability in soil texture, soil genesis, and precipitation patterns warrant further investigation into both shallow groundwater and soil property alteration following conversion from row crop to prairie. Further study will assess the applicability of integrating prairie vegetation as a wide-spread conservation practice