Degree Type

Dissertation

Date of Award

2008

Degree Name

Doctor of Philosophy

Department

Natural Resource Ecology and Management

Major

Environmental Science

First Advisor

Richard C. Schultz

Second Advisor

Thomas M. Isenhart

Third Advisor

Timothy B. Parkin

Abstract

Denitrification is recognized as the major mechanism for reducing nitrate (NO3-) in riparian buffers and thus diminishing non-point source pollution (NPS) of surface water bodies subject to high N loads. However, increasing denitrification rates in riparian buffers may be trading the problem of NPS pollution of surface waters for atmospheric deterioration and increased global warming potential because denitrification produces nitrous oxide (N2O), a greenhouse gas also involved in stratospheric ozone depletion. Also N2O produced in the denitrification process can be dissolved in groundwater and is eventually emitted into the air when groundwater flows into a stream or a river. Riparian buffers restored from cultivated crop fields may have significant capacities as sinks or sources of CH 4. It therefore is important to quantify the fluxes of N2O, CH4 and dissolved N2O, identify the source of N 2O from different kinds of riparian buffer systems and evaluate the significance of N2O and CH4 sources. We measured N inputs, weather conditions and N2O and CH4 fluxes from soils in forested riparian buffers, warm-season and cool-season grass filters, and a crop field located in the Bear Creek watershed in central Iowa. We measured concentrations of NO3-, dissolved N2O, and other chemical properties in groundwater under a multi-species riparian buffer, a cool-season grass filter, and the adjacent crop field. The forest buffer and grass filter soils had significantly lower bulk density; and higher pH, total carbon (TC), total nitrogen (TN), and ammonium (NH4 +) than those in the crop field. Nitrous oxide emissions from soils in all riparian buffers (1.8-4.5 kg N2O-N ha-1) were significantly less than those in the crop fields (7.2-16.8 kg N 2O-N ha-1), but no differences among different kinds of riparian buffers were observed. Our results indicate that the emission factor (ratio of N2O emission to N inputs) of soils in riparian buffers was smaller than the crop fields. While N2O peak emissions followed by rewetting dry soils and thawing frozen soils significantly contributed to annual N2O emissions from soils in the crop fields, soils in the riparian buffers were less sensitive to such events. Soil incubation with inhibitors indicated that the main sources of N2O might be nitrifier denitrification and denitrification in the crop field soil and nitrifier denitrification in the riparian buffer soils. The ratio of N2O to N2 in riparian buffer soil (0.88-6.8) was less than that found in crop field soil (16.5). These results suggest that N2O emissions from soils in all riparian buffers were significantly less than those in the crop field. In both a multi-species riparian buffer and a cool-season grass filter, NO 3- concentrations in groundwater were significantly decreased in comparison to those in the crop field, by 48-59%. However, dissolved N 2O concentrations in groundwater did not differ among locations (6-14 mug L-1). These results indicate that the riparian buffers decreased NO3- concentrations in near-surface groundwater, without increasing N2O losses. Methane fluxes in crop field soil were not observed to be significantly different from those in the forest buffer and grass filter soils, and no significant difference in CH4 flux was found between the forest buffer and grass filter soils. Annual CH 4 flux was -0.80 kg C ha-1 yr-1, -0.46 kg C ha-1 yr-1, and 0.04 kg C ha-1 yr-1 in the crop field, forest buffers and grass filters, respectively. The annual CH4 flux in forest buffers and grass filters were not significantly different from zero and these three amounts were not significantly different from one another. These results suggest that (1) N2O emissions from soils in all riparian buffers were significantly less than those in the crop field, (2) the riparian buffers decreased NO3 - concentrations in near-surface groundwater, without increasing N 2O losses, and (3) CH4 flux in the crop field, forest buffers and grass filters were not different and CH4 flux was not changed in the forest buffers and grass filters soils, despite that soil properties have changed significantly since the planting of the forest buffers and the grass filters. (Abstract shortened by UMI.)

DOI

https://doi.org/10.31274/rtd-180813-7425

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Dong-Gill Kim

Language

en

Proquest ID

AAI3316230

OCLC Number

271642680

ISBN

9780549688723

File Format

application/pdf

File Size

124 pages

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