Available Nitrogen and Denitrification in Soil Altered by Ground Cover and Nutrient Source in an Organic Apple Orchard

A shift in public demand towards more organic and locally produced fruit and vegetables has been occurring across the United States in recent years. A common practice in organic fruit production is the application of organic ground covers to supply nutrients while enhancing other soil properties. A need for research exists in the southern region of the U.S. examining the effects of regionally applicable ground cover and nutrient management on nitrogen availability and the microbial community to provide information to organic farmers in the region. Two studies were conducted to determine how 12 treatment combinations of four ground covers (compost, wood chips, paper mulch, and mow-and-blow) and three organic fertilizers (poultry litter, organic commercial fertilizer, and a no-fertilizer control) applied every year in April from 2006 to 2013 affected soil properties. In the first study, soils from March 2007 and 2013 were analyzed to determine the long-term effects of the treatment combinations on soil chemical and biological properties at the 0-10 and 10-30 cm depths. In addition, denaturing gradient gel electrophoresis (DGGE) was performed on soil microbial DNA to determine if treatment additions over time had altered the denitrifying community. In the second study, soil biological and chemical properties were measured at the 0-10 cm soil depth before (March) and after (May) yearly ground cover applications (April) to determine how nutrient contents and microbial populations responded to additions immediately (May) and long-term (March) and if responses were the same each year or changed through life of the orchard. Organic matter increased through time regardless of ground cover treatment, with compost resulting in the greatest increase from 1.84 % in 2007 to 5.29 % in 2013. Soil water content, electrical conductivity, microbial biomass nitrogen (N), ammonium (NH4-N), and nitrate-N were all greater in 2013 than in 2007. Microbial species richness (R) was greatest in 2013 in soil receiving compost and wood chips compared to the other ground cover treatments and R in those two ground covers also increased significantly from 2007 to 2013. Shannon-Weaver index of diversity in 2013 progressed from greatest to least in the order of compost ≥ wood chips ≥ paper ≥ mow-and-blow control with diversity in wood chips significantly increasing from among the lowest diversity in 2007 to among the highest diversity in 2013. The second study revealed many treatment differences that were not apparent in the first study when comparing only the beginning and end of the study. Soil organic carbon (C) and N, microbial biomass C and N, NH4-N, and enzyme activities increased through time, peaked during 2009-2011, and declined to levels with relatively few differences between 2007 and 2013 values. Denitrifying communities (nirK) analyzed by DGGE, were a sensitive indicator of treatment effects responding to ground cover treatments in 2007. The trends through time in dissolved nutrients and microbial biomass suggest that the microbial community was not growing continually over time, but shifting in composition and diversity of nirK-containing organisms and possibly other groups facilitating N-cycling. Acknowledgements I would like to thank my advisor, Dr. Mary Savin for the opportunity to continue my education and conduct research at the University of Arkansas and for continued guidance along the way. I would also like to thank the members of my committee, Dr. Kristen Gibson and Dr. Rick Norman for their time and help with my research and thesis. A special thanks goes to my committee member Dr. Curt Rom and his research program, who laid the foundation for this research orchard and especially Jason McAfee for assisting with sampling efforts and coordinating data collection. I could not have completed my data analysis without Dr. Edward Gbur and Jeff Velie, who were consistently a tremendous help. I would like to thank the Southern Sustainable Agriculture Research and Education for funding. I was very lucky to have amazing fellow graduate students, Katy Brantley and Suhartono, and will always be grateful for their assistance and support. I am extremely thankful for the help I had in the lab from many others along the way including Kamela Mitchell, Brina Smith, Connie Moloney, and Ryan Macleod. I am also thankful for the technical support from USDA Poultry Waste Lab at the University of Arkansas and the use of Dr. Ken Korth’s lab equipment. My family and friends provided me with endless love and support and I could have never made it through this journey