Measuring Root Growth Response to Nitrogen Fertilization Rates in Young Pecan Seedlings Using the Minirhizotron Method
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Pecan is an important nut crop in Texas and the United States. Increased demand for pecan exports has created an interest for new and current growers to plant new orchards. Nitrogen (N) application is an important step in establishing a productive pecan orchard, but few studies have determined how N affects the establishment of pecan tree roots, specifically fine roots (0-2 mm) responsible for nutrient uptake. The objectives of this study are to: 1) determine how fine root growth is impacted by five rates of N fertilizer and 2) pinpoint times in the growing season when fine root production peaks to time fertilizer applications for maximum N absorption. The minirhizotron method was used to observe root growth through time at two soil depths, 14-28 cm and 98-112 cm, from February 2010 to June 2012. Images were collected every two weeks, roots were traced individually, and date of birth and death, diameter, and length were recorded. Trees receiving 229.5 kg N ha^-1 (1N) had the greatest standing root length throughout the study at both depths observed. The 2N treatment showed decreased standing root length compared to most other treatments. Two peaks in root growth were observed, in March 2010 and April 2011, when trees began to come out of dormancy. Living root length steadily declined throughout the rest of the growing season. The 1N treatment had greater cumulative root growth than the other treatments and there was a depth effect observed. Root lifespan was influenced by both N treatment and depth. Roots receiving higher rates of fertilizer (1N and 2N) and those living in 14-28 cm of soil had a higher risk of mortality. Root birth season and diameter also affected lifespan. Roots born in spring and those with smaller diameters had a higher risk of mortality. These results support current extension service recommendations that 229.5 kg N ha^-1 is an appropriate amount of fertilizer and does not negatively affect root growth. However, application time could be moved to earlier in the season (March-April as opposed to May) to target the peak time for fine root growth and thus, N absorption.
Graves, Paige (2014). Measuring Root Growth Response to Nitrogen Fertilization Rates in Young Pecan Seedlings Using the Minirhizotron Method. Master's thesis, Texas A & M University. Available electronically from