Minirhizotron Studies: the Effect of Nitrogen Fertilization on Root Phenology on Hog Island Dunes.

Everett Weber and Frank P. Day.

Little work has been done on the phenology of root growth and senescence largely due to the methodological difficulties. The application of minirhizotron technology has enabled the tracking of individual roots through an entire growing season. As a result, direct measures of turnover, root growth, and an analysis of cohorts were made. Small plots on a 36 year old dune of Hog Island were fertilized with nitrogen. Minirhizotron tubes were installed in each fertilized and control plot. Each tube was sampled monthly for nine months, March through October of 1992. Root length density increased throughout the growing season with the highest root length density in the top 0 - 20 cm of the soil profile. The fertilized plots had higher root length densities (14.05 mm cm-2) than the unfertilized plots (2.88 mm cm-2). The turnover was higher in the unfertilized plots only in the top 0 - 20 cm of the soil profile (fertilized = 0.020 percent loss per day, unfertilized 0.024 percent loss per day). The cohort analysis found that the largest loss of roots for a cohort occurs within the first month. There was also a decline in root loss in the last sampling of the last cohort potentially indicating the roots were preparing for the winter months. The overall low turnover rate, the decreased turnover rate with fertilization and the decreased turnover in the last cohort imply that roots tend to be conserved in this nutrient poor system.