Verdure: I’m not dead; I’m dormant
When should fertilizer be applied to replenish the nitrogen reserves that fuel the green-up and growth of bermudagrass breaking dormancy?
Beth Guertal, Ph.D.
In many regions, bermudagrass likes to take a lengthy timeout, going dormant in colder weather. After a few months of hanging out and being brown, the bermudagrass will begin to break dormancy, greening up and growing. But when do we fertilize that emerging bermudagrass? Does it immediately need an application of fertilizer nitrogen, or is the residual nitrogen hanging out in the stolons/rhizomes (stems) and roots sufficient for use by the grass? That’s the question researchers at North Carolina State University set out to answer, using labeled nitrogen in a growth chamber study to determine which pool of nitrogen was providing the growing bermudagrass its nitrogen nutrition.
To trace the source of the nitrogen, researchers first collected dormant, established common bermudagrass and then cut individual nodes and internodes (with attached roots) from washed stolons. (A node is the growth tissue, from which new shoots emerge.) The plant material was moved to a greenhouse, and half of the nodes were grown with labeled nitrogen (15N). Labeling allows the nitrogen to be tracked as it is taken up and moved around within the plant.
All the harvested nodes were allowed to grow for 21 days, with plants destructively harvested at three, six, 10, 14 and 21 days as the bermudagrass grew and broke dormancy. At each harvest time, the plantlet was separated into the node, internode, old root, new roots and individual shoots, and each part was analyzed for labeled (the 15N) and unlabeled nitrogen contents.
As the bermudagrass grew and broke dormancy, new growth occurred in two places: shoots and new roots. The dry weight of the internodes dropped during this time, and the weight of the nodes was unaffected. Although shoots that received nitrogen fertilizer were bigger than those that did not receive nitrogen, the weights were not significantly different at 21 days. Also, shoots appeared at the same rate over time, regardless of nitrogen fertilization. As for the roots, their length and number were affected by nitrogen fertilization, with fertilized bermudagrass having a greater number of new roots. However, although fertilized plants had more roots, those roots were shorter, and individual roots in plants that received no nitrogen were often longer.
So, where was the nitrogen going in each plant? Even when nitrogen was not added, nitrogen was being transferred from the internode to new shoots and roots. Pools of nitrogen in the node and old roots were small and tended to stay constant throughout growth. Supplying external nitrogen changed how the plant accumulated nitrogen. In the first 10 days, the plant was still obtaining nitrogen from the stored internode nitrogen. After that, the old roots began to take up fertilizer nitrogen, and it also accumulated in the shoots. By day 14, most of the nitrogen in new roots was from the fertilizer nitrogen. By the 21-day harvest, about 28% of the nitrogen in the internode and 40% of the nitrogen in the nodes and old roots was from the fertilizer. Thus, the growing bermudagrass used fertilizer nitrogen to replenish the existing nitrogen (in the internodes).
This work demonstrates that internal nitrogen reserves (primarily in the internodes) were sufficient to provide nitrogen for bermudagrass as it broke dormancy. That supply was sufficient to promote shoot growth for only the first 10 to 14 days, after which supplemental nitrogen was needed to continue growth. This work supported the standard recommendation that nitrogen fertilization should start at about two weeks after first green-up, when the growing bermudagrass is ready to use that applied nitrogen.
Source: Sermons, S.M., B.G. Wherley, C. Zhang, D.C. Bowman and T.W. Rufty. 2017. The role of internal and external nitrogen pools in bermudagrass growth during spring emergence from dormancy. Journal of Plant Nutrition 40:1404-1416.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.