Bermudagrass Spring Root Development and Uptake of 15NH415NO3 Following Application of Various Preemergence Herbicides.

Numerous studies indicate that preemergence herbicides can influence root establishment of sprigs or sod, but few have examined their effects on root growth in established turfgrasses.  Preemergence herbicides may move downward in soil following application, thus inhibiting roots of germinating seedlings but also those of established turfgrasses.   Warm-season turfgrasses could be particularly vulnerable to root inhibition from preemergence herbicides where seasonal growth and dormancy cycles occur.  Warm-season turfgrasses experience significant dieback of old roots and replacement by new roots arising from existing rhizomes or stolons during the spring greenup period, generally coinciding with the presence of pre-emergent herbicides.  Restriction of root development in established turf could have numerous implications including decreased nutrient uptake efficiency.  Preemergence herbicides oxadiazon [2-tert-butyl-4-(2,4 dichloro-5-isopropoxyphenyl)-2-1,3,4-oxadiazoline-5-one], prodiamine [N3,N3-Di-n-propyl-2,4-dinitro-6-(trifluoromethyl)-mphenylenediamine], and dithiopyr [3,5-pyridinedicarbothioic acid, 2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-S,S-dimethyl ester] were applied at labeled rates to plots of established ‘Tifdwarf’ bermudagrass (Cynodon dactylon Pers. x C. transvaalensis Burtt-Davy) 16 February 2009.  The plots were mowed at 1.3 cm and had never received preemergence herbicides.  Over the following months, 81 cm² x 30 cm deep polyvinylchloride cylinders were periodically driven into plots and a 5 A% ¹⁵NH₄¹⁵NO₃ solution was syringed into the turf at 4.9 g N m^-2.  After 24 hours, cylinders were removed and soil rinsed from plant tissues.  Plant tissues were analyzed using mass spectrometry to determine amounts of ¹⁵N acquired.  Effects on root development were evaluated by measuring changes in root mass and root length density to a 30 cm depth.  Although not detectable early on, restriction of root growth became evident in prodiamine-treated plants, resulting in decreased N uptake 8 WAT.  Conversely, oxadiazon actually enhanced root growth; although this did not result in greater N uptake.  By 12 WAT, no differences in N uptake could be detected between treatments, as plants acquired ~40% of the applied N within 24 hours.