Monitoring Root Development in Forage Mixtures to Investigate the Effects of Defoliation and N Fertilization Source

Our understanding of how the root system of perennial forage crops changes during the growing season and how it is affected by crop management is limited. Our study aimed at investigating the effects of N fertilization sources and forage defoliation on root development of perennial forage grasses, in eastern Canada. A mixture of timothy (Phleum pratense L.) and tall fescue (Festuca arundinacea Schreb.) was seeded in spring 2016 and defoliated once in 2016 and three times in 2017. Ammonium nitrate and dairy cattle slurry were compared as N sources. Split N applications totalized 100 kg and 150 kg available N ha^-1 in 2016 and 2017, respectively. Root growth was monitored from autumn 2016 to autumn 2017 by taking root images weekly with a minirhizotron camera in transparent tubes installed into the soil at a 60-cm depth. Images were analyzed for root length, area, and diameter. Root dry biomass was also measured every autumn by soil coring to a 45-cm depth. Root length increased every week until the third defoliation in 2017, while average root diameter decreased, indicating an increasing proportion of fine roots. After the first defoliation in 2017, the daily growth rate of root area decreased by 68 %, probably due to the utilization of root carbon for aboveground regrowth. Root biomass was mostly located (73 %) in the upper 15 cm of soil, while root length was evenly distributed over the soil profile, suggesting that deep roots are thinner than shallow roots. From autumn 2016 to autumn 2017, root biomass increased by 40 % and root length by 369 % (0-45 cm). Fertilization source did not affect root biomass and morphological traits in 2017. Improving our knowledge of root growth and senescence will allow refining estimates of root turnover and carbon input to soils by forage crops.