Root Distribution of Temperate Forage Species Subjected to Water and Nitrogen Stress

Root allocation and distribution patterns can influence forage yield during periods of moisture or nitrogen stress, as deep-rooted species access water and nutrients found deeper in the soil profile. In a greenhouse study, we examined rooting characteristics to a depth of 50 cm for 21 cool-season forage species (9 grasses, 6 legumes, and 6 forbs) exposed to drought and low N conditions. The goal of this research was to compare root distribution under uniform growing conditions for common grass, legume and non-leguminous forb species found in humid-temperate pastures of the northeast United States. Nitrogen or water stressed grasses generally had greater root biomass and a greater proportion of roots in the 30 to 40 cm soil layer than did stressed legumes or forbs. Low N significantly reduced root weight, but to a lesser extent than the decrease in shoot weight, resulting in an increase in root/shoot ratio. Drought stress reduced shoot weight but had no effect on root weight, also resulting in a greater root/shoot ratio. Drought stress also increased the proportion of deep roots compared with controls, whereas, N stress did not. Comparisons with previously published field studies suggested that information from container-grown plants could provide insights into field results, and also suggested that inherent species differences in root distribution could explain some, but not all, rooting patterns observed in multi-species forage mixtures.