ARS | Publication request: Root distribution of temperate forage species subjected to water and nitrogen stress

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 11, 2010
Publication Date: July 23, 2010
Citation: Skinner, R.H., Comas, L.H. 2010. Root distribution of temperate forage species subjected to water and nitrogen stress. Crop Science. 50:2178-2185.

Interpretive Summary: Plants with deep root systems can often tolerate drought better than more shallow-rooted species because of their ability to access water deep in the soil profile. At the same time, drought and nutrient stress often induce plants to extend their roots deeper into the soil than they normally would in search of additional water and nutrients. Inclusion of species in pasture mixtures with either inherently deep roots, or with the ability to easily adjust rooting depth could, therefore, improve stress tolerance and productivity of the pasture. We conducted a pot study using 21 forage species, including grasses, legumes, and forbs, to examine the relative rooting characteristics of individual species and functional groups typically found in temperate pastures. Grasses that were stressed generally had larger and deeper roots than did legumes or forbs that were similarly stressed. Nitrogen stress plants had smaller root systems than well-fertilized plants. However, drought stress did not reduce root weight compared with controls. Drought stress also increased the proportion of deep roots, whereas, nitrogen stress did not. Results from this pot study were able to help explain some, but not all, of the differences in root distribution that we have previously observed in field-grown multi-species forage mixtures.

Technical Abstract: Root partitioning patterns can have an important influence on forage yield during periods of moisture or nitrogen stress, as deep-rooted species can potentially access water and nutrients found deeper in the soil profile. Interactions with other species can also influence root distribution. In a greenhouse study, we examined rooting characteristics of 21 species (9 grasses, 6 legumes, and 6 forbs) that are commonly found in northeastern USA pastures. Species examined covered a range of responses to defoliation frequency, drought, and nitrogen availability. This report examines root production and distribution within the soil profile of defoliated forage species in response to water and nitrogen stress. Nitrogen or water stressed grasses generally had greater root biomass and a greater proportion of roots in the deepest soil layer than did legumes or forbs. When averaged across functional groups, nitrogen stress significantly reduced root weight, but to a lesser extent than the decrease in shoot weight, resulting in an increase in root/shoot ratio compared with controls. 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, whereas, nitrogen stress did not. Comparisons with previously published field results suggested that inherent species differences in root distribution could explain some, but not all, rooting patterns observed in multi-species forage mixtures. Rooting depth of taproot forming species especially appeared to be underestimated because of slow growth of the taproot over the relatively short duration of the study.