Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/29/2008
Publication Date: 5/28/2008
Citation: James, J.J. 2008. Effect of soil nitrogen stress on the relative growth rate of annual and perennial grasses in the intermountain west. Plant and Soil Journal. 310:201-210. Interpretive Summary: The ability of a plant to grow fast is expected to come at a cost, namely in terms of stress tolerance. The hypothesis that low nutrient availability will negatively affect the growth of fast-growing weeds more than slow-growing native plants was tested. Low nutrient availability did not decreased relative growth rate or associated morphological and physiological parameters more than native plants. While the idea of using carbon amendments to lower soil N on rangeland and shift the competitive balance toward natives has generated much interest, this study suggests managing these ecosystems for low N availability alone will unlikely inhibit the spread of weeds.
Technical Abstract: A trade-off between inherent relative growth rate (RGR) and tolerance to low nutrient availability is a central theory in plant ecology and is predicted to be a key factor influencing invasion resistance in nutrient-poor systems. Specifically, low nutrient conditions are predicted to favor native species with low RGR while increases in nutrient availability are predicted to favor invasives with high RGR. While there is much evidence indicating that invasives achieve higher RGR than their native counterparts under nutrient-rich conditions, the hypothesis that low nutrient availability decreases RGR of invasives disproportionately more than natives has remained largely unexplored. The broad objective of this study is to examine the degree to which soil N stress affects RGR and RGR components of invasive annual and desirable perennial grasses. It was hypothesized that 1) invasive annual grasses would demonstrate a greater proportional reduction in RGR as soil N stress increased compared to perennial grasses, and 2) the mechanism in which low N decreases RGR of annual grasses more than perennials would depend on the severity of N stress. Three annual and three perennial grasses were exposed to three levels of N availability (0.04, 0.4 and 4.0 mM N). RGR and components of RGR were quantified over four harvests. RGR of all species decreased with decreasing N supply due initially to N stress effects on leaf mass ratio (LMR) and then at more severe stress to effects on net assimilation rate (NAR). There was no evidence that N stress affected invasive annuals more than native perennials. Instead, annuals had greater biomass, tiller production, leaf nitrogen productivity and RGR than perennials at high and low N supply. While the idea of using carbon amendments to lower soil N in disturbed or weed-infested areas and shift the competitive balance toward natives has generated much interest, this study suggests managing ecosystems for low N availability alone will unlikely inhibit the spread of these annual invaders.