Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2004
Publication Date: May 1, 2004
Citation: Blank, R.R., Young, J.A. 2004. Influence of three weed species on soil nutrient availability. Soil Science. 169(5): 385-397. Interpretive Summary: Soil nutrient availability is an important component in determining competitive outcomes among plants. We hypothesized that weeds might be able to increase soil nutrient availability to facilitate their success. To test this hypothesis, we studied the effect of the highly invasive weeds, perennial pepperweed, cheatgrass, and yellow starthistle, on soil nutrient availability. Two initial soil types were investigated; a high fertility soil presently being invaded by perennial pepperweed and a low fertility soil with low base saturation and low pH. To simulate the effect of declining soil resources each plant was subjected to three successive growth cycles. Relative to unplanted controls, growth of the weed species increased availability of Ca+2 and Mg+2, increased N-mineralization potentials, and soil enzyme activities; but initial soil fertility level and growth cycle moderated the effect. Our data supports the working hypothesis.
Technical Abstract: Weeds use a multitude of strategies to facilitate their invasiveness. We hypothesized that weeds may increase soil nutrient availability to elevate their competitive stature. The hypothesis was tested in the greenhouse, examining and contrasting plant-soil relationships among downy brome (cheatgrass), perennial pepperweed (tall whitetop), and yellow starthistle. Two soil types were evaluated: a high resource (HR) soil presently being invaded by perennial pepperweed and a low resource (LR) soil from a high elevation Sierra Nevada montane meadow. To simulate declining soil resources, individual plants were grown in the same pots, harvested, then replanted for a total of 3 growth cycles. Attributes measured after each growth cycle included aboveground and belowground plant mass, aboveground plant tissue nutrient concentration, available soil N, potentially available soil N, bicarbonate-extractable soil P, soil-solution nutrients, and soil enzyme activities. Aboveground mass was greatest in the HR soil in which perennial pepperweed and yellow starthistle accumulated greatest biomass after the second growth cycle and downy brome accumulated greatest biomass after the third growth cycle. In the LR soil, all plants had the greatest aboveground mass after the third growth cycle. The ratio of aboveground mass in the HR soil to the LR soil was significantly (P=0.05) higher for perennial pepperweed and yellow starthistle than downy brome, and for all plants, this ratio declined significantly in the third growth cycle. Overall, nutrient concentration in aboveground tissue significantly declined with growth cycle. Tissue K and P concentrations were highest from plants grown in the HR soil, whereas Ca and N were highest from plants grown in the LR soil. Relative to unplanted controls, all three species facilitated an increase in soil-solution Ca+2 and Mg+2 in the HR soil, greater N-mineralization potential in both soils, and greater enzyme activities in the LR soil by the third growth cycle. Relative to several nutrient availability pools the species differed considerably in their proportion of uptake by soil resource level and growth cycle. The species displayed distinctly different ratios of nutrient uptake and concentration in aboveground tissue with perennial pepperweed having much higher tissue Ca/Mg concentration ratios and uptake ratios of Ca/Mg, Ca/N, and Ca/P than yellow starthistle or downy brome. Stepwise regression indicated that the ratio of N uptake to the total available N pool was the strongest factor explaining plant mass of all three weeds species. That soil-solution Mg+2 was also a significant factor explaining the growth of yellow starthistle and perennial pepperweed suggests soils with high levels of available Mg+2 may have a greater risk for invasion. These data support the working hypothesis that these three weeds can increase soil nutrient availability.