Influence of three weed species on soil nutrient dynamics

The hypotheses that weeds increase soil nutrient availability and differentially alter soil nutrient dynamics were tested in the greenhouse by comparing and contrasting plant-soil relationships among Bromus tectorum L. (cheatgrass, downy brome), Lepidium latifolium L. (perennial pepperweed, tall whitetop), and Centaurea solstitialis L. (yellow starthistle). These species are highly invasive and produce dense stands that reduce or eliminate native species. A high resource (HR) soil and a low resource (LR) soil were evaluated. To simulate declining soil resources, individual plants were grown in the same pots, hair-vested, then replanted for a total of three growth cycles. Aboveground mass was greatest in the HR soil in which L. latifolium and C. solstitialis accumulated greatest biomass after the second growth cycle and B. tectorum accumulated greatest biomass after the third growth cycle. In the LR soil, all plants had the greatest aboveground mass after the third growth cycle. Overall, nutrient concentration in aboveground tissue declined significantly with growth cycle. Tissue K and P concentrations were highest in plants grown in the HR soil, whereas Ca and N were highest in plants grown in the LR soil. Nutrient uptake overall decreased with growth cycle for the HR soil and increased with growth cycle for the LR soil. Relative to unplanted controls, all three species facilitated an increase in soil-solution Ca and Mg in the FIR soil, greater N-mineralization potential in both soils, and greater enzyme activity in the LR soil by the third growth cycle. The species displayed distinctly different ratios of nutrient uptake and concentration in aboveground tissue, with L. latifolium having much higher tissue Ca/Mg concentration ratios and uptake ratios of Ca/Mg, Ca/N, and Ca/P than C. solstitialis or B. tectorum. Stepwise regression suggests soils with high levels of available Mg may have a greater risk for invasion by C. solstitialis and L. latifolium. These data support the working hypotheses.