Influences of mycorrhizae and phosphorus on belowground competition between two old-field annuals

Abstract

We studied the effects of inter- and intraspecific belowground competition, infection by vesicular-arbuscular mycorrhizae (VAM), and phosphorus supply rate on the growth of ‘target’ ragweed ( Ambrosia artemisiifolia) plants in sand culture. Competition came from 0–16 ragweed or black mustard ( Brassica nigra) ‘neighbors’ in each pot; these neighbors were tied back from the targets to eliminate aboveground competition. In the absence of competition, VAM-inoculated ragweed had a 20-fold greater shoot and total mass, greater total P mass and P concentration in shoots and whole plants, greater P uptake efficiency, and greater root length than uninoculated ragweed. In the presence of belowground competition, target performance generally decreased with increasing neighbor root mass, regardless of the species of the competitor. In general, the responses of VAM-inoculated targets to neighbor root mass at the high P supply rate were best described by linear functions when the neighbors were Ambrosia and by log-linear functions when the neighbors were Brassica, but coefficients of determination were generally low. When we reanalyzed the data using only the maximum target response in each of 16 intervals of neighbor root mass, r 2 values increased and the majority of the regressions were statistically significant. Thus the upper boundaries of target performance were strongly correlated with competitive pressure. The results of these experiments suggest that (1) annual ragweed is obligately mycorrhizal under these soil conditions, (2) belowground competition can strongly affect ragweed growth, adn (3) under some conditions a non-mycorrhizal competitor can have a stronger per-gram belowground competitive effect on the growth of a mycorrhizal plant than a conspecific, mycorrhizal neighbor.