THE IMPORTANCE OF NEIGHBORS, SOIL-PH, PHOSPHORUS, AND NITROGEN FOR THE GROWTH OF 2 C-4 GRASSES

In the coastal prairies of Texas, C-4 grasses dominate. However, reaso ns for spatial differences in composition of these prairies are uncert ain, and the reasons for codominance of Schizachyrium scoparium and Pa spalum plicatulum are unexplained. We completed a greenhouse experimen t that examined the effects of neighbors, soil pH, added P, and N on t he growth of P. plicatulum and S. scoparium, two C-4 grasses. Intraspe cific competition was essentially equal to interspecific competition, and soil factors seem to control dry mass of these two species. For P. plicatulum, addition of P to native soil increased above- and belowgr ound dry mass 1.8-2.5 times, while P and N together increased dry mass 2.9-6.5 times. The response was pH dependent, with the greatest dry m ass at low pH. Root:shoot ratios changed from 0.3 to ca. 1.0 as the pH decreased in native soil regardless of the P level. When N was added, ratios were ca. 0.3 across all pH and P treatments. For S. scoparium in native soil, there was little change in aboveground dry mass when P was added: however, belowground dry mass was augmented 1.5-1.9 times, with the greatest dry mass at pH 7.0. Additions of P and N increased aboveground dry mass 2.2-2.7 times, with the greatest dry mass at pH 7 .0. N additions caused reductions of 79%-89% in belowground dry mass c ompared with that of native soil, at all levels of P and pH. Root:shoo t ratios were 5-10 times higher in the native soil compared with N add ition treatments and increased to 1.0 as pH decreased. Ratios were abo ut 15% higher with P added to native soil and remained constant at ca. 0.1 in all the N addition treatments. These two C-4 grasses seem to b e competitively equal. Their growth was not determined by the presence of neighbors but by levels of soil resources and soil pH.