The importance of neighbors, soil pH, phosphorus, and nitrogen for the growth of two C-4 grasses

In the coastal prairies of Texas, C-4 grasses dominate. However, reasons for spatial differences in composition of these prairies are uncertain, and the reasons for codominance of Schizachyrium scoparium and Paspalum plicatulum are unexplained. We completed a greenhouse experiment that examined the effects of neighbors, soil pH, added P, and N on the growth of P. plicatulum and S. scoparium, two C-4 grasses. Intraspecific 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 belowground 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 mass 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 compared with that of native soil, at all levels of P and pH. Root:shoot ratios were 5-10 times higher in the native soil compared with N addition treatments and increased to 1.0 as pH decreased. Ratios were about 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 be competitively equal. Their growth was not determined by the presence of neighbors but by levels of soil resources and soil pH.