Abstract
This study was conducted to determine the colonization capacity and levels of vesicular-arbuscular propagules in a Costa Rican soil (Ultisols, Tropohumult) under grass (Pennisetum purpureum Schumach. var. King), coffee (Coffea arabica L.), and mature forest cover. The study was conducted on experimental plots at a tropical wet lower montane forest site on the Pacific side of the Talamanca Mountains. An extractive bioassay with Sorghum vulgare Pers estimated the most probable number (MPN) of endomycorrhiza fungus propagules present. Per cent endomycorrhiza colonization in the roots of field-planted Sudax (S. vulgare Pers. x Sorghum sudanense Stapf) was used to compare in situ colonization capacity of mycorrhiza propagules. Sudax height, top, and root weights were used as relative measures of colonization effectiveness. The bioassay yielded the same value of 110 propagules per gram of moist soil for all soils as the MPN of viable endomycorrhiza propagules in the soils. At four weeks, per cent root colonization in the field averaged 48, 93, and 96% for the coffee, grass, and primary forest soils, respectively. In the coffee soil, root colonization was significantly lower (P = 0.001), plant height was significantly greater (P = 0.012), and P soil content was significantly greater (P = 0.053), than in grass and mature forest soil. Soil P content correlated negatively with per cent root colonization (r = -0.82, P = 0.007) for all soils. At 8 and 14 weeks, root colonization was not significantly different among the three field plots. These results suggest that there was no major difference in endomycorrhiza inoculum potential among the three cultural practices, but that early root colonization was negatively correlated with soil P content.
