Plants retard litter decay in a nutrient-limited soil: a case of exploitative competition?

Litter was incubated in pasteurized and unpasteurized soils, with and without the presence of prairie grasses (Bouteloua gracilis or Schizachyrium scoparium), to determine if competition between plants and saprophytes altered rates of litter decay. A soil pasteurization treatment was included to ascertain if the presence of vesicular-arbuscular mycorrhizae inocula would affect the competitive interaction. Analyses of variance detected significant effects (P<0.5) of soil pasteurization, plant species and litter presence on root mass, and significant effects of plant species and litter presence on shoot and total plant mass. The degree of mycorrhizal colonization varied little between plant species but mycorrhizae were entirely absent in pasteurized soils; soil pasteurization usually reduced plant growth. Analyses of covariance detected (1) a highly significant negative relationship (P<0.01) between litter mass loss and plant growth (as covariates), when controlling for the effects of time, and (2) a significant positive relationship (P<0.05) between litter nitrogen content and mass loss (as covariates), when controlling for the effects of time and soil pasteurization. These relationships would be expected if exploitative competition for nutrients existed between plants and decomposers such that nutrient uptake by plants reduced concomitant litter decay.