Soil depth and fertility effects on biomass and nutrient allocation in jaraguagrass.

Abstract

The African perennial C4 grass Hyparrhenia rufa (Nees) Stapf has successfully invaded the lowland non-flooded savannas of Venezuela except in isolated sites with a shallow lithoplinthic hardpan. To study the mechanism of this invasion process, an experiment was designed to determine the effect of soil fertility and depth of the lithoplinthic hardpan on growth, biomass, and nutrient allocation of H. rufa. The main treatments were fertilization with nitrogen, phosphorus and potassium and mechanical disruption of the lithoplinthic horizon prior to seeding with H. rufa at the beginning of the rainy season. Soil fertility rather than soil depth is the predominant abiotic variable regulating the invasion and growth of H. rufa in savanna sites with a shallow lithoplinthic horizon. H. rufa exhibited flexibility in phenology, morphology, productivity and biomass allocation patterns in response to nutrient availability. These responses are typical of successful invader plants. Fertilization significantly increased plant growth through increased tillering and leaf production. Fertilization increased total and organ biomass by approximately 1,000% and the highest proportion was allocated to reproductive tillers. In unfertilized plants, live leaves comprised the highest fraction (approximately 40%) of total biomass whereas the root/shoot ratio was about 0.3 in all treatments. N concentration was approximately 50% higher in roots and rhizomes than in other organs at the beginning of the dry season and under all treatments. Live leaves of unfertilized plants had higher N concentration than leaves of fertilized plants. Phosphorus and K concentrations were similar among vegetative organs but approximately 400% greater in reproductive tillers of fertilized plants. Fertilized plants bad the greatest total content of mineral nutrients due to increased biomass production.